back firing 340 hp

[Ed S.]

Re: back firing 340 hp

Frank, thanks for your response. Back fire is at carburetor, not out the exhaust. Any other thoughts?
Bruce

Is it actually "back firing" through the carburetor - popping back or is it more of an ignition miss fire? The only way, as far as I am aware that you will get detonation back through the intake manifold and up the carb is if one or more intake valves are partially or completely open. Bucking or "missing" ignition spark will cause some popping sound but not back through the carb. If you really are getting explosions back through the carb an intake valve must be open (or the timing is so far off it is ridiculous - but you would likely not even be able to start the engine no less wind it up to 5000 + RPM). Do you have stock valve springs or do you have some after market dual spring setup? Weak or worn springs will allow the valves to float some at higher RPM - in this case you would get that popping back through the carb. I ask if you have a dual spring set up - if you do and one spring on one valve is broken it will run fine at lower RPM - but fail to close the valve fast enough at higher rev ranges.

[Bruce W.]

Re: back firing 340 hp

Gene, i have some work to do . will check out these items.
thanks Bruce

[Bruce W.]

Re: back firing 340 hp

Ed, interesting. heads were rebuilt about 1000 miles ago. no double springs. might be a miss fire?? I think it is electrical. the research continues.
thanks for your help
Bruce

[Bruce W.]

Re: back firing 340 hp

Gene, electronic ignition was done by supposedly an expert. it is the single wire version. wires are packard solid wires. also car has ballist resistor. as far as the rotor is concerned how would i know if it is the shorter version? and where do i get the correct one. appreciate your help.
Bruce

[Ed S.]

Re: back firing 340 hp

Ed, interesting. heads were rebuilt about 1000 miles ago. no double springs. might be a miss fire?? I think it is electrical. the research continues.
thanks for your help
Bruce

Bruce, you likely have an ignition problem. As for the "backfiring" or popping sound. If the ignition system is malfunctioning what is likely happening is that one or more spark plugs are not receiving sufficient voltage to spark and ignite the air / fuel mixture in the compression chamber. That un-burned mixture is then pumped out of the cylinder and into the exhaust manifold on the subsequent exhaust stroke - when the mixture arrives in the exhaust manifold it is ignited by the hot exhaust gasses from other cylinders. That is what is producing the popping or backfire sound - the absence of a full power stroke by one or more cylinders is what causes the bucking or slight jerking of the car.

Back in the day (and even today) cars with breaker point ignitions would do this (at higher RPM ranges) if any of the following conditions exist: 1) worn (pitted) or incorrectly gaped points 2) fouled, worn or improperly gaped spark plugs 3) old spark plug wires 4) failing ignition coil.

The phenomenon is that the ignition system is able to (appear) to function normally at lower RPM ranges and when the engine is not under a load - when just driving around town all seems fine - but stand on it and you get the bucking and popping - that is almost always an ignition problem.

[Gene M.]

Re: back firing 340 hp

Bruce, try the longer rotor tip. It is an inexpensive part available (I think advertised in drive line) about $8. It is about 1/16” longer as were the old style rotors in the 50’s and 60’s. A short one is NG for higher RPM’s.

This might be all the issue you have.

A short one along side a long one is noticeable but otherwise difficult to discern.

[Bruce W.]

Re: back firing 340 hp

Gene, thanks will give it a try and let you know
Bruce

[Bruce W.]

Re: back firing 340 hp

Ed, it was indeed an ignition problem. Changed the coil and it fixed the problem. Thanks for your insight. you were dead on!!!
Bruce

[Ed S.]

Re: back firing 340 hp

That is great! Glad you found the problem - replacing a coil is an easy fix. May I ask, what brand, model etc coil did you install, and where did you get it?

I ask because, from what I have heard (on this forum as well as other places), is most "vintage" type coils that are manufactured to original design specs are far and few between. Most are made "overseas" and quality is typical "overseas" if you know what I mean. I would like to replace the coil in my '64 L76 because the one in there is a no name no marking unit of unknown age and I am just suspicious of its quality and reliability.

[Frank D.]

Re: back firing 340 hp

Gene, electronic ignition was done by supposedly an expert. it is the single wire version. wires are packard solid wires. also car has ballist resistor. as far as the rotor is concerned how would i know if it is the shorter version? and where do i get the correct one. appreciate your help.
Bruce

If you have a Pertronix conversion you cannot use solid core wires; if its an M&H Breakerless SE they are designed to work with everything else stock: wires, ballast resistor, plug gap and ignition coil...

[Bruce W.]

Re: back firing 340 hp

Ed, i just bought a replacement coil from NAPA. cost about 50 bucks. I have had a bad experience with the GM replacement. they are made in China. My search will begin looking for a correct coil. will be watching ebay. originals are as scarce as hens teeth. thanks again for your help.
Bruce