Jetting Webers on 2.7 911 engine., Tuning. Options

[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 (IMG:style_emoticons/default/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]

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)

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]

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)

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 (IMG:style_emoticons/default/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]

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.

(IMG:style_emoticons/default/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]

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 (IMG:style_emoticons/default/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