Backfire under acceleration how do I fix it?
Senior User
Joined: May 2007
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Sounds like too rich to me, but maybe not. How is your choke adjusted? Which holley is it?- 1850 600 cfm, 1310 750cfm, one of the avenger series. A backfire can happen any time there's a misfire, I get a condition like you're describing when i have the (manual) choke too far out. I would try to find someone who is more experienced with engine tuning to help you out.
I think more advance than 6 degrees would be ok, especially if you live at high altitude.
I think more advance than 6 degrees would be ok, especially if you live at high altitude.
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Elder User
Joined: Feb 2010
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Its a 600 cfm Holley, that's all I know about it. The choke is all the way open. I cant adjust it inside the truck. Ive had a few people tell me to go with a 750 Holley. I can drive really slow without a backfire but if I try to get on it it usually does once or twice. Its running great other than the backfire. smooth idle, plenty of power, no smoke now either, though I think that was oil burning off the exhaust from the oil leak I had. Positive pressure pushed oil out of the dipstick tube but that is fixed now.
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Elder User
Joined: Feb 2010
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I can do 80 mph if I want. If I take off slow and slowly accelerate to the speed I want all is fine but if I try to take off faster (like most people drive normally) it will backfire once or twice. As I said it runs great except for the backfire. It has 0 fluctuation at idle. its idling steady at 1,200 now that I have the timing set right. What should the idle be set at? I was told 600 but it seems a little low to me. I like 800-900 but that's not right I'm sure.
As far as the vacuum advance goes I am only getting about 15 at the carb as tested with my vacuum gauge on page 2. Is that an issue? Do you think its leaking around where the carb meets the intake manifold? I did use a used gasket and didn't use anything to seal it. I was thinking about buying a spacer that comes with 2 new gaskets (one for spacer/intake one for carb/spacer).
As far as the vacuum advance goes I am only getting about 15 at the carb as tested with my vacuum gauge on page 2. Is that an issue? Do you think its leaking around where the carb meets the intake manifold? I did use a used gasket and didn't use anything to seal it. I was thinking about buying a spacer that comes with 2 new gaskets (one for spacer/intake one for carb/spacer).
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Elder User
Joined: Feb 2010
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In my tags at the bottom of the page is the number "54" and the word "Taurus". I didn't put these tags in there, what the heck?
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Senior User
Joined: Sep 2009
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From: Goldsboro Nc
Is your accelerator pump linkage adjusted to were it just sits on the pump arm without pushing it down? You should take your idle adjustsment screws all the way in and, then 1 1/2 turns out, adjust to highest vaccum using the vacuum gauge. holley's website has some good info on adjusting your pump shot and troubleshooting. Your pump nozzle may be to small and you may need a bigger one. Did the carb work before the rebuild?
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Elder User
Joined: Feb 2010
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The carb did not work when I bought it. I paid $75 to have it rebuilt. He said he had to sand something down so it wouldn't stick at WOT. My idle screws are set at 2 turns out now. I know nothing about the accelerator pump but I can check that.
New User
Joined: Feb 2011
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My truck has a 360 with 390 heads and was rebuilt about 2200 miles ago. I swapped the intake for an Edelbrock performer 390 and I put a freshly rebuilt Holley 4bbl 600cfm on it. Now when I take off it backfires if I give it more than a tiny bit of gas and seems to have no ***** even at full throttle but burns rubber at almost idle. I know the carb isn't adjusted right (I don't have a vacuum gauge) but someone told me the carb was too big for the engine and someone else told me it wasn't big enough. WHO DO I TRUST? Neither is what I guess. Anyone have any input on this? It runs great at steady rpm (on the highway or sitting at idle). It smells like its running SUPER rich! Any help would be great. I want to drive it more.