Another Jetting Post, 1911 short stroke motor. Options

[Alphaogre]

1970 914-4 "1.7"

Motor spec:
• 1.7 built into short stroke 1911cc motor.
• Stock 1.7 stroke, with 96mm KP pistons.
• Web 86 cam.
• 1.7 Heads machined by Headflow Masters.
• Tuna Can for extra oil.
• Carbs: Dual Weber 44's (i know should have 40's) Car came with them and always ran well with them.
• Idle Jet: 50
• Emulsion Tube: F11
• Main Jet: 115
• Air Jet: 200
• Lower Venturi: 28mm
• Upper intake above Venturi: 38mm
• Velocity stacks: 52mm tall, Upper Diameter:61mm / Lower diameter:40mm
• Electric fuel pump with a fuel pressure regulator at about 4.5lbs


Car use:
Sunday Funday canyon driving.
Intermittent AX (not trying to win a championship)
Intermittent Track time Trials (not trying to win a championship)
with that It has a front Adjustable Sway bar, rear has Adjustable Bilstine HD C/O's

The Questions:
• When Warm (driven hard) it has trouble keeping idle...I've adjusted the idle crews but 50 seems small right? maybe up to 60/65? (especially since thats your fuel flow up to 3K RPM's)

• I have A Lot of exhaust popping on throttle lift-off. I thought it was just over Jetted since it cam with the 44's when I bought the car as a project... but 115 seems low, right? but so do the 28mm venturi for a now 1911 motor? So, now I'm thinking it's under fueled? (IMG:style_emoticons/default/huh.gif)

I don't think the popping is exhaust leaks, I have the copper washers in the exhaust manifold with them all good and tight and stock exhaust manifolds. The popping really started after the motor build and lack of carb matching to the motor.

I never really messed with it too much because the car pulls well, no major complaints, and has gone through some pretty long/hot track days with no issues. I've just had an "if it's not broke..." attitude.

I've been distracted with other projects and have been driving it here and there, but have an itch to get back into some AX/track days. So, I'm trying to take a look at it now and go down another part of the car that I somewhat neglected in the past. I've been learning a lot with doing some research but thought some of the gurus here can school me some more (IMG:style_emoticons/default/biggrin.gif)

Any parameters suggestions or numbers to look at as target would be appreciated, I can then start working towards them. I don't want to just start buying jets and messing around with it all in vain because of a main facet negating everything...

I guess my main questions to start in the Idle Jet Number, Main Jet Number, and Venturi Number in relation to listed specs and applications.

[Tbrown4x4]

I am NOT a jetting expert, but I wanted to reply so you don't feel ignored.

The best advice I can give is to install a wideband O2 sensor. I put a PLX in the car, and it helped tremendously in setting up the jetting. It's so much nicer to see actual numbers than to try to "feel" and hear how your jetting changes affect how the car runs.

[michael7810]

I agree a wideband is a great tuning tool. It might be worth the expense to take it to a reputable shop with a dyno to get a good baseline tune. I think it cost less than $200 when I did it 8 years ago.

[rhodyguy]

How many turn out from lightly seated idle air mixture screws? Not ness all the same. So, unable to get the car to idle at 950ish rpm when warmed up? When warm, use ONE idle speed screw (dr side is most convenient) to try and bring the idle up. The other screw should be off the stop. Now measure the flow on the front Venturi side to side. Any linkage slop will be evident. Flow being equal with both idle speed screws on the stops at idle is great, but doesn't mean much after throttle tip in. Your jetting requirements should be about the same as a 1971cc(2.0). 38mm Venturi? Is that number visible on the top edge? Rather large if so.

[Alphaogre]

Thanks for the feedback, I like the idea of the airflow sensor, that would be easy to incorporate. I would just need to know what parameters to hit once I would be getting reading though...

The diameter above the lower venturi is 38mm, the venturi down lower has 28 stamped on it, so I am assuming its 28mm...

I have the carbs balanced, and it will idle cold to warm (cruising around) but when I beat on it, at AX for example, and get back in line I have to keep blipping the throttle or it will eventually just die. That's the part that is throwing me off...

[porschetub]

These large carbs will work ok as you have 28mm venturies,you are rather lean on idles and mains however,I would suggest starting with 55- 57 idles and 125-130 mains ,before you do this change drop your fuel pressure back to around 3 PSI,you are way to high @ present,you can't start with any baseline if your pressure is too high.
Good luck.

[barefoot]

Could you have fuel boiling in the float bowls after hard running ?
Phenolic spacers installed under intake manifolds ??

[Alphaogre]

Yea 130 Main Jets is what I was thinking too, I called Redline Weber and they suggested 130 main jets and 60 for the idle jets. They said the f11 elmusion tube is and 200 air jet is good.

From there, they suggested adjusting the choke tubes (venturi) to a 30mm or 32mm since the motor is not really a full 2000cc but bigger than a 1875cc, along with the fact that I im in CA and our gas we pay $4.50 a gallon for sucks!

