loosecannon

I have a 911SC with Webers, headers, MSD ignition and 964 cams. Venturis are 36mm, F3 emulsion tubes, 160 main jets, 175 air correction jets, 65 idle jets. I am running the PMO pressure regulator and it's set for 3.5 psi. The jetting, fuel pressure, carb cleanliness, plugs, wires, rotor and timing are all good. The only problem is that at very small throttle openings, like just above idle or while cruising under light load, the car stumbles and sputters and the a/f ratio gauge I have shows really lean. Applying slightly more throttle clears up the sputter completely. What could cause this? What is the fix?

VNTGSPD

If it only happens at throttle tip-in, I would check the accelerator pump circuit. You can do this by taking a small vial (I used a small dropper from a medicine bottle and sealed the end with a lighter) and attaching a wire to it. Lower the vial down into the venturi and slide it under the accelerator pump jet. With the engine off, pump the throttle to WOT twice and measure the output. Do this for all six venturis and ensure they are all about the same. Spec is .6 to 1.0cc I think.

If they are uneven by more than .2cc, you may have a clogged or leaking jet (unlikely), broken pump diaphragm (definitely possible if old rebuild), or failed check valve in the float bowl.

Good luck!

loosecannon

No, the problem is not at tip-in, it's under steady state but very small throttle openings only. Example, driving 60 mph down a level highway with no headwind and the stumbling will go on and on for miles. If going up a hill or driving a little faster or in a headwind which need a little more throttle, the problem does not occur.

Steve Weiner-Rennsport Systems

Put in some 60 idle jets and readjust your fuel mixture, first,...

loosecannon

I'll give that a try and post the results.

911pcars

Fuel level in carbs? Have you checked?

Sherwood

loosecannon

Yes, fuel level and synchronization between carbs and even air/fuel ratio with a wideband have all been done. I took Steve Weiners advice and put in 60 idle jets and it seems to have cured the problem, which is strange because my narrow band O2 sensor was showing a lean condition when the car sputtered. Oh well, whatever the case, it has great throttle response, lots of power and I couldn't get it to sputter no matter what I did-thanks Steve

Jay Laifman

Given my recent postings trying to get my CIS to work, which now does, and the comments I received that the solution was to go carbs, I feel I have to return the favor: Dump the carbs and go CIS!

VNTGSPD

Glad its solved! If my head is on straight, everything you are saying is correct. By reducing the idle jet from 65 to 60, you are reducting the amount of air entering the circuit and therefore richening the mixture coming out of the circuit. Therefore, the lean condition registering on the O2 sensor gets richened up and voila, runs great!

Enjoy.

BTW, Steve W is the best!

loosecannon

I would put imperfect Webers against a perfect CIS any day. And I think if you experienced the power and throttle response of my car, you would be forever hooked.

Umm..no, the jets control fuel flow not air flow. Reducing the jet size reduces the amount of fuel. Anyways, I think what was happening was that the car ran super rich at small throttle openings and so much fuel was getting dumped into the intake that it was washing across the O2 sensor, causing a false lean reading.

RoninLB

excess fuel cools combustion temps.

It's common to run cooler than necessary acceleration circuit temps without bogging.


heat is power