Help - Search - Members - Calendar
Full Version: Jetting Webers on 2.7 911 engine.
914World.com > The 914 Forums > 914World Garage
Pages: 1, 2
914Toy
I recently installed a "double" Innovative air/fuel (A/F) gauge with sensors in both headers of my 1977 2.7 911 engine, which is stock except for: ignition, Weber carbs, reground cams (modified SC) for spirited street performance with the carbs, and exhaust headers (MB911). The A/F gauge has assisted final tuning, including fine balancing of the carbs (well worth it).

Ignition is Clewett crank fire with ignition timing set at: 10 deg. idle 800 rpm, 29 deg at 3000 rpm, and 33 deg at 6000rpm.

Carbs are Weber 40IDA's with 34 main chokes and tall secondary chokes. Jets are: 145 main's, 180 air correction's, F3 emulsion tubes, and 60 idle's.

Engine is running very smoothly at all rpm's, no carb "spitting" or exhaust popping, no hesitation under any acceleration, and instant accelerator response with plenty of power. In other words, running great. However, A/F's are not perfect with 10.5 at idle, 12.5 to 13 at cruising, and high 13's up hill WOT. On a recent uninterrupted 180 miles run on 101 at 3400rpm (78mph), fuel consumption was 24miles per gallon smile.gif

I have discussed the A/F ratio issue with several carb "experts" - mostly Old School guys. Consensus is my numbers are as good as it gets vs. near perfect 14.7 A/F one can expect from EFI.

Any comments will be welcome.
gereed75
Agree with "experts". Those A/F's are what I get on my 2.4 mod solex motor. Dont think you will do a lot better without a LOT of screwing around for minuscule gain. Run it!!
TravisNeff
Is this an S spec engine, or standard? (35mm ports vs 32mm)?
914Toy
QUOTE(TravisNeff @ Oct 5 2018, 01:46 PM) *

Is this an S spec engine, or standard? (35mm ports vs 32mm)?


Purchased this 1977 engine with CIS thus assume stock port size. Checking with builder actual size.
TravisNeff
I got a vent and jet kit from PartsKlassic, they suggested 32mm vents for my 2.7 (74 std small port and low 8.0 compression)
TravisNeff
I got a vent and jet kit from PartsKlassic, they suggested 32mm vents for my 2.7 (74 std small port and low 8.0 compression)
TravisNeff
I got a vent and jet kit from PartsKlassic, they suggested 32mm vents for my 2.7 (74 std small port and low 8.0 compression)
PanelBilly
Sounds like you’ve found a sweet spot. I’ve been reading a thread on the pelican site about tuning PMOs. I’m lost when it comes to carbs.
larryM
see jetting chart here
http://forums.pelicanparts.com/porsche-911...g-question.html

also see https://members.rennlist.com/webercarbs/Web...et_Settings.htm

i did the dual AFR on my 2.7Rs & 2.8 long ago - it's the only way to go!

remember that the Weber IDA mostly run on the idle jets up to 3000 rpm, so focus there for hiway cruising AFR

(fwiw my sense from many yrs of tuning IDA's is that you are too rich, but it helps prevent WOT detonation if CR is above 9.5)


QUOTE(914Toy @ Oct 5 2018, 09:52 AM) *

Carbs are Weber 40IDA's with 34 main chokes and tall secondary chokes. Jets are: 145 main's, 180 air correction's, F3 emulsion tubes, and 60 idle's.
Any comments will be welcome.
914Toy
QUOTE(larryM @ Oct 6 2018, 10:18 AM) *

see jetting chart here
http://forums.pelicanparts.com/porsche-911...g-question.html

also see https://members.rennlist.com/webercarbs/Web...et_Settings.htm

i did the dual AFR on my 2.7Rs & 2.8 long ago - it's the only way to go!

remember that the Weber IDA mostly run on the idle jets up to 3000 rpm, so focus there for hiway cruising AFR

(fwiw my sense from many yrs of tuning IDA's is that you are too rich, but it helps prevent WOT detonation if CR is above 9.5)


QUOTE(914Toy @ Oct 5 2018, 09:52 AM) *

Carbs are Weber 40IDA's with 34 main chokes and tall secondary chokes. Jets are: 145 main's, 180 air correction's, F3 emulsion tubes, and 60 idle's.
Any comments will be welcome.



