strange carb problem

[steve cowan]

i had my 950 ultra hp carb apart tonight doing some fine tuning after the weekend at the track,so i do my changes and prior to re-fitting carb to intake i crack the throttle wide open and there is fuel in the accelerator pump and it discharges from the squirters as you would expect but on releasing the throttle back to closed the squirters are discharging fuel and air,i do this several times and same thing.
never seen this before - strange???
so i remove the squirters and replace the pull over needles- same thing.
i removed the 30cc accelerator pumps and inspect diaphragms and rubber check valve,i cannot see any problems anywhere.
i replaced both diaphragms and rubber umbrellas and problem has gone.i have never seen this problem,the car ran well all weekend and showed no signs of a carb related problem on the AFRs or by feel and on the track it just does its thing.
anyway just wanted to share my findings and it may help somebody one day

[bill jones]

---I'm wondering if you are having to deal with the small syphon hole in the pump passage?

[NormS]

If the check needles(under the squirters) are not sealing on their brass seats properly, the accel pumps will draw air back into the upper part of
those circuits, through the squirters,when the throttles are closing. The next time that the throttles are opened, that air has to be pushed back out before you get a good stream of fuel. Replacing the check needles usually doesn't solve the problem.

I have also found that many of the billet metering blocks use a set screw to seal the upper end of the angled accel circuit passage, and that those set screws are not sealing well. And because they are above the fuel level, that "leak" can draw air into the circuit when closing the throttles. Both these problems are usually easy to solve.

One or both of these problems usually cause a stumble on acceleration, unless the transition circuits in the carb are calibrated too rich. Because the accel pump circuits and the transition circuits work together as the throttles are being opened, a weakness in one can be compensated for by the other, during the opening of the throttles.

[steve cowan]

If the check needles(under the squirters) are not sealing on their brass seats properly, the accel pumps will draw air back into the upper part of
those circuits, through the squirters,when the throttles are closing. The next time that the throttles are opened, that air has to be pushed back out before you get a good stream of fuel. Replacing the check needles usually doesn't solve the problem.

I have also found that many of the billet metering blocks use a set screw to seal the upper end of the angled accel circuit passage, and that those set screws are not sealing well. And because they are above the fuel level, that "leak" can draw air into the circuit when closing the throttles. Both these problems are usually easy to solve.

One or both of these problems usually cause a stumble on acceleration, unless the transition circuits in the carb are calibrated too rich. Because the accel pump circuits and the transition circuits work together as the throttles are being opened, a weakness in one can be compensated for by the other, during the opening of the throttles.

thanks for the post Norm
i found it strange this to happen on both squirters,been using this carb on a few different combos over the last 5 years,carb has been on and off alot of fine tuning and testing different parameters.
i found it strange by replacing the umbrella and diaphragm cured the problem??
maybe not
looking like Bill might be on to it by above comment.

[Walter R. Malik]

Not common but,
on a few carbs I have seen the pump lever relax a bit at WOT
because the pump cam profile allows that very slight pump lever return.

That pump shot when returning to idle is very minimal though.

[cjperformance]

Not common but,
on a few carbs I have seen the pump lever relax a bit at WOT
because the pump cam profile allows that very slight pump lever return.

That pump shot when returning to idle is very minimal though.

You hit the nail on the head Walter. Yes, pump arm going over the highest point of the acel pump cam before WOT , and with umbrella one way valves not quite sealing 100% is just pulling a tiny bit of air in as the arm relaxes toward WOT , then as the throttle is released from WOT the arm rides back up the pump cam and pushes out a small ammount of aur/fuel then comes back down the cam to closed throttle, the more aggressive arm movement on the way TO WOT is enough to seal the umbrella.
Possibly if tried as it was before Steve fixed it and opening the throttle very very slowly there would have been a delay or lack of pump shot.

[steve cowan]

Walter and Craig,
Thankyou for your input, always appreciated.
I have thought a bit more of last weekend at the track but prior to that I put a blue cam on primary and kept the pink cam on secondary, I run 1:1 throttle linkage.
My thoughts were the blue cam is a bit more aggressive and the pink cam carries the pump shot right up to full throttle.
I am particular about the adjustment as I make sure the pump arm does not over extend the pump diaphragm if that makes sense.
Even with this low hp combination I am running at the moment I am still using my 5600 converter, I leave at above idle and it flashing to 5250-5300 at the hit carb has been working well BUT I do remember it had a slight stumble on the last run of the day and after I packed up and hooked my trailer on the car it was a bit hard to start as in to much fuel, the car normally fires right up after initial cold start of the day.

[fdicrasto]

Did you remove the squirters when you had the carb apart?

[steve cowan]

Did you remove the squirters when you had the carb apart?

not initially,i had carb off after meeting to fine tune transfer slot JET AND PVRC jetting,only took squirters out after finding problem
what are you thinking??