CarbHelp

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ojh
Posts: 29
Joined: Tue Mar 30, 2010 6:39 pm

CarbHelp

#1 Post by ojh » Sat Jul 03, 2010 9:26 am

I have an interesting problem on an O/T engine, it is an original ford 406 with factory 3 X 2 setup on factory intake. I have rebuilt the carbs (and i do carbs all the time for circle track racing, esp 2 barrell) and when they are on the engine they are fine with the fuel pump running - it is a holley red - but when you start the engine after a brief period the center carb starts dumping fuel thru the vent as if the float is too high. I have the carb on my bench now, i can test it to 9psi and the needle & seat are holding. I am thinking that when the engine starts it is pressurizing the bowl somehow and forces the fuel out the vent tube. I am looking for the source of positive pressure and how it could get in there. I tested the power valve and it is ok under vacuum and opens at rating so i am thionking it is pressure tight.
In brief, 3 carbs all fed with common fuel log and all see same fuel pressure. With electric fuel pump running the carbs are filled and needle & seat close proper. When the engine starts the center carb has fuel exiting the vent tube, all three are exposed to same fuel pressure and sit on stock intake with common plenum. Carbs are same except the center carb has choke mechanism.
Anybody have any idea what is going on? Thanks, oj

sc racing
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Joined: Sun Jun 11, 2006 1:45 pm
Location: Sahuartia Az

Re: CarbHelp

#2 Post by sc racing » Sat Jul 03, 2010 7:14 pm

This may sound a little out of left field but when the car is running is there any "wind" from the fan going over the carbs? Are you sure you got the correct metering block gasket it may have blocked a passage.just some thoughts. I cant see where pressure could occur in the bowl.

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jmarkaudio
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Location: Florida

Re: CarbHelp

#3 Post by jmarkaudio » Sat Jul 03, 2010 9:04 pm

Sent a PM on YB, in addition can you set the float level low enough to not come out the sight hole, then start it with the sight plug out. See if the fuel creeps up out of the sight plug. Try swapping fuel bowls, and make sure there are vent whistles on the metering blocks.
Mark Whitener
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ojh
Posts: 29
Joined: Tue Mar 30, 2010 6:39 pm

Re: CarbHelp

#4 Post by ojh » Sun Jul 04, 2010 7:43 am

Thanks guys. I did a mess of rigging and have ruled out pressure. The floats in all three carbs are on the low side - so here's a thought: The bowl used isn't the normal bowl for this carb, the bowl is actually from a 4160 and there is a plug driven into the crosstube port that would normally supply the rear bowl and has a larger brass float than i have seen in other side hung carbs. What if the volume isn't great enough to overcome the weight of the float and close the needle & seat, remember this carb was purpose built 50 years ago for this car and the fuel had lead and likely a greater specific gravity and the float would rest higher in the liquid? And the reason it is the center carb being stupid is because the idle circuit is using fuel so that the float drops and can't fully reseat itself?
I believe i'll add a regulator to the fuel and drop the incoming pressure to about 5psi, then i'll be able to raise the float level back up to proper and the needle & seat should close. What do you guys think? Thanks guys, oj

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