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altune 12-23-2007 08:33 PM

carb leaking primary's
Ok all you brainiac carb guy's out there, I need some help. Carburator is a holley 750 double pumper 4150.
when at idle the carb is leaking out of one of the venturi's. now I have seen carbs do both but never one only.
Float level is fine.
not sure on gaskets but what could cause it on the gasket?
power valve has been changed, rechanged, etc.
anybody got any ideas? carb is pretty much new.
my idea, maybe that butterfly is slightly bent or open.
not sure.

hammertime 12-23-2007 09:48 PM

A float is stuck more then likely, take them out and make sure there is nothing in them.

lookingaround 12-24-2007 03:04 AM

i agree with the float.

jmarksdragster 12-24-2007 08:13 AM

If it is only one, it may be a blockage in the main air bleed or in that path in the metering block. Try the needle & seat first, but if that doesn't fix it, pull it apart and flush the air bleeds in the main body and the metering block.

bjuice 12-24-2007 04:36 PM

Al what kind of 4150 you the shelf Holley or aftermarket carb..

reason i that my wife had an off the shelf 4151 750 double pump..i sent it off to Blakes be worked and they milled the horn down and did soem trick stuff to the metering blocks etc... and the car picked up 2 tenths.....very,very,Mild 406ci

if its a stock type carb and you can spare a few spare bucks over the next few would be money well spent to send her off to a good aftermarket carb company for some magic work to be done her..


27keith 12-24-2007 05:03 PM

This goes along with the stuck float theory. It could have a sunken float. or a pin-hole in the float. Proabably only if you have brass floats though. It would not be stuck but would sink when in fluid. When you disassemble the bowl look to see if there is a wet spot forms anywhere on the float it could be fuel leaking out of a hole. It only takes a tiny hole also. Try checking float hieght while the engine is running to verify its corect. If it is not then replace the float.

edvancedengines 12-25-2007 01:46 PM

Too much fuel pressure?

Or float or float level

Or needle and seat are leaking.

Take carb off car. Drain all fuel. With carb upside down by mouth blow into carb inlet it should be all restricted and you can't blow through. If you can blow through the flaot is not closing the needle to the seat or the needle and seat are bad. TRy first spraying some spray lube on them after you remove them and re-install and upside down the carb and again blow. It should seal. IF not buy new needle and seat and buy either the steel or the viton. Viton is best with most fuels.

When you get the needle to seat so you can not blow freely through with carb upside down. That part is done.

Next you should be able to adjust the needle and seat up or down to postion the float so float is slightly below the sight plug. If not you will need to mabe bend a little tang on the float some so it will adjust right.

When you get it right the fuel level will be just barely below the sight plug. You get this by adjusting to barely see the fuel at bottom of the sight plug. Then turn the flaot adjuster down by 1/2 turn.

The right fuel pressure is the pressure that is just below where the fuel blows by the seats.


altune 12-25-2007 02:39 PM

The float level is fine, remove the plug, rock the car , it bearly dribbles out. I got that. I tend to go with the other guys theory of the air bleeds being plugged.
what else can cause it?
damn thing picks up rpm, then starts to stagger cause the fuel is just dripping out.

edvancedengines 12-25-2007 03:49 PM

If air vents are stopped up the fuel will not easily siphon from the fuel bowl. The air comes in and displaces the fuel leaving. Bowl vents are important.

Air bleeds if stopped up will cause it to run excessively rich because you will not get the air needed to emulsify the mixture and give a proper air/fuel balance. That will not cause it to pull fuel from the fuel bowls upwards and out while sitting still or going down the track. It will cause an excessive rich mixture that defies tuning.

You could have an internal vacuum leak somewhere. For fuel to get up to the bossters.


hammertime 12-25-2007 06:22 PM

Originally Posted by altune
The float level is fine, remove the plug, rock the car , it bearly dribbles out. I got that. I tend to go with the other guys theory of the air bleeds being plugged.
what else can cause it?
damn thing picks up rpm, then starts to stagger cause the fuel is just dripping out.

I was not saying float level, I am saying something is stuck in the float itself. I've seen this problem a 100 times at the track when people say float level is fine and cant get it to stop leaking.

altune 12-26-2007 02:20 PM

Carb is 3 months old, can that still happen?
if the float is filled or partially, wont the level be high.
not sure what you are trying to say.

cepx111 12-26-2007 11:18 PM

I agree with you if you say you've checked the float level while running and its BELOW the hole I beleive you. I'd lower the float down some anyway for good measure and see what happens. If the needle was stuck or sticking or the floats were heavy the dam thing would have fuel pouring out it like 90 going north. Fuel coming out of the boosters can only come from 2 places. #1 the power valve circuit, something is either making the power valve cicuit open up I. E. low vacuum due to high lift cam or a severe vacuum leak or #2 its being pushed through the main jet circuit via a high float level or by being pulled out via venturi vacuum affect from the engine which is not present at an idle.
How high of an idle are you running? what size power valve?
Sometimes I've seen Holley's pull fuel out of the boosters if idled up to high which lowers the vacuum that holds your power valve closed. Try lowering your idle down a bit and see if the problem goes away.
You mentioned you've changed out the power valve several times but did'nt mention if you had tried running a different one it should have 6.5 from the factory,you made need to go with a higher rated one if you have a high lift cam that kills idle vacuum, or just do what most people do and plug it and go up about 4 to 6 jet sizes on the primaries.
Goodluck >< Cp

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