Thanks for the chart. Comforting to see I am right at the recommendation after a lot of testing beerchug.gif
IronHillRestorations
You can lean it out a bit by just opening the air bypass screws a little. Do you have a STE air flow meter? If so, start by making sure they are pulling the same amount of air, and then open up 1/4 - 1/2 turn each and see if it helps.

Alternately you could close the mixture screws a very small amount each, like 1/8 a turn.

To be honest though, I'd say you are in the range especially if your gas isn't the best. As long as you aren't fouling plugs, I'd say don't do much, if anything. Your ignition is a primo system, which is a plus for carbs.

Years ago 12.5 - 13 was considered very good, but that was before you could put an AF meter in the dash, which kinda makes it easier to think you can get it better, when it's really pretty good as is.
larryM
more info

check this site - Mike Pierce is "Mr. Weber" www.piercemanifolds.com

"The secret to understanding the critical nature of the carburetor set up and the advantages of a WEBER over other carburetors is the Idle circuit. Referred to as the low speed circuit by Weber, this circuit is responsible for 80% of the driving operation."

Mike told me MANY yrs ago related to my 40 IDA's "idle screws 3/4 to 1-1/2 turns out" - "if other, need to rejet"..."

(i then had the benefit if living in Gilroy, so it was real easy to go chat with him)

re: drilling your own jets - see this http://www.914world.com/bbs2/index.php?showtopic=333749
porschetub
QUOTE(IronHillRestorations @ Oct 7 2018, 03:11 PM) *

You can lean it out a bit by just opening the air bypass screws a little. Do you have a STE air flow meter? If so, start by making sure they are pulling the same amount of air, and then open up 1/4 - 1/2 turn each and see if it helps.

Alternately you could close the mixture screws a very small amount each, like 1/8 a turn.

To be honest though, I'd say you are in the range especially if your gas isn't the best. As long as you aren't fouling plugs, I'd say don't do much, if anything. Your ignition is a primo system, which is a plus for carbs.

Years ago 12.5 - 13 was considered very good, but that was before you could put an AF meter in the dash, which kinda makes it easier to think you can get it better, when it's really pretty good as is.

agree.gif think the OP is in the right range,on the rich side is a safe move with any aircooled engine on carbs IMO.
Perry made good points,if your idle jets are spot on its important to stay in the range of 2.5 turns for the mixture screws ,the air bleeds are handy for that final low speed balance between throats, adjustments are usually small .
The low speed circuit is far more importance to get sorted because that is your driving range.
Don't attempt to tune carbs unless the engine is tuned well to start with,ask me how I know ????.
GeorgeRud
Paul Abbot’s Performance Oriented site also has amazing info on properly tuning Weber carbs.
gereed75
i Am not a “Weber guru”. I have worked pretty diligently to get mine set up and tuned pretty well and have studied a lot. Paul Abbot really is a guru and has made a huge amount of info available on his site

In my experience, it is very hard to nail the idle jet size. Although my car runs at numbers very close to OP’s (rich at idle, slightly rich throughout range) on 60 idle jets, it would not run at all on the next smaller idle jet (55)

In talking to and reading Paul Abbot, he found it necessary to ream idle jets to “between” sizes on his personal (perfect I am sure) car..

Again, that is why I said that you might get slightly better AFR’s, but it will take lots of work and will only improve slightly.

I think you will find that the leaner 14:1 ratios seen with EFI are achieved at part throttle, steady state, low load cruise conditions. That is where EFI can shine in getting better mpg. As soon as load increases or throttle is opened, the EFI’s will start to richen back towards 12-13:1.

