Pretty nasty backfiring.

Ever since I replaced the headers and changed the sparkplugs, my '53 M38-A1 with a Buick 3.8 has a nasty backfire. Yesterday was the first time I had started it in about a month. (It has only been about 15 degrees for a week here). It backfires worse until it warms up a little, then you hear a "pop" about every 4 or 5 seconds. It doesn't backfire much at all when you're on the throttle however. Only when you're ideling or letting off.

I've been told to try turning the distributor cap a little to see if that helps. I've also been told it could be the plugs I put in. Bosch Platiums. It had some autolites in it before. I just thought that it would never be easier to change the plugs then when the headers were off. It didn't backfire BEFORE the new headers/exhaust though. At least I couldn't hear it if it did.

So, would you guys think it's plugs, or the dist cap maybe?

The plugs are all gapped to what it was specified.

Dondo

 

Backfire through the exhaust or through the carburetor? If it's through the exhaust, it means you have fuel coming out the cylinder and being ignited in the exhaust. I'd think the timing and idle mixture is most likely ... try a little more advance and leaning out the idle.

 

Plug wires are in the right order.. That was one of the first things I thought of.

It is backfiring though the exhaust. I did replace the fuel line from the pump to the carb. It was a crappy homemade metal tube (started leaking around the couplings), so I replaced the section of line with rubber fuel hose. It might have been a little bigger diameter and I'm getting too much gas into the carb. Didn't think about that. Think that might be the problem?

 

Hmmm new headers and backfiring, sounds like a jetting issue to me... 02

 

No, the carb float meters the fuel from the pump. The float only lets in as much fuel as is needed to replenish the demand.

 

$ sink -- What is "jetting" and how can it be resolved? Does backfiring actually hurt anything in the engine?

 

Tim
Just my experience dealing with bike engines and headers: headers tend to lean out the fuel/air mixture the engine is getting (better flow in and out of the engine) most header installs are accompanied with slightly larger jets. This is not to say that you need to rejet, but leakage into your carb/manifold system through worn throttle shaft, vaccum hose cracks, poorly sealed gaskets, whatever, may be compounded by the addition of headers. a good rebuild of the carb and examination of the hoses/gaskets, with attention to any possible sources of leakage would not be a bad use of your time. once again only my .02 Randy

 

Ah yes, Randy, I had heard that headers will lean out the mixture. I wonder whether that's really a lean condition, or actually less exhaust gas dilution. For example, if you remove an EGR valve, the combustion chamber tempurature will increase. The EGR dilutes the fuel-air charge with exhaust gas, lowering combustion temperature and reducing NOx emissions. After EGR valve removal, the engine will require a richer mixture to get back to the same combustion temperatures as before. Same thing should happen if you more efficiently remove exhaust gases from the cylinder, either through less back pressure or pressure-wave extraction.

I said idle mixture because the popping happens on decelleration. It's an easy test - just turn both idle screws in a 1/4 turn or so. Actually, I'd see what the number of turns on the idle screws is, and then adjust