Mild backfire through mufflers

[furyus2]

To up date, the car now backfires through the mufflers, and bucks on acceleration, hot or cold. Any RPM. I verified firing order. Number one plug looked good. Will check the others. Carb is an older Holley, with an RPM intake. I think the exhaust crossover is open, no choke. This setup ran fine for years. Then it just started. Put in a new pertronix dist. since the old one was original. No change. It also has the Street Fire MSD ignition box. I'm wondering if it's the coil. How do I check it?

 

[nm9stheham]

You can measure primary and secondary resistance to the spec sheet for the coil but that will not tell you if it is breaking down internally at times. Unless you have a test jig, then replacement is the typical 'test'.

 

[furyus2]

Well, it's not the mixture screws, or the vacuum advance, or the coil, or the fuel pressure.

 

[justinp61]

Have you checked the timing?

 

[yellow rose]

I'll agree on a carburetor with a choke, without a choke and it'll be lean until it warms up. I run a 950HP and the afr's at idle cold are in the 16-17 range, after it warms up 13.8-14.1.

If you non choked carbureted engine idles great cold it's pig rich warm.

Yup, this is true. I live where it is piping hot in the summer and colder than a witches mammary in the winter and I don't use a choke. I have the heat cross over blocked as well. When it is cold it take a few to warm it up but that is much better than a choke.

 

[furyus2]

Have you checked the timing?

Tried timing in several positions. When this started, it was timed and running correctly for years. Put in a new dist, reset timing, then tried a couple different settings. no change. Thinking of going to another carb I have, and see. First I want to pull the plugs, and check the wires.

 

[furyus2]

For those that are following, I found the issue. It turned out to be a bad plug wire. I checked them with an ohm meter, and noticed one didn't show a reading. Apparently, it fired that cylinder well enough, until under a load. Oddly, I didn't have a dead cylinder, just the backfire, and jerking. I put the meter on the plug wire terminals, and got nothing. removed the terminal that I installed when making the wires up originally. Checked the wire where it used to touch the terminal. Got nothing. Cut it back, got a reading, and put on a new terminal, and all is good.

 

[yellow rose]

For those that are following, I found the issue. It turned out to be a bad plug wire. I checked them with an ohm meter, and noticed one didn't show a reading. Apparently, it fired that cylinder well enough, until under a load. Oddly, I didn't have a dead cylinder, just the backfire, and jerking. I put the meter on the plug wire terminals, and got nothing. removed the terminal that I installed when making the wires up originally. Checked the wire where it used to touch the terminal. Got nothing. Cut it back, got a reading, and put on a new terminal, and all is good.

So what do you think caused the plug wire to fail? What brand are they?

 

[furyus2]

The are the Accel spiral wound wires. The failure point seemed to be where I stripped the insulation back, and folded the wire core back to put the terminal on. I may have nicked the wire core when assembling the wire, causing it to fail over time. I'm still going to put a new set of wires on it soon, just cause these are a few years old. Thanks to every one who helped.

 

[nm9stheham]

Good for you....! Sounds like it could jump the gap in the wire AND the plug gap under certain circumstances, but not jump both under others.

Everyone should keep in mind that the voltage required to strike an arc across a plug gap varies considerably with the mixture, the cylinder fill, gap distance, and the cylinder pressure, all of which change with load and RPM. That is why looking for a spark to arc the plug gap in open air is a meaningless test; the voltage required to make it arc in the cylinder is muuuuuch higher.