chevelle backfiring at acceleration

by Dead Man Sun Nov 18, 2012 1:48 pm

Well, every weekend now Ive been driving my 73 chevelle ss sbc 350, holley 600, new plugs and wires, idles great and has been running really well, last night it done fine too. well today I go for a spin in it, and its backfiring at acceleration, I mean I barley push the gas down at all and it will backfire sounds like its coming from under the hood, if I just ease the peddle down it will be ok but I have to go very slow or it backfires and almost dies, but it still idles just fine and I can rev it up just fine and sounds great, just seems like its only doing this under load. Im still on stock points. Im hoping this is an easy fix like maybe just up grading to hei. any help would be great thanks.

by BlackChevelleSS Sun Nov 18, 2012 2:39 pm

check timing first, also check the dwell. if its still backfiring, need to check the cam make sure its not going flat

by ant7377 Sun Nov 18, 2012 3:47 pm

Does it just continully back fire or just when you first step on the gas then it goes away and is normal?

by pila Sun Nov 18, 2012 5:43 pm

First question: is it back0firing through the intake, or the exhaust ??
One item you can check is the distributor cap, for carbon tracks, where spark may be jumping between the terminals, and maybe the rotor for cracks.

Of course if it's running lean, like fuel-starved, it will back-fire through the carb.
Have you ever checked the fuel filter ?? Have an inline filter on that fuel line to the Holley ?? You could disconnect the line from the carb, spin it over with the starter & catch the fuel, to see how much the fuel pump is putting out (with the wire from the coil pulled out of the dist first).. If it's pumping plenty of fuel, you will have to assume that the fuel pump is OK......
Converting to an HEI distributor is the best idea, since you still have a point type. Nothing to do with your problem in my opinion.....unless that old coil is giving up.....

by Dead Man Sun Nov 18, 2012 5:44 pm

ant7377 wrote:Does it just continully back fire or just when you first step on the gas then it goes away and is normal?

well if I step on it kinda fast it will do it but if I just ease into it, its ok. but no matter what speed im at it will do it if I put it down to fast. other than in park then it will rev just fine and as fast as I want it.but no not one pop after another just once right when I throttle it to fast. ( even though I wouldnt call it fast at all because if your not going down with it super slow it will pop.)

by Dead Man Sun Nov 18, 2012 5:46 pm

pila wrote:First question: is it back0firing through the intake, or the exhaust ??
One item you can check is the distributor cap, for carbon tracks, where spark may be jumping between the terminals, and maybe the rotor for cracks.

Of course if it's running lean, like fuel-starved, it will back-fire through the carb.
Have you ever checked the fuel filter ?? Have an inline filter on that fuel line to the Holley ?? You could disconnect the line from the carb, spin it over with the starter & catch the fuel, to see how much the fuel pump is putting out (with the wire from the coil pulled out of the dist first).. If it's pumping plenty of fuel, you will have to assume that the fuel pump is OK......
Converting to an HEI distributor is the best idea, since you still have a point type. Nothing to do with your problem in my opinion.....unless that old coil is giving up.....

Im assuming its coming from the carb because its under the hood for sure. no fuel filter under the hood going to the carb, I going to add one of those now you mention it. but I will check and spin it over make sure its pumping good.

by pila Sun Nov 18, 2012 5:55 pm

Pushing the pedal down slowly lets the fuel that's available, keep up with the engine, while pushing hard requires more fuel to accelerate, so it does kinda sound like a fuel problem.....
Maybe some debris in the carb, if there's no inline filter....

by Dead Man Sun Nov 18, 2012 5:59 pm

pila wrote:Pushing the pedal down slowly lets the fuel that's available, keep up with the engine, while pushing hard requires more fuel to accelerate, so it does kinda sound like a fuel problem.....
Maybe some debris in the carb, if there's no inline filter....

makes sense, I will check that tomorrow with day light, any thought on why it only does it under load though/while in gear.

by JiMi_DRiX Sun Nov 18, 2012 6:15 pm

I have a similar problem with the 75 I bought, it had been sitting a while, so I'm thinking its related to that. Maybe a gummed up carb or something. I havent had time to try and diagnose it yet.

by ant7377 Sun Nov 18, 2012 7:09 pm

73ssbb wrote:
ant7377 wrote:Does it just continully back fire or just when you first step on the gas then it goes away and is normal?
well if I step on it kinda fast it will do it but if I just ease into it, its ok. but no matter what speed im at it will do it if I put it down to fast. other than in park then it will rev just fine and as fast as I want it.but no not one pop after another just once right when I throttle it to fast. ( even though I wouldnt call it fast at all because if your not going down with it super slow it will pop.)