Unless you want to be a guru, relax, have fun, Run it!!
porschetub
QUOTE(gereed75 @ Oct 8 2018, 02:43 AM) *

i Am not a “Weber guru”. I have worked pretty diligently to get mine set up and tuned pretty well and have studied a lot. Paul Abbot really is a guru and has made a huge amount of info available on his site

In my experience, it is very hard to nail the idle jet size. Although my car runs at numbers very close to OP’s (rich at idle, slightly rich throughout range) on 60 idle jets, it would not run at all on the next smaller idle jet (55)

In talking to and reading Paul Abbot, he found it necessary to ream idle jets to “between” sizes on his personal (perfect I am sure) car..

Again, that is why I said that you might get slightly better AFR’s, but it will take lots of work and will only improve slightly.

I think you will find that the leaner 14:1 ratios seen with EFI are achieved at part throttle, steady state, low load cruise conditions. That is where EFI can shine in getting better mpg. As soon as load increases or throttle is opened, the EFI’s will start to richen back towards 12-13:1.

Unless you want to be a guru, relax, have fun, Run it!!


Good points there,I didn't fully understand the changes necessary when I went to larger vents (30mm) and have a better understanding now,and no my idle jets weren't easy to get right,went for the common #55's as a starter,engine didn't respond well on the idle screws and was very finicky balancing on the air bleeds,I dropped to #52' and was amazed what that small change made,went back to stock idles as an experiment which worked out rather lean with lots of spitback even with a warm engine.
You learn as you go along,one thing that was obvious was when the idle jetting is wrong which ever way you get poor response when tuning and balancing ,my high speed jetting was somewhat easier went 115 to 130 then back to 125,fuel economy certainly improved.
My testing has been on Zenith carbs but the principals are basically the same,any tuning is a waste of time unless valve setting, timing and ignition are totally up to scratch.
914Toy
Thanks for all the responses. After carefully adjusting all of the linkage, including the accelerator pump linkage ( to snug fit), I have settled at where I started with this thread except I have replaced the 60 idle jets with 55's, accepting the slightest hesitation with sudden acceleration some of the time. I now know 60's will eliminate all hesitation, but I prefer better fuel consumption, more power(?), and less potential for carbon build up. Engine on my test hill climb has a little more power and idle A/F's are a little better at high 10's. I may look for some drill bits to ream the 55' out to say 57.
IronHillRestorations
If I resort to reaming jets, I use a Sharpie on the reamer to mark the starting point, in this case lets say 55, and then if I have the next size up (60), I use that to make another mark, the top mark being only a reference. Then I barely go past the first mark with all the jets and test. It's a tedious and time consuming process to do it right, but it can help dial it in.

edited

If you get reamers, it's also good to get the plug gauge set. FWIW, though a 55 plug gauge won't go into a 55 jet though, so opening to that point can be a good starting point. You can use the Sharpie on the reamer, and then a scribe to gently mark a line.
914Toy
Thanks Perry. This process is above my paygrade sad.gif
DaveO90s4
Hi Toy914 and all,

Did you use the Innovate "DLG-1 Dual Lambda (Air/Fuel Ratio) Gauge" with the 8 foot long leads? If so were the leads long enough to mount the gauges somewhere sensible?

I need to do the same for my Corvair-powered 2.8L engine - which has O2 sensor bungs in each exhaust right at the back of the car.

I'm running zenith 40TINS with 30mm chokes, running way too rich. May copy your jetting and see how I go.

Your reply much appreciated,

DaveO
72hardtop
With regards to jetting....

If you're using a centrifugal only distributor shoot for 13:0 - 13:5 across the board.

If you have a vacuum advance distributor you can go leaner but stay out of the 'Deathzone'....14:0 - 15:0 AFR zone At WOT. This is where EGT & CHT temps are the absolute highest.

Go leaner than 15:0 and the temps are cooler than 13:0 or so (partial throttle). Ideally 15:5 - 17:0 at cruise speed light load/cracked/part throttle = GREAT MPG's! This can only be done with a vacuum advance distributor or mapped EFI set up.