I think the idle issue is mostly 50 idle jet being too low, no matter of adjustment would resolve it, it's just not enough fuel.

[type2man]

Try one thing at a time. I would do the 55 idles first. The popping under acceleration is usually running lean.

[Alphaogre]

Try one thing at a time. I would do the 55 idles first. The popping under acceleration is usually running lean.

The poping is on deceleration when you lift off of acceleration in the higher range. Yea, I'm going to do the idle Jet first, then look at the main... pending how that goes, I can go from there with a better baseline of where the jetting should theoretically be with the increased displacement.

[porschetub]

You will need to do your mains also they are too lean,I think 130 would be pretty close,no need to change the venturies,if they are indeed 28mm they will give you good vacuum signal for the size of your engine and be more suited to your cam.
By upping the idle jets you will notice a real change as they supply most of the fuel in your normal driving range and RPM.
Please address your fuel pressure before proceeding,I have found around 3psi is good for Webers,Dellorto's and the Zeniths on my "6".
Good luck.

[VaccaRabite]

Change one thing at a time for sure.

On my 2056 (IDF 40s with 28mm vents) I think I was using 55 for my idles.

Also, tune for hot running, not easy starting (cold engine).

Zach

[Gatornapper]

Please see this post:

http://www.914world.com/bbs2/index.php?s=&...t&p=2755419

Didn't post it here as I didn't want to hi-jack this thread.......

GN

[Alphaogre]

I started this thread because I struggled to find a good jetting thread on a 1911cc motor build with a cam. So, I intend to post the progress as it goes. Any ideas or things to consider should be added for anyone down the road who might be chasing their tails and gain some good insight or at least things to consider.

I popped in the 60 idle jets and 130 main jets yesterday. I didn't have too much issue balancing them and getting it to idle, definitely much better than before. The pull feels more linear especially as it transitions from the idle jets to the mains.

The big problem now is that I have not been driving the car as recently so I'm trying to go off the memory of how it felt before. I'm going to get some seat time in and get reacquainted (IMG:style_emoticons/default/biggrin.gif) before I move on to any other adjustments.

The popping went away for the most part, and now when adjusting the idle I can tell the difference in a 1/4 or even 1/8 of a turn, which makes more sense now.

When I spoke with Redline Weber he said the main issue will be that the 1911cc with my cam kind of falls between the 40 and 44 in terms of the venturi's, he thinks to get final tuning it will be an in-between size like a 32m or 34mm venturi. Especially at peak RPM with a cam pushing higher RPM than stock.

Next, I'm going to look into the timing and maybe even upgrading the distributor to a more controllable option in terms of advancing or retarding the timing...then as I drive it maybe look into the venturi options as well...

[Alphaogre]

So, I have been doing laps around the block and through an industrial area near me and I noticed that when I push it, let off and roll to a stop, it's taking a bit for the idle to drop down and settle.

Im thinking something with the floats? or Maybe the idle/air mixture? It almost seems like when you don't have a strong enough spring on there to snap it closed, but the spring I have on there is definitely doing that, in fact it's harder to push the pedal now...

Maybe too strong of a spring? I would think that would choke it out, not slow down the return to idle... (IMG:style_emoticons/default/confused24.gif)

[yeahmag]

There are general settings you can use based on venturi size and type of carb, but I can guarantee you will benefit from a wide band O2 meter. I have been learning more and more over the years on how to get the best out of a carb thanks to this simple device.

[Alphaogre]

I have a new issue now.

I have been re-setting the base number of turns for the idle screws and idle stops according to redline weber, I can get them balanced and at the right idle.

1) I still have a slow return to idle.

The weird one...
2) I went to go test it around the block, and when I put the clutch in, my idle went up to 3500...then when I let the clutch out (not in gear) it comes back down.

Should I set everything up with the clutch cable loose, then set the clutch and adjust as needed?

Other than changing the jets, everything is the same as it was when it was under jetted, so I'm not understanding why everything is fighting me... (IMG:style_emoticons/default/dry.gif)

[yeahmag]

Sounds like your accelerator and clutch cable are tangled. Very common.

[IronHillRestorations]

You need to journal every single change you make, and what it does to your problem. As others have said, one thing at a time or you'll be chasing your tail.

Your idle problem could possibly be a sticking advance mechanism in the distributor.

You don't list your distributor, or exhaust.

FWIW I had a car in my shop that I just couldn't get rid of the backfire off acceleration, turned out to be the (new) muffler. I put a stock banana muffler on the car and it went away.

[Alphaogre]

the car was driving, the only change has been the idle jets and main jets. Those two changes were just from redline Weber saying my old jetting was way too low. Other then that I’ve been trying to re-set the carbs to idle and running.

The only other thing I can think of is if the timing was advanced/retarded to account for the old jetting, and now it’s off or the distributor sticking is a good call.