what causes the lean stumble when the secondaries come in too soon. Is it that there is not enough signal to pull the fuel thru the mains? 750CFM on a SBB. Cheers!
You posted this in the Holley section, so I guess you have a Holley on it? they have different vacuum diaphrams to bring in the secondaries sooner or later. You might need to experiment with them to find the best one for you.
Actually it's Quickfuel 4150. I know how to set it up with the springs. My question is why does it bog when the spring is too light. OK the secondaries come in to soon but what in the circuit causes it to bog and go lean. If it was a 750 dbl pumper, it wouldn't bog. uzzled:
Engine has not reached enough RPM to pull fuel out of the booster. Also, I had the same problem from the V6 Buick engine in my Triumph TR7 bracket car and although I have an adjustable vacuum pod (Quick Fuel) on the carb, no matter where (how) I adjusted the spring screw the motor bogged (nosed over) right after launch. The problem turned out to be that the front fuel bowl level was to high (only to the bottom of the inspection hole) and the fuel was coming out the vent tube and into the front of the carburator. I thought this was impossible as the car only had a 1.88-1.89 60' time but a friend of mine who works at Quick Fuel (there down the street from me) told me he had the same problem on one of his Mopar motors. I kept lowering the fuel level until the problem went away even though my better judgement told me the fuel level was to low, but it wasn't. Hope this helps you.
What QF carb, one with sight glass? How low did you need to go and it didn't affect the rest of the curve? Mine is a q750pv, the vacuum screw is 1/4 turn out, any more and it has lean bog.
I wonder if a larger high speed air bleed in the secondary would allow the circuit to flow sooner allowing the seconds to come in sooner without a lean curve. i'll give it a try, right now i'm at 1/4 turn out on the vacuum.
I've got a Holley 650 vac secondary with Quick Fuel vacuum pod and a rear plate with jets and jet extensions. I don't know how low the fuel level is but you can shake the car and fuel won't spill out of the inspection hole. I wanted to try a 750 vac secondary Holley but the rear fuel bowl won't clear the hood latch (front hinged hood). My little car went 8.850 last time raced with a 1.888 60' time @ 75.96 mph but it only weighs 2363 lbs (I'm 200 lbs) with me on board. Its like driving a go cart.