WOT = 100% load

Load = Throttle position

Part throttle = light load

See where the EGT and CHT temps go beyond peak....

Click to view attachment
gereed75
Hardtop,

that is a very good graph and if you understand it you are a long way towards understanding mixture control. A couple of problems though when trying to apply it to this automotive carbureted applications=

With Weber’s there are only so many adjustments available to control mixture. Within the limits of these adjustments I see know way you could achieve lean of peak operations under part throttle/ light load conditions and then dependably transition back to rich of peak under load, no matter what kind of distributor you are running.

We got no mixture control knob. With EFI you could program for lean of peak, but this ain’t EFI.

That’s why you have to settle for 12.5 - 13.5 or so. As you know those AFR’s are all Rich of peak.

I think we are ranging well beyond the scope of the original question
914Toy
QUOTE(DaveO90s4 @ Oct 7 2018, 10:51 PM) *

Hi Toy914 and all,

Did you use the Innovate "DLG-1 Dual Lambda (Air/Fuel Ratio) Gauge" with the 8 foot long leads? If so were the leads long enough to mount the gauges somewhere sensible?

I need to do the same for my Corvair-powered 2.8L engine - which has O2 sensor bungs in each exhaust right at the back of the car.

I'm running zenith 40TINS with 30mm chokes, running way too rich. May copy your jetting and see how I go.

Your reply much appreciated,

DaveO


Yes, and here is where I placed the gauge.
Click to view attachment
72hardtop
QUOTE(gereed75 @ Oct 8 2018, 07:20 AM) *

Hardtop,

that is a very good graph and if you understand it you are a long way towards understanding mixture control. A couple of problems though when trying to apply it to this automotive carbureted applications=

With Weber’s there are only so many adjustments available to control mixture. Within the limits of these adjustments I see know way you could achieve lean of peak operations under part throttle/ light load conditions and then dependably transition back to rich of peak under load, no matter what kind of distributor you are running.

We got no mixture control knob. With EFI you could program for lean of peak, but this ain’t EFI.

That’s why you have to settle for 12.5 - 13.5 or so. As you know those AFR’s are all Rich of peak.

I think we are ranging well beyond the scope of the original question



That's what makes them great they are very tune-able compared to stock carbs. You have more control over them. It all boils down to flow and knowing when one jet finishes and the next picks up.

The distributor load senses.

As the load increases (slight road increases) in a fixed throttle position flow decreases (timing backs off) but fuel rate stays steady. The flow thru the carb allows for the mix to richen as the airflow drops. And if you put your foot into it....it will richen further.

This is not beyond the scope of the thread. Not at all.

After tuning my 2056cc in my Westy it will with a cracked throttle hang 15.5-17 AFR or so on the highway. Any incline as the speed drops the AFR richens to 13 range. Give throttle to maintain speed....AFR holds highs 12's low 13's.

Around town driving with part throttle light load cruise high 15-mid 16 AFR. Get into the throttle low 12's to 13. WOT 12-low 13's

It is possible to do.

Want a good read....

https://www.thesamba.com/vw/forum/viewtopic...hlight=wideband
jfort
I have a recently completed 2.7 with DFI, PMOs, JEs at 10.8, and Dougherty cams DC30/Mod. Solex.

Info from build sheets: Manifold 40/36mm. Venturies 40/34. Main Jets 145. AC's 185. Idle Jets 50 (we changed the initial 55, tried 60). Idle Airs 130. E-Tubes F3.

The DFI is set at 2° INIT, 25° 3000, 5° 8000.

This was after Richard Parr and Jay Kjoller collaborated extensively. Runs great now.

I understand about half of this, but I hope it helps.
gereed75
Hmmm. Interesting. Did not think that the Weber’s could do that.

Can’t say that I ever collected any data for sustained light load part throttle ops. I was only concerned with safe mixtures under load.

Following your thought process, I would think that if your Weber’s are tuned to produce 13 AFR under load that they will intrinsically yield leaner mixtures under light loads part throttle. By there design, I Don’t see how you could have one without the other unless you were way off on jetting.