What kind of carburetor do you have? It sounds like your accelerator pump on the carb might be a little out of adjustment and the backfire you hear is just a lean pop.

by Brainstain Sun Nov 18, 2012 7:24 pm

Sounds like a carb issue to me. I'm having almost the exact same problem with the 76 I just bought.

by ant7377 Sun Nov 18, 2012 7:32 pm

If you have a holley heres something that can help you

https://www.youtube.com/watch?v=w3F3ssOb8lg

by ant7377 Sun Nov 18, 2012 7:34 pm

A q-jet will have to have the top open and the little cup on the accelerator pump rod (goes up and down on the right front of the carb) you usually will see fuel around the little pocket there.

by Dead Man Sun Nov 18, 2012 9:34 pm

ant7377 wrote:If you have a holley heres something that can help you

https://www.youtube.com/watch?v=w3F3ssOb8lg

thank for the link, at the 250 mark in that vid it sounds just like mine, so it looks like I have some checking to do. it just bugs me that it was running just fine the night before lol. Only thing different in the vid is mine will only pop liek that while in gear, park and neutral it revs up fine with no problems.

by 77mali Sun Nov 18, 2012 9:45 pm

The inline fuel filter is a good idea as Pila said even if it has nothing to do with your issue.

by Dead Man Sun Nov 18, 2012 10:02 pm

oh yeah, gonna do that first thing

by ant7377 Sun Nov 18, 2012 10:51 pm

73ssbb wrote:
ant7377 wrote:If you have a holley heres something that can help you

https://www.youtube.com/watch?v=w3F3ssOb8lg
thank for the link, at the 250 mark in that vid it sounds just like mine, so it looks like I have some checking to do. it just bugs me that it was running just fine the night before lol. Only thing different in the vid is mine will only pop liek that while in gear, park and neutral it revs up fine with no problems.

Yeah but go under the hood take off the air cleaner and listen carefully while you move the throttle bracket gently just get it past the accelerator pump.I bet you hear it go lean a little (sound like its sucking air) . Do you see in the video the pump cams? Maybe the little screw backed out and the arm isnt moving correctly or the bolt adjustment backed off (again this is all based on if you are running a holley but you get the idea its all similar) . Look down in the carb with the car off and see if you see gas shoot as soon as you move the throttle bracket (you should) .

by Dead Man Mon Nov 19, 2012 1:07 am

yes its a holley 600, will check it out tomorrow with some updates. thanks guys

by Dead Man Mon Nov 19, 2012 12:26 pm

Update, it is backfiring in park after all, I took the breather off and reved it by hand to see if it was spraying gas like it should and it seems to be spraying just fine, but when I rev it hard it backfires through the carb I can see a flash of fire from the front part of the carb, the accelerator pump seems to be fine.

by Dead Man Mon Nov 19, 2012 7:26 pm

another update, went by my grandfathers body shop and told him what was going on so he wanted to check it out, we revved it, drove it like we stole it grandma the car ext ext, and it would not backfire at all it seems magically fine now.

by ant7377 Mon Nov 19, 2012 7:26 pm

Check to make sure your distributor hasnt moved. It sounds like it may be out quite a bit. Look at your ignition system,maybe the coil is starting to go bad. You have a backfire but do you have a miss while its running? Im still thinking its a fuel issue.

by Dead Man Mon Nov 19, 2012 10:07 pm

no miss it runs really well and strong.I think im gonna get a new fuel pump with hei upgrade if it acts up again. but I agree I think its a fuel problem.

by Dead Man Tue Nov 20, 2012 7:09 pm

well put the filter on today and it only backfired once, but its still doing it here and there, so guess Its time for the hei upgrade and go from there.

by 77mali Tue Nov 20, 2012 7:16 pm

Maybe put in a fuel treatment like Gum Out or Seafoam w/ a freash tank of gas & see if it stops altogether. It might have just been the gas...you never know.

by Bruisr Tue Nov 20, 2012 10:39 pm

Check your vacuum plugs, and base of carb. you may have developed a vacuum leak causing a lean backfire.. Increase in airflow with no increase in fuel flow will cause a lean backfire.

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