Idle question for carb guys

[rewguy]

Im no carb wizard, obviously, lol.... I have a problem with a holley 750 HP that I just purchased brand new as a put together kit. Ive put together several carburetors like this one and never had a problem until now. It was bought for a 355 sbc street car, 425 hp or so. It will not idle whatsoever. Ive taken it apart three or four times, checked for gasket alignment between the main body and baseplate, .025 of transfer slot showing on all four holes. .073 idle air bleeds in place. It just seems to move no fuel at idle. Ive used carb cleaner and blown out all holes with compressed air and no passages seem to be blocked..........Any ideas? For the engine to stay running, it is up near 2500 rpm and runs VERY rough. Idle circuit plugged I suppose but I cant figure where. Ive had two other carburetors on this engine and they've been fine.

[Procision-Auto]

If you've set the idle mix (~ 1.5 turns CCW initial setting), and the float
levels are fine, then the primary idle air bleed size might be your problem.

I have found that restricting the air bleeds solved my idle issues.
Have a read through this thread. It just might help you as well:

viewtopic.php?t=3086

Another thing to consider is the power valve rating. What is the manifold
pressure at idle? The power vavle might be open and dumping too much
fuel.

Start with a PV that is two values lower than the vacuum reading at idle.

[My427stang]

Take a close look at the metering surface on the carb body as well as the witness marks on the met plate gasket. You could be pulling air across channels killing your idle circuit response

PV shouldnt do it, PV circuit is loaded with vacuum, but fuel is dead until air flows across the boosters.

Best bet is to do what you are doing, spray carb clean through passages as you assemble.

- base plate through idle hole and transition slot
- then body and base plate through metering plate holes
- through air bleeds
- Through IFR and air feed in metering plate
- Through idle circuit in met plate

If all of those are open, it should work, my guess is you will find something leaking, cracked or plugged

[Procision-Auto]

I respectfully disagree:

PV shouldnt do it, PV circuit is loaded with vacuum, but fuel is dead until air flows across the boosters.

http://www.hardtail.com/techtips/carbtuning.html

If the power valve flutters open at idle, it can act as a pump, and push extra fuel into the main well, causing a drip from the booster venturis.

http://www.holley.com/data/TechService/ ... valves.pdf

A competition or race engine which has a long
duration high overlap camshaft will have low manifold vacuum
at idle speeds. If the vehicle has a manual transmission, take
the vacuum reading with the engine thoroughly warmed up and
at idle. If the vehicle is equipped with an automatic transmis-
sion, take the vacuum reading with the engine thoroughly
warmed up and idling in gear. In either case, the power valve
selected should be 1/2 the intake manifold vacuum reading
taken. EXAMPLE: 13” Hg vacuum reading divided by 2 = 6.5
Power Enrichment System.
power valve. If your reading divided by 2 lands on an even
number you should select the next lowest power valve. EXAM-
PLE: 8” Hg vacuum reading divided by 2 = 4 power valve.
Since there is no #4 power valve you should use a 3.5.

[My427stang]

I suppose with an unstable vacuum at idle and the right conditions it could pump fuel, but I havent seen it. Not saying you are wrong, but I havent seen it (and I havent seen MANY things LOL).

Without air flowing past the boosters, it still seems pretty tough for me to figure out how it'd get out there

However Holley's recommendation on how to pic a PV has always been hogwash in my book. I know you are just the messenger, but I think its a poor way to bake a cake.

1/2 of idle vacuum as you go lower and lower, can result in a PV too lean under roll-on throttle applications and one that shuts off pretty quick when rpm comes back up. I much prefer to start with a higher number and work down

[Procision-Auto]

Agreed. I don't like the 'half' method either, that is why I suggested a -2 value
under manifold vacuum.

The idle isn't much of an issue I guess, unless the pumping action is
occuring as suggested by one article. Just offering all the angles.

Part throttle cruise however - a totally different book!

[crazycuda]

When you say kit are you meaning just using the 750 hp body and reusing your old baseplate/metering plates etc.? Or just getting a new usassembled carb.
I am sure you probably did this but i will repeat it
Did you check the main body metering plate gaskets to be sure all of the holes are corrrect and in alignment. When you thigten the bowls up is the gasket compressing and blocking any of the holes. (had a fiew sets of the moroso rubber metering plate gaskets do that to me.)

If it has 4 courner idle setup ,on the base plate there are 2 slots cut from the primary to the 2ndary. (they follow the outside of the bores) There is suspose to be a plug or stop to seperate the 2 idle circuts.

This is a long shot but are the throttle blades correctly mounted in their respective bores. I have seen one that the primary blades were taken from another holley and they were to small for the bore causing all sorts of idle problems.

A final long long shot is does the car have an electric fuel pump or are you filling the bowls manually. when you try to start it. I know you said it runs like junk at 2500, but could it be your float level is real low and not allwoing the bowls to fill with enough fuel at idle rpm. Maby a blockage in the bowl itself?
The last idea is can you return the carb kit and exchange it for another one.

[flyin ryan]

if the carb is a sound peice, i'd go straight to smaller idle air bleeds.

[ZEROMAN91]

Sometimes when every thing looks good with the carb, the idle problem can be lack of initial advance. When the cam, size ( overlap) increases the manifold vacum goes down, increased initial and less total advance can help. Try advancing the timing at idle and see what happens to your engine vacum, justy don't rev the engine to high, if that helps your situation send your distributor out for a recurve.

[Doug Schriefer]

If you can keep the engine running at 2500 RPM lower the speed, and cover the idle (outside) air bleeds with your fingers. This should cause the engine to pull more fuel and continue to run. If it does not you may not have the proper T/B gasket between the baseplate and main body.