Looks like good info in the samba thread- all 97 pages!! That will have to wait for another time.

In the mean time - drive!
72hardtop
QUOTE(gereed75 @ Oct 8 2018, 08:54 AM) *

Hmmm. Interesting. Did not think that the Weber’s could do that.

Can’t say that I ever collected any data for sustained light load part throttle ops. I was only concerned with safe mixtures under load.

Following your thought process, I would think that if your Weber’s are tuned to produce 13 AFR under load that they will intrinsically yield leaner mixtures under light loads part throttle. By there design, I Don’t see how you could have one without the other unless you were way off on jetting.

Looks like good info in the samba thread- all 97 pages!! That will have to wait for another time.

In the mean time - drive!



What's to be avoided is 14:0 - 15.5:1 AFR WOT condition.

Under 3/4 throttle positions there is not enough heat combined with load to be an issue. Even though the AFR is hotter than 13:1

16-17AFR is where you want to be during part throttle cruise (cracked throttle) for MPG's (vacuum advance distributor). And cooler running temps.

Your foot dictates load. Even with a 009....which will run lean (jetted right) at ~14-15AFR no more (w/light throttle). With a cracked throttle (low load cond.) you will not be on the main circuit (even at 60+mph) You'll be on the idle/progression phase.
Larmo63
Earlier this year, I had a guy I know tune my Webers after 914Toy (Keith) and I futzed around with smaller chokes and different idle jets.

He got them pretty close and now I don't touch them. I'll probably screw them up if I do.

driving.gif
72hardtop
QUOTE(Larmo63 @ Oct 8 2018, 09:47 PM) *

Earlier this year, I had a guy I know tune my Webers after 914Toy (Keith) and I futzed around with smaller chokes and different idle jets.

He got them pretty close and now I don't touch them. I'll probably screw them up if I do.

driving.gif


Any idea what jet/s, vent size etc....?
gereed75
Spent some time with the Samba thread - very informative. you can get much of the benefit from that thread in the first 4 pages or so.

Yes it appears that the webers do produce mixtures over a much larger range than I thought.

Nailing idle jets is still very important and difficult.

Although my car runs pretty well, this reminds me that it could be better - alas this will be a project for another day.

In the mean time I am confident that I am running with good AFR's in the critical 2/3's and above load conditions. Starts well, idles rock steady but probably richer than optimum in some regimes. Probably similar case to Toy and many others. The "published" jetting charts are pretty good, but fine tuning is probably necessary to be optimal for each motor, depending on ignition timing, cams, compression, piston type, fuel etc.

Lots of good info here but is just a basis for further work. Thanks
Larmo63
2.4S spec Euro 7R block
36mm heads & manifolds
Daugherty DC30 cams (between E and S spec)

Here are my carb specs:

Weber 40IDTPC3C & C1, we changed 34mm venturis

32mm venturis
60 idle jets
145 mains
140 air correction
F26 emulsion

I'm running the Clewett ignition system as well. This photo was taken while I was doing the wiring, so it doesn't appear finished.
Click to view attachment
Larmo63
Click to view attachment
72hardtop
QUOTE(914Toy @ Oct 7 2018, 03:41 PM) *

Thanks for all the responses. After carefully adjusting all of the linkage, including the accelerator pump linkage ( to snug fit), I have settled at where I started with this thread except I have replaced the 60 idle jets with 55's, accepting the slightest hesitation with sudden acceleration some of the time. I now know 60's will eliminate all hesitation, but I prefer better fuel consumption, more power(?), and less potential for carbon build up. Engine on my test hill climb has a little more power and idle A/F's are a little better at high 10's. I may look for some drill bits to ream the 55' out to say 57.


High 10's at idle? Get the idle jets down. You should be ~14AFR at idle

There is no load at idle on the engine. The 14-15.5:1 AFR is to be avoided during high load (3/4 throttle or WOT) Cruising at an AFR of 16-17 is optimal and cooler running than 13:0
gereed75
Larmo,

That is exactly what I am running on maybe the exact same motor. What pistons/compression are you running?? I have 9.5:1 with JE domed pistons, single plug, running about 30 degrees advance.

