Dual Carb Setup Questions

[Archrms]

I have dual holley 1850 600cfm carbs on my 427 s/o. Been having some issues with stumble off idle, and gas dripping into intake after engine is shut down. Rebuilt both carbs and found some issues I was not aware of and want to either confirm whether they are correct or need to be fixed. I assume that both carbs should be set up identical to each other:

1. Main jets (both of mine were 65)
2. Accelerator Pump Discharge Nozzle (front was 35, rear was 31)
3. Accelerator Pump Cams are both the same profile and both set on #1 holes.

The front carb engages the accelerator pump almost immediately. The rear carb has a delay due to the progressive linkage. The rear pump does not seem to have as much movement on the pump lever as the front does - is this normal?

Never really paid much attention to the carb setup, as it worked fine up until now. How are yours set up in comparison to mine, and what needs to be changed/adjusted?

Thanks, Ron

[ERA Chas]

You don't mention floats-check for porous one and level of both. Lower is better than higher. Look for a needled seat that may be grooved by dirt.
With 2.250 intake valves and moderately big cam I used 65 jets and 37 squirters (up from 31).
Check the vacuum diaphrams in the secondaries for tears or leaks, they may have caused the spring rate to change causing a bog off idle.
If all checks are OK, try the next hole on the pump cam on the second carb to bring the pump in a bit quicker. If bog gets worse, go back.
Do one check at a time, testing between so you don't lose where you started from.

[FWB]

sometimes when the accel pump is adjusted too tight it will dribble.
you may see it only when shut down.

i run about .010-.015 clearance between the lever and the pump pin

[Archrms]

Originally Posted by FWB

sometimes when the accel pump is adjusted too tight it will dribble.
you may see it only when shut down.

i run about .010-.015 clearance between the lever and the pump pin

Is that measurement taken with the throttle plates at idle or wide open?

[ERA Chas]

Originally Posted by Archrms

Is that measurement taken with the throttle plates at idle or wide open?

Only at idle-at any throttle above that it's fully depressed.

[patrickt]

Originally Posted by ERA Chas

Only at idle-at any throttle above that it's fully depressed.

Uhhh, surely you realize that Holley specifies adjusting the accelerator pump at WOT. Like this:

First you will need to check the adjustment on the pump. To do this you will open the throttle all the way (WOT). Push the pump arm lever down and then adjust the pump override spring to obtain .015" clearance between the pump arm and lever.

Source: Holley Performance Products Frequently Asked Questions

[ERA Chas]

Originally Posted by patrickt

Uhhh, surely you realize that Holley specifies adjusting the accelerator pump at WOT. Like this:
]

I apologize for the brain fade.

[patrickt]

Originally Posted by ERA Chas

I apologize for the brain fade.

[undy]

Is the fuel dripping from your bowl vents??

If so, excessive fuel pressure (above 6 psi) is the mortal enemy of Holley's bowl float needle valves. They will lift and dump the further north of 6psi you get. Make sure your floats aren't set too high also.

[FWB]

thank you patrick for showing us how much you know.

the adjustment for a pump shot is at WOT. for a stumble


but to get back to the problem of fuel dribbling at shut off the pump lever should not be in contact with the pin on the pump. to adjust that at WOT will not help this.

some slop .010-.015 at idle should be there or the off idle circuit will be sending fuel to the venturies and cause a dribble. you can hear it dribbling sometimes in the exhaust at idle....a little fluctuation in tone

[patrickt]

Originally Posted by FWB

thank you patrick for showing us how much you know.

Well, it would take more than a short post to do that.... But, Chas. and I have taken a solemn oath to always correct one another when a momentary brain fart is spotted. And this one was like finding a C-Note on the sidewalk while walking the dog.

[Gaz64]

Originally Posted by FWB

thank you patrick for showing us how much you know.

the adjustment for a pump shot is at WOT. for a stumble


but to get back to the problem of fuel dribbling at shut off the pump lever should not be in contact with the pin on the pump. to adjust that at WOT will not help this.

some slop .010-.015 at idle should be there or the off idle circuit will be sending fuel to the venturies and cause a dribble. you can hear it dribbling sometimes in the exhaust at idle....a little fluctuation in tone

The .010-.015 is to make sure the pump diaphragm doesn't bottom out.

The pump override spring serves a dual purpose in this regard.

You should have NO slop in the linkage to give immediate fuel delivery on tip-in.
The pump cavity is vented back to bowl to keep it vapour free and bleed some fuel back on slow throttle openings.

[Archrms]

Originally Posted by undy

Is the fuel dripping from your bowl vents??

If so, excessive fuel pressure (above 6 psi) is the mortal enemy of Holley's bowl float needle valves. They will lift and dump the further north of 6psi you get. Make sure your floats aren't set too high also.

It was dripping from the vents at first, then the boosters. I will try to check the fuel pressure tomorrow. What should a normal pressure be for a system with a mechanical pump?

[patrickt]

Originally Posted by Archrms

It was dripping from the vents at first, then the boosters. I will try to check the fuel pressure tomorrow. What should a normal pressure be for a system with a mechanical pump?

Usually 5 to 7 psi. You can not rely on in-line liquid filled pressure gauges that are under there all the time (just in case you have one) -- they're just for show.

[undy]

I always run a bypass fuel system on my "hot rods". The fuel pressure seems to be more constant over a "dead head" single line system which quite often results in fuel pressure creep. You'll need a return fuel line back to the tank though. There are other fuel related benefits reaped from a system like this too, like cooler fuel etc...

[Archrms]

Checked the fuel pressure last night, it was a steady 6-1/2 psi at both carbs. Waiting for new nozzles to arrive so that the front and rear carbs match.

[Archrms]

Swapped out the mis-matched 31/35 nozzles for 40s. No more backfire off idle, also no more dripping fuel since rebuild. Need to put some miles on it now, but so far so good.