Thread: Chevy 350 Engine Backfire

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I have an aftermarket chevy 350 engine in my 68 camaro. It has iron Vortec heads, hedders, a 750cfm double pumper 4160 holley carb, and a hydraulic cam and has been running great until today. It started backfiring out of the carb when I rapidly throttle up. It occurs around 2000-2500 rpm. I have already checked the firing order, ensured #5 and #7 spark plug wires do not touch each other, checked the valve lash, and pushrods, idled the engine with the valve covers off, and I don't see any problems. Also, if i smoothly bring the engine up to speed, i can max out the rpm's without a backfire. It only happens when I rapidly advance the throttle. Could a valve, lifter, or pushrod be hanging up? Or could the timing be off? Any help would be greatly appreciated.

First step I would do is put a timing light on it and see what your set at. Your distributor might have spun loose(maybe hold down came loose)

Well, I checked the timing and that doesn't seem to be the problem, engine still backfires. With the vacuum advance disconnected, it is set at 10 degrees at 1,000 rpm, and at 2600 rpm the total timing is 30 degrees. At 2600 rpm with the advance connected, total timing is 52 degrees, which all fall within the reasonable range for the engine. I guess my next step is going to involve taking apart the motor...

Originally posted by macfarlb
Well, I checked the timing and that doesn't seem to be the problem, engine still backfires. With the vacuum advance disconnected, it is set at 10 degrees at 1,000 rpm, and at 2600 rpm the total timing is 30 degrees. At 2600 rpm with the advance connected, total timing is 52 degrees, which all fall within the reasonable range for the engine. I guess my next step is going to involve taking apart the motor...

1. Check to see if the carb floats are working properly and the butterfly

2. Check all of the ignition (Spark Plugs, Wires, cap, rotor, points)

3. The go after the timing again

4. You also have a really good chance that you have blown the power valve in the holly and if so you can buy a one way power valve from summit or jegs for around 12 dollars to fix the holly

5. Dont tear down the motor until you get to the final and last resort

be sure the carb. pump is working. could have a cam going flat, which will make it pop out of the carb. a spit out of the carb is a miss fire or fire at the wrong time.

That's my plan...when I was checking the timing it took me a while to get a consistent flasher on the number 1 wire, and some of the spark plug wires are worn, so i'm going to replace them, the cap, and the rotor as well as checking the carb. I'm pretty sure the problem lies in one of those two areas.
Thanks!

I would check the carb. I would replace the plug wires, time, replace the plugs. Look at the plugs for consistancy. Spark plugs are a great thermometer.
A point an old racer showed me, do not run the plug wires parallel. They can induce misfire under certain conditions by induction. I always cross mine at least twice from spark plug to distributor. I would look for a weak lifter.
It sounds like it is one of those things.

Last ,but not least, check the power valve in that double pumper.

A point an old racer showed me, do not run the plug wires parallel. They can induce misfire under certain conditions by induction.

that was about 50 yrs ago and it was on a ford 292 or 312 cu.in. and it would do that because ford had 2 cylinders that was side by side in the cap and side by side in the head. if you ran them parallel to each other they would cross fire or jump fire some times. hey, dave or denny or one of you ford guys, get me stright on one of fords better ideas.

I don't know if this'll help you or not, as Im a bit of a newbie to the restore game myself, but the 350 in my truck stared backfiring through the carb about 2 weeks ago, and then a day or so ago, the carb went out in it, just a thought...

Hey ya'll...thanks for all the replies...I think I fixed the problem.
I replaced the spark plug wires, put a comp cams adjustable vacuum advance kit on, put a new power valve in the Holley, and put some 93 octane gas and it works like a champ, no backfire. I actually think that I may have put some bad gas in it...may have had some water in it or something...needless to say i'm not going to buy any more gas from that particular place! Anyway, thanks again