I do get some spitting/sniffing at anytime and some backfire on deccel. Suspect I have some air leakage somewhere. Gaskets are all good and I thought the throttle shafts were tight when I set up the carbs per Performance Oriented guide, but who knows.

How much of that do you get?? Thought maybe some is attributable to the cams.

SWEET street motor huh!!! And I had no problem running with the Boxster/Cayman/SC/E36 crowd on track.
72hardtop
QUOTE(gereed75 @ Oct 9 2018, 08:10 AM) *

Spent some time with the Samba thread - very informative. you can get much of the benefit from that thread in the first 4 pages or so.

Yes it appears that the webers do produce mixtures over a much larger range than I thought.

Nailing idle jets is still very important and difficult.

Although my car runs pretty well, this reminds me that it could be better - alas this will be a project for another day.

In the mean time I am confident that I am running with good AFR's in the critical 2/3's and above load conditions. Starts well, idles rock steady but probably richer than optimum in some regimes. Probably similar case to Toy and many others. The "published" jetting charts are pretty good, but fine tuning is probably necessary to be optimal for each motor, depending on ignition timing, cams, compression, piston type, fuel etc.

Lots of good info here but is just a basis for further work. Thanks



Easy enough to do....

Pull the main stacks out entirely....everything and run on the idle jets alone.

Note on your tach when it falls on it's face.
914Toy
My engine is running well at all speeds, except for an occasional small hesitation when accelerating vigorously from idle, after changing from 60's to 55's idle jets. This change has helped increase idle A/F's from 10.4 to 11 at 800rpm. My idle air adjustment screws are several turns open, and air/fuel mixture screws are turned out only 1/4 turn.
So I am continuing trials and researching options to; 1) eliminate the occasional small hesitation when accelerating from idle, and 2) increasing the idle A/F ratio into the 12 to 13 range.
1) I am waiting for a set of "Hatchet" carb acceleration fuel pump cams, and a set of 175 air correction jets to replace the 180's installed, to address acceleration hesitation.
2) I have a range of idle jet sizes and have discovered that the idle jets that are 55's, 60's, or 65's (maybe even smaller and larger sizes) all have the same diameter side air inlet holes, that introduce air into the idle jet metered fuel, with this emulsified fuel mixture descending to the air/fuel mixture screws that control the volume of this fuel mixture into the carb throats. Thus, I conclude that if I replace the 55's in my carbs with 50's and open up the air /fuel mixture screws to regain lean best idle, this may result in a higher A/F ratio at idle.

Comments?
gereed75
I am not sure where I got these but they graphically show how the jetting and float level inter-relates and operate as the throttles are open. Thanks and credit to whoever authoured these. Click to view attachment

Click to view attachment

Click to view attachment


Understand all of this, apply it in 30 or 40 applications, and you are on your way to weber guru status
72hardtop
QUOTE(914Toy @ Oct 10 2018, 01:03 PM) *

My engine is running well at all speeds, except for an occasional small hesitation when accelerating vigorously from idle, after changing from 60's to 55's idle jets. This change has helped increase idle A/F's from 10.4 to 11 at 800rpm. My idle air adjustment screws are several turns open, and air/fuel mixture screws are turned out only 1/4 turn.
So I am continuing trials and researching options to; 1) eliminate the occasional small hesitation when accelerating from idle, and 2) increasing the idle A/F ratio into the 12 to 13 range.
1) I am waiting for a set of "Hatchet" carb acceleration fuel pump cams, and a set of 175 air correction jets to replace the 180's installed, to address acceleration hesitation.
2) I have a range of idle jet sizes and have discovered that the idle jets that are 55's, 60's, or 65's (maybe even smaller and larger sizes) all have the same diameter side air inlet holes, that introduce air into the idle jet metered fuel, with this emulsified fuel mixture descending to the air/fuel mixture screws that control the volume of this fuel mixture into the carb throats. Thus, I conclude that if I replace the 55's in my carbs with 50's and open up the air /fuel mixture screws to regain lean best idle, this may result in a higher A/F ratio at idle.

