I recently, by accident, picked up about 3mph at the dragstrip by inadvertently running less fuel pressure than I intended. I usually run about 7lbs, but on one particular pass I ran about 5.5lbs and picked up the speed.

Does this mean the car was running too rich and the lower pressure leaned it out on top?

I'm still learning about carbs but I'm assuming that the relationship is that less pressure = leaner mixture. I know that's the relationship with EFI; is it the same way with carbs, or is a carb not as dependent on fuel pressure?

TIA,
RSR

 

no,
the fuel pressure has nothing to do with adjusting A/F ratio with a carburetor.

 

What effect does lowering the pressure have? How might too much pressure affect things?

RSR

 

A carb draws fuel into the mixture from a fuel bowl with a float in it. As the fuel level in the bowl drops, the float opens the needle valve to let more fuel in. Too much pressure can force the needle off its seat and cause the bowl to overflow and flood the carb. Too little pressure usually means too little flow which fills the bowl too slowly and under high demand the fuel bowl can be drained, causing fuel starvation.

 

I believe float level can definitely effect jetting and sometimes idle for the aforemention reasons. Higher level = slightly richer mixture. Also on a hipo car you can get fuel from the primary fuel vent as fuel rushes to the back of the bowl. That's what float whistles are for.

 

RSR,

You don't mention what kind of carb you're running. If it's a quadrajet, you might have had trouble with the higher pressure "lifting" the needle off the seat too early. Matter of fact, at that kind of pressure, mine blew the needle off the seat entirely.


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"Beware he who would deny you access to information, for in his heart, he dreams himself your master"

'84 Black Camaro ZZ4 M5 (V-6 in a former life) -- street beast
'91 Medium Blue Metallic Z28 A4 (UB-HSTRY's old ride) -- currently broken
'99 Navy Blue Metallic Firehawk #120 M6 (99.44% stock) -- daily driver

 

Many thanks to those of you who provided polite, info-filled answers. "Politeness" is so rare these days! I definitely appreciate the info!

The carb is a Demon 750, mech secondary. I'll do some testing this weekend at the strip at the lower pressure to see what the results are.

Thanks again!
RSR

 

I'm also running a 750 Speed Demon on a 400+ F**d pickup. It's not a Chevy but it was built to run off and hide from my stock R/S. I'm still tuning it but I like the carb. If you run faster with lower float levels/fuel pressure I would think you might be jetted to rich for max power.

[This message has been edited by greearhr (edited March 21, 2001).]