Backfiring through the carb - Team Camaro Tech
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post #1 of 16 (permalink) Old Jul 24th, 01, 04:16 AM Thread Starter
 
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Hi all, I have a 78 camaro, it has a 468 big block, it has had some bottom work done. I have have edelbrock 750 carb. I have checked the ignition, timing, the plugs, tuned the carb... but what happens is everytime I step on the gas it backfires through the carb... I have pulled off the valve covers in an attempt to see if it could be due to a bent pushrod or broken valve spring... As far as I can see there are no bent pushrods, and I can't tell about the springs unless I remove them. Before it started backfiring the car was falling flat on its face at the 2000 to 3000(roughly) rpm range. This is what instigated the carb tuning. Finally I got the power back out of the motor and then took one mild run wraped it up to about 5000 rpm, ran great, then on the way home crusing at about 2000 - 2500 rpm I romped on it and BANG!... It starts backfiring like crazy... we rechecked everything(plugs, ignition, carb, timing) and it was all perfect. Any ideas???? Help me bring my baby back to life!
Thanks!
JLW.

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post #2 of 16 (permalink) Old Jul 24th, 01, 05:59 AM
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I would recheck the accelerator pump on the carb to make sure that you are getting that extra squirt of gas. If it is malfunctioning or not working at all, your engine is getting a big gulp of air and that will make it pop thru the carb ... there is an adjustment on the edelbrock to alter the the "size" of the stroke of the accelerator pump
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post #3 of 16 (permalink) Old Jul 24th, 01, 12:13 PM
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I agree. It sounds like your mixture might be a little lean when you step on the throttle. Check your accelerator pump which was previously said and also check your plugs. If they are tan in color, (look in any shop manual for a picture of a plug out of an engine that is tuned properly)I would look at the accelerator pump area. If they are indicating a lean condition, you might look at the jetting. My car did the same thing until I had it jetted right, especially in the winter when the mixture needs to be a little richer.
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post #4 of 16 (permalink) Old Jul 25th, 01, 05:27 AM
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I would take a really good look at your ignition system. Do you have a good ground for the motor? Make sure you have good spark plug wires that don't touch each other. And make sure your firing order is right...I've seen this once before. If everything seems good still...did you do a compression test on all the cylinders? Wet & dry if any are low? Are you using HEI, points, MSD add-on?
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post #5 of 16 (permalink) Old Jul 25th, 01, 05:48 AM Thread Starter
 
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I am using an HEI ignition, accel plug wires. I have checked all of the pushrods and they are straight, hooked up a tach and tuned the carb. The plug wires are all seperated by wire looms.

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post #6 of 16 (permalink) Old Jul 25th, 01, 06:43 AM
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DId you change the carb? or is this the same one that was on the car? When did this problem start? I think it could very well be acceleratop pump related, but I would check my squirter size and cam (plastic cam on the carb that actuates the accelerator pump). I am assuming if you accelerate slowly everything is fine, but when you try to romp on it, that's when you get the backfire. You may also want to check for vacuum leaks.

Royce

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post #7 of 16 (permalink) Old Jul 25th, 01, 04:50 PM
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Do you have a vaccuum advance on the HEI? I had similar problems and found my vaccuum advance was sticking open and, at other times, closed. I'd re-time it with the vaccuum hose plugged (but the advance unit stuck open), and then get strange results when it would suddenly "unstick" while I was driving. Since I'd recently re-jetted my carb, I looked at about everything before I figured it out. Steve Laabs
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post #8 of 16 (permalink) Old Jul 25th, 01, 05:48 PM
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my camaro is also popping through the carb and im going to jet mine also i think with the heads we put on it , it was suppose to add 50 to 60 horse power so im thinking i am running lean
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post #9 of 16 (permalink) Old Jul 25th, 01, 07:58 PM
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A lot of distributor hold downs dont hold correctly.
check your timing again, the high speed run might have retarded the timing.
David

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post #10 of 16 (permalink) Old Jul 26th, 01, 03:33 AM Thread Starter
 
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This is exactly whats happening!!! Does anyone think that this could be caused by a problematic intake valve or broken valve spring? The carb is the same one I've had on it since the motor was built(a 750 edelbrock)
The problem started like this... I had no power through 2000 to 3000rpm's, we thought it to be a carb tuning issue. So I tuned the car(plugs, timing, oil, etc...) and we tuned the carb. I made one pass and she ran great, but on the way back I was crusing at about 2500rpm's and hammered it, thats when the backfiring started. If I slowly depress the gas she runs fine, but if I put any sort of a load on her then she starts poping like crazy. I have rechecked the timing(with the vac adv. unhooked) and hooked up a handheld high and low rpm tach up and rechecked the carb at an idle and everything appears to be fine. But she still backfires....


<BLOCKQUOTE>quote:</font><HR>Originally posted by camaroman7d:
DId you change the carb? or is this the same one that was on the car? When did this problem start? I think it could very well be acceleratop pump related, but I would check my squirter size and cam (plastic cam on the carb that actuates the accelerator pump). I am assuming if you accelerate slowly everything is fine, but when you try to romp on it, that's when you get the backfire. You may also want to check for vacuum leaks.

Royce
<HR></BLOCKQUOTE>



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post #11 of 16 (permalink) Old Jul 26th, 01, 04:40 AM
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Elvis,

If it's a popping sound, and not a full-fledged backfire, I think it is a broken exhaust valvespring or an exhaust cam lobe is down. Especially if it only happens during hard acceleration. Also, is it continuous (keeps popping while you accelerate), or does it backfire only once upon WOT, then it stops? If it's just once, it could be an accelerator pump problem. If it's continuous, it could be timing, but it's probably a spring or lobe. Just my 2 cents. Take care.

Shane
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post #12 of 16 (permalink) Old Jul 26th, 01, 04:58 AM Thread Starter
 
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This is exactly right... It keeps poping continuously during acceleration. (not gentle acceleration, but mild to heavy accleration).


<BLOCKQUOTE>quote:</font><HR>Originally posted by 69SSRS:
Elvis,

If it's a popping sound, and not a full-fledged backfire, I think it is a broken exhaust valvespring or an exhaust cam lobe is down. Especially if it only happens during hard acceleration. Also, is it continuous (keeps popping while you accelerate), or does it backfire only once upon WOT, then it stops? If it's just once, it could be an accelerator pump problem. If it's continuous, it could be timing, but it's probably a spring or lobe. Just my 2 cents. Take care.

Shane
<HR></BLOCKQUOTE>



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post #13 of 16 (permalink) Old Jul 26th, 01, 05:49 AM
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elvis, i had the same problem and it turns out my timing was to retarded (due to the balancer had slipped)i readjusted my timing by ear and the backfire went away.
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post #14 of 16 (permalink) Old Jul 26th, 01, 06:07 AM
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If you sort of fixed the problem by tuning then it shouldn't be valve springs or a bad cam {your symptoms indicate it should} Make sure the power pistons in the carb are not stuck or you left out the springs. Do not continue to drive the car like it is what you are experiencing is destroying the engine.

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post #15 of 16 (permalink) Old Jul 26th, 01, 09:29 AM
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oger,

He states that he tuned it because it wasn't running right, wouldn't rev between 2-3k rpm. After he tuned it, it made one good pass, then started backfiring. The tuning was done before it started backfiring. My guess is once he finally got it to rev, a spring let go. Just a guess based on his symptoms.

I don't think it's a balancer problem, because he states it runs fine as long as he's not accelerating. Only then does it start to backfire, if I read his posts correctly.

Shane
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