Comments?


Running Weber IDF's?

IDF's start with combined with vacuum advance distributor. No lean tune cruising without one.

47.5 ilde jets
200 air corrector jets
120 main jets
F11 tubes
Float height 10mm - 11.5mm (ball not depressed gasket in place) Critical
Float drop 28-32mm (less critical)
Fuel pressure - no more than 3.5lbs (no dial regulator allowed) Critical

Once the carbs are bench set you should start with no more than 1/2 in from contact with the idle speed screws. This is a starting point only. It will very likely run like a tractor initially. Opening to far and you will expose the 'progression circuit'

Once it's running....LBI each screw to the smoothest highest idle speed with the air mixture screws. Once done...use a snail and check each carb (2 & 4). Note your idle speed (engine warm) bring high flow down to low flow carb.

Now LBI each air mix screw again.

Once done check carb sync. The same? If so now check idle speed get it to roughly 800 rom or so by bring each carb up slightly being sure to keep them synced until you reach idle speed.

Now LBI each screw again.

Once done check your carb sync again. If the same....Done. Now you can work on jetting.

With regards to jetting start by driving around with only the idle jets (no main stacks installed) This allows you to see and feel when it falls on it's face.

A bigger air corrector tips the main jet in sooner. A smaller air corrector delays the mains from coming in.
914Toy
After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?
72hardtop
QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.

The graphs you posted to are a bit incorrect.

The first graph say fuel enters main jet into idle circuit. That is incorrect. It enters the idle jet circuit. The main jet does not meter fuel for the idle jet or circuit.

On IDF's the air entering the idle circuit flows in from the idle air bleed ports not idle air correction jets as the graph refers to.

The main jet on IDF's is at the end of the emulsion tube.
914Toy
QUOTE(72hardtop @ Oct 11 2018, 02:40 PM) *

QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.


After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.
72hardtop
QUOTE(914Toy @ Oct 12 2018, 07:49 AM) *

QUOTE(72hardtop @ Oct 11 2018, 02:40 PM) *

QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.


After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.


Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.
914Toy
QUOTE(72hardtop @ Oct 12 2018, 09:32 PM) *

QUOTE(914Toy @ Oct 12 2018, 07:49 AM) *

QUOTE(72hardtop @ Oct 11 2018, 02:40 PM) *

QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.


After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.


Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.


Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.
72hardtop
QUOTE(914Toy @ Oct 13 2018, 07:31 AM) *

QUOTE(72hardtop @ Oct 12 2018, 09:32 PM) *

QUOTE(914Toy @ Oct 12 2018, 07:49 AM) *

QUOTE(72hardtop @ Oct 11 2018, 02:40 PM) *

QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.


After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.


Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.


Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.


A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.
914Toy
QUOTE(72hardtop @ Oct 13 2018, 07:54 AM) *

QUOTE(914Toy @ Oct 13 2018, 07:31 AM) *

QUOTE(72hardtop @ Oct 12 2018, 09:32 PM) *

QUOTE(914Toy @ Oct 12 2018, 07:49 AM) *

QUOTE(72hardtop @ Oct 11 2018, 02:40 PM) *

QUOTE(914Toy @ Oct 11 2018, 11:43 AM) *

After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?


I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Yes, I have considered and will source some 50's to try this. Will report back.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.


After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.


Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.


Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.


A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.
914Toy
[quote name='914Toy' date='Oct 13 2018, 10:26 AM' post='2656998']
[quote name='72hardtop' post='2656964' date='Oct 13 2018, 07:54 AM']
[quote name='914Toy' post='2656955' date='Oct 13 2018, 07:31 AM']
[quote name='72hardtop' post='2656881' date='Oct 12 2018, 09:32 PM']
[quote name='914Toy' post='2656698' date='Oct 12 2018, 07:49 AM']
[quote name='72hardtop' post='2656524' date='Oct 11 2018, 02:40 PM']
[quote name='914Toy' post='2656463' date='Oct 11 2018, 11:43 AM']
After further research and inspection of the carbs, it seems logical to me, as suggested by some on PP, that the idle air jets are too small, as there is almost no change in A/F's when reducing the idle jets from 60's to 55's, which is actually a significant reduction in jet cross sectional area, about 14%. Thus I am considering drilling these air jets out a little as some others have done. The air jet holes are close to 1.3mm in diameter. What increase in diameter does anyone suggest?
[/quote]

I'm against reaming jets. No matter how careful one is it never seems to work out well in the end.

Your idle jet meters A/F during the progression phase. The mixture screws only adjust the A/F mixture when idling. No other time.

Yes, I have considered and will source some 50's to try this. Will report back.

Bigger air corrector brings mains in sooner smaller delays the mains. Best to pull the whole stack and drive with only the idle jets. You'll know exactly when it fall on it's face. Be conservative with the accel pump adjustment during this time.
[/quote]

After further thought and discussion with Paul, I will not drill these air jets. I have now checked the accelerator pumps which are working well, and will acquire a set of "hatchet" cams for the acceleration pumps in an attempt to eliminate the small occasional acceleration hesitation. If that does not eliminate this issue, I will replace the 55' with 60's idle jets.

Thanks for all suggestions made.
[/quote]

Down on the idle jets. After LBI you should be ~14:1 AFR

One can be really rich with a 40 idle jet or really lean with a 60. It's all in the AFR screw adjustment. Remember the A/F mixture screw will ONLY adjust the mix for idle speed setting. Nothing else.

Your progression phase is dictated via the idle jet size which is already pre-mixed via the air coming from the air bleed port/s.
[/quote]

Issue is with 55's and mixture screws turned out 1/2 turn, I have smooth best idle speed at 800 rpm, but A/f's of 10.5. Checked throttle plates at this idle are above the idle port and below the transition ports - where they should be. I am suspicious of the absolute accuracy of the A/F meter, which is consistent but may not be factory calibrated correctly.
[/quote]

A larger idle will require less turns out from lightly seated position. Go down on the idle jet size to 50 and start with the screws 2 turns from lightly seated position. Then LBI each screw.
[/quote]

Yes - i will try 50's and keep you posted.
914Toy
Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue

I have today replaced the 55's with 50's idle jets and this has increased the idle A/F's from 10.3 to 11.5 at 800 rpm's - a good improvement. A test drive and seat-of-the-pants judgement, there is no noticable change in power in the midrange mode with no change in WOT mode. Will stay with this set up and see what miles per gallon I get.
larryM
y'all may also find this thread relevant

http://forums.pelicanparts.com/911-engine-...arb-tuning.html
72hardtop
QUOTE(914Toy @ Oct 16 2018, 07:40 AM) *

Update.

Installing "hatchet" shaped accelerator pump cams has solved the hesitation in acceleration issue

I have today replaced the 55's with 50's idle jets and this has increased the idle A/F's from 10.3 to 11.5 at 800 rpm's - a good improvement. A test drive and seat-of-the-pants judgement, there is no noticable change in power in the midrange mode with no change in WOT mode. Will stay with this set up and see what miles per gallon I get.

Down on the idle to 47.5. LBI should be ~ 14:0
914Toy
Update:

I installed the "hatchet" cams on the carb accelerator pump linkages - hesitation is eliminated!! Also installed 50's idle jets - too lean. Back to 55's and engine is running smoothly under all rpm's and varying power ranges.

A/F's still at 10.5 at idle at 800 rpm Have ordered a set of metric jet reamers - two weeks delivery. Will then ream out the small pressed in idle air jets 0.1 mm and report back.
This is a "lo-fi" version of our main content. To view the full version with more information, formatting and images, please click here.
Invision Power Board © 2001-2024 Invision Power Services, Inc.