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Discussion Starter #1
The car idles and revs. fine, but as soon as you drive, the carb. pops or back-fires. Does not back-fire from the tail pipes.
68 Camaro 350/300 w/m21 4spd. and holley 670 carb.

Thank you for your expertice.
 

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Perhaps it could be the timing and/or intake gaskets are going bad. Is the car running "rich?" Meaning the air/fuel mixture? Look for vacuum leaks too.

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1969, X-44, Export Model, Hugger Orange, Std. Int., BC Rear(soon to be a BT 12 bolt), PDB, GMPP 350HO, M22 Supercase, SSM Lift Bars, AirGap, Holley 750, FlowMaster American Thunder Exhaust, FlowTech Headers
 

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Is this happening when it's slightly cold? Mine did it until I adjusted the choke...no more backfire, Previous post is correct, might check your timing or vacuum also...Good Luck Mike
 

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Discussion Starter #5
This problem happens when the car is warmed up. Also the engine compartment seems too hot, the timing light connector to #1 cyl. was melted, but the temp. gauge reads normal. Makes me wonder if I have a header gasket leak, also the fumes are bad, but I see no smoke.
When the car is parked, the cars revs without a problem, as soon as I put it in gear and give it some gas, the carb backfires or pops, but there is no back fire from the tail pipes.
 

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hey sounds like it could be running lean with the popping thrut the carb, but it really sounds like you have a wire or two that has melted on a header and is arcing out under load, i had a chevelle that ran great while sitting and reving but under load(the coil puts out more spark) the spark would jump through the small melted part on the wire right to the header and it would pop like crazy if i were passinf someone or was going 40-50 and then punched it, but goodluck
jake
 

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you need to advance your timing, when i installed an msd coil in my 69' it did that till i had advanced my timing and regapped my plugs, do you have any performance coils you might have in there. because the plugs need to e at 45 thousands or higher to run that and will give you that problem. also check your cap and rotor and air/fuel mixture
 

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Discussion Starter #8
The last thing I did was retard the timing and adj. the air/fuel mixture screws on the carb. The coil might be original, still has GM markings, and the plugs and wires are new. Looks like I need to re-adjust the carb. and timing.
Question, on the air/fuel mixture screws-after adj. screws all the way in, do you unscrew them 1 1/2 turns out?

Thanks for the advice.
 

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1 1/2 from full in is a good starting point for the idle mixture. Set idle speed and then put a vac guage on a manifold vac source and adjust the screws to achieve max vac. I have the same problem with the pop thru the carb and am in the process of replacing the accelerator nozzels with a larger one to richen the mixture when I get on the throttle. I retarded my timing to 8 degrees static with the vac on a ported source and that helped a lot, but still need to tweek the pump shot. I'll let you know how mine turns out.

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68 convertible
 

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I had a similar problem one time with one of my Camaro's. I was running an HEI and the coil was going bad. It ran good stopped with no load but would spit back through the carb and run bad under a load. Make sure you have a good blue spark. You can also take some windex or some water and spray on the plug wires while someone stalls up the engine and see if you can spot and archs or if the engine runs even worse than before you sprayed it. Easy way the find a bad plug wire.
 

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Does it pop when you try to accelerate? I would be willing to bet the carb is too lean. I would guess the pump shot is not large or long enough to cover the sudden hole you have when you ope the throttle up. When you are above (rev'd up) or below (idle) the hole you won't see any problems. I would try a larger pump shot (larger squirter) and if that doesn't do it I would try another accelerator pump cam (larger). This is a common problem, just a matter of tuning the carb..

Royce
 

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Discussion Starter #13
Yes this happens only when I excellerate. I can stomp on the excel. when I am parked and do not get the "pop". I have the day off on Thur. and will readjust the carb.
FYI this is a new two week old 670 street avenger holley, I might have a warranty issue, but hope/pray it is only adj. problem.
I will let you know if it fixed the problem.

Thanks again!
 

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FLS68 I am sure a little carb tuning will cure your problem.

Jeff, you may have a different problem. Are you saying you car pops through the carb on deceleration? Are you sure it's not through the exhaust? In your case I would look for a vacuum leak or timing issues.

Royce
 

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Discussion Starter #15
I just hooked up the vaccum gauge and the motor was reading 6lbs w/air cleaner. I changed the power valve with holley part #125-85. This was a big improvment. I now have 13lbs. The carb. was readjusted with the vac. gauge conn. Adj. the air/fuel for max vac. and re-adj. the timing. All my friends tell me it is a timing issue, but I have adj. the timing every which way but loose. Still no success. This is a single point dist., could a bad distributor and or a coil cause this problem?

Thank you.
 

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Discussion Starter #16
I almost forgot, if I let the car idle on its own power out of first, I get no pop out of the carb. So if I take off with out touching the accel. pedal I get no pop out of the carb., but as soon as I give it the gas, out of first and only "first gear" the carb. goes pop. No pop from carb. going to 2nd. or 3rd. or 4th.

Thanks again.
 

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Discussion Starter #17
This problem is fixed!

Replacing the "65"jets with "68"jets solved this problem.

Could not have done it with out you.
Thanks for everyones help!
 

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Discussion Starter #18
This problem has been resolved.
The fix was installing bigger jets in the carb. I replaced the 65 jets with 68s.

Thanks again for everyones help!
 
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