bradzap

I need some help from you carb experts out there. Over the winter I rebuilt the Quadrajet on my 72 vette. The carb wasn’t in bad shape, but the accelerator pump was intermittent in function and I had a terrible bog off idle and during the transition between the primaries and the secondary.

I took my time and to my amazement it came apart and went together quite easy.(no extra parts).

I reinstalled it and it ran fine….with no engine load. I could rev it in the driveway with no problem. When I was finally able to drive it, it backfired like no tomorrow under load. I checked the timing and the plugs/wires with no found problems. The accelerator pump is working fine. I adjusted the air valve so the tension is just enough to close the valve.

Can someone please help?

Thanks

73, Dark Blue 454

Backfiring through the carb is almost always a timing issue. Double check your timing, the vacuum advance (disconnect the VA hose from the carb and suck on it to make sure the diaphragm is still good) and your mechanical advance (make sure the weights move freely), etc.

Another thing,..sometimes the outer ring on the harmoic slips and causes inacurate readings. Is the harmonic original? Is the timing cover and timing tab original as well?

bradzap

I'll double check the timing. I had a steady 8 degrees advance last time, but I will give it another shot. As far as the balancer goes, I believe it is original.

Thanks again.

olescarb

the problem could be low flow level, the power piston stuck down so the carb is always lean, is the spring under the power piston? most times the sec spring is set at 1/2 to 3/4 turns tighter after the spring just closes the valve.

bradzap

Thanks for input. When I manually press the power piston down it feels sticky..like it is hanging in the bore, or on the gasket.

I will adjust the sec spring tighter and see if that helps.

olescarb

there is no ball that goes under the power piston, at times the bore for the power piston does get distorted so you can clean it up with a 11/32 ream just do not take out too much material. the top gasket slips thru/around the metering rods and rod hanger and should not touch the rods or hanger once it is in place

bradzap

Your right about the ball. I was mistaken for the pump discharge ball next door.

Thanks again

lars

Backfire through the carb is not a timing issue - it's a lean condition.

DaveL82

First I would tight up the secondary air valve to take it out of the problem (so it doesn't come in too fast).

Check to see if the carb sprays fuel in from the accel pump when this is occuring.

I have had this occur when the HEI module went bad but did not show up until distributor came up to engine temp. Appeared as a lean backfire condition.

Dave

bradzap

Thanks Dave.

I'm glad you mentioned that. I don't seem to have an issue until I get up to temp., but then again I don't apply enough throttle to open the secondaries until it is up to temp. How did you diagnose the bad HEI module? (I changed my points to HEI a few years ago).

Avette4me

listen to lars...lean is the word..