Backfiring at wide open throttle

[63comet]

Heh everyone, what would make a car backfire and stumble at wot only? Car starts and runs perfect with good throttle response at idle and up to half throttle, but when you stand on it, it stumbles big time and sometimes backfires.

 

[addo]

Does it happen if you build up speed slowly, too?

 

[63comet]

I don't believe it does Addo, I really haven't had the opportunity to take it out and check it out. I do not have it inspected. But, when it goes slowly, or if I back off the throttle to half way, it doesn't seem to do it.

 

[addo]

Could it be a weak accelerator pump shot? If it were transitions in the fuel circuits, you'd probably get the same symptoms with similar rates of venturi flow.

William (wsa) is probably the guru for dialling in a Holley, including pump shots and the pump cam selection.

Regards, Adam.

 

[63comet]

I'm not really sure it would be the accelerator pump, due to the fact that it continues to stumble as long as I hold the throttle wide open. It almost seems like it is running out of gas at WOT, when I back off to half throttle it responds fine. If I only rev it half throttles, it revs fine.

 

[pedal2themetal45]

I'd check the timming.. maybe tomuch advance in the dizzy.. might need to be recurved.
tim

 

[Anonymous]

Backfire thru the carb equals lean. Backfire at WOT usually equals either vacuum leak or too small main jet.

 

[63comet]

Heh everyone, where are you supposed to take the vacuum from for the duraspark dizzy, (manifold or ported), I know it's been discussed here before, but I can't remember. I have set the timing at 8* and I have also tried it at 6* and 12* initial.

 

[63comet]

Is it possible to have the dizzy 1 tooth off and still have it start and respond good to half throttle? I changed the timing to about 30*, (not sure, my pointer doesn't go all the way around, but it's about 3" from the top of the pointer bracket), and the car will rev and no more backfire or stumbling at WOT. I'm thinking when I dropped the dizzy in, I'm off by a tooth.

 

[pedal2themetal45]

HI
As long as you can line up the timming marks with it running (you know 4,6 or 12 with it runing.) it shouldn't matter. Unless you are talking about the tinmming gears then yes it would matter..
As for the vacuum I would think that above base plate would be the better place.. If you hook it up to the one in the base plate you should maybe time it with that plug in to the carb., Becaues when you plug that one in it well pull the advance and move your timming at idle..
Tim

 

[63comet]

That's what I am saying, when I set the timing according to the pointer,(6,10,12, degrees) the car starts fine and runs good to half throttle, but when I dump it, it stumbles and sometimes backfires. But, yesterday, I set the timing advanced way beyond the pointer, and the car runs great when I dump it, (can smoke the tire right off it) with no backfire or stumbling. But it probably is too much advance as it pings pretty good under a load and binds on the starter when hot.

 

[pedal2themetal45]

HI
The dampner may have slipped... If it runs good but ping a little or hard to restart try turnning the timming back just a couple of degs. at a time untill its not pinging or hard to start.. a couple of deg well make a differance..
tim

 

[63comet]

thanks, I guess that will be my plan for now. Move it a couple of degrees at a time, and do not pay attention to the marks.

 

[thinman56]

try running without the vacuum advance, mechanical only, and see if it makes a difference. if the vacuum advance is too aggressive (like mine, 20 degress at 14" vac), it may run too advanced in the low/mid-range at light/medium throttle if tuned for WOT. i'm waiting to get my new head on to re-connect and sort out my vacuum advance.