Technical Backfiring issue

Good evening everyone!

I am having a backfiring issue with my '59 GMC pickup with a '72 Chevy 350 in it. It's always started and ran good, but after I put a brand new Holley 4412 2 barrel carb on it it wants to start backfiring after it warms up. It starts by just making some faint popping noises from the exhaust; then if I rev the engine or go to a higher rpm than idle it makes a really loud popping noise from the exhaust. Could this be the engine running to rich and its not done burning the fuel on the exhaust stroke, or could the timing be off since I put this new carb on it. It had a 4412 on it before, so I would think it would be the same. I could be wrong. Any suggestions or tips would be appreciated!

Thanks,
Tex

 

18436572. Check your 5 7

 

Yes, firing order is correct. That was my first initial thought.

 

All the same ignition as before tare down? Headers? If yes, bad plug and fuel collecting then firing off?

 

Look at the jets in the old one and look up the jet size the new one is built with. See if those are the same. Also, make sure you don't have a vacuum leak.

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Vacuum leak especially at carb base to manifold. Also is the carb base gasket correct ? Any other vacuum lines or hoses that would have been disturbed in the replacement. Also is carb base tight where it bolts/screws to main body\bowl of the carb.

Good luck

 

I'm not the one who tore it down. My grandpa did 2 years ago he died and never got the new carb put on it. But yes ignition is the same as the old carburetor.

 

Shouldn't be any vacuum leaks. I double checked all that. All ports are plugged that are supposed to be plugged and the base gasket it correct (I compared it to the old one). It does seat tight on the intake.

 

Can having jets bigger or smaller jets than the old carb make it backfire like it does? (I'm not questioning your knowledge, I'm genuinely curious.)

 

Too rich or too lean can cause back firing... Usually lean more than rich.

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So if I stick my vacuum gauge on the port where the vacuum advance goes and re-adjust the mixture screws it might fix it?

 

Mixture screws will only effect idle. Check the jets and start there.

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check your ignition for a weak condenser .

 

Is the choke adjusted properly?

 

What else was torn down and put back by your grandpa BEFORE you got it.

 

He said he overhauled it and converted it to electronic ignition. I replaced the ignition module when I inherited it and first got it running because it wasn't getting spark.

 

Please excuse my ignorance, I'm no mechanic, but could someone explain how bigger/smaller jets make a difference on an engine? I know what they do, how do different sizes make a difference? (Not being a smartass, just wanting to know)

 

The jets control the maximum amount of fuel. The engine moves the air that it moves and you match the jet to stay in the optimal air fuel ratio.

 

Broken valve spring or flat lobe.

 

I don't think it's that since he used this as a daily driver for a while and I heard it the last time it ran. it ran really good, no clicks, knocks, or popping to be heard

 

It all started after I put this carburetor on.

Could it be that the engine was not yet all the way warmed up yet? It was like 50 degrees when I started it. I only let it run about 5 or 6 minutes when I did this because it was getting dark out and I'm in a shed with no electricity. But usually when it's cold engines backfire up through the carb and not through the exhaust, right?

 

There you go. Start there because that's the only change aside from tuning. Which is always done at full running temperature.

 

Thank you all for the replies, I really appreciate it! I'm learning step by step haha.

 

Was there an intake manifold change?

Quicky guessing help and hints.
If it's a lean miss, you can close the choke and it should go away. If it's a rich misfire giving it a vacuum leak will make it stop. the plugs would be black as well. Exhaust leaks close to the engine will cause popping in the exhaust.
Also remember that most carb issues are electronic in nature meaning ignition problems.

Double triple check the firing order.

 

Put the old carb back on it. If it runs good then, you know where to look.

 

Did it sit for 2 years after Grampa tore it down? And why did he want to change carbs? Was it backfiring then too? If so. I go with the cam lobe wiped idea. It is a small block Chevy, after all.......

 

Generally, backfiring through the carb is a lean condition or retarded timing. Backfiring through the exhaust is usually ignition issues. I would pull the plugs first and see if there is a problem there, check cap and wires also.
Bill

 

Does it have 2 or more year old gas in it? That can cause misfiring also.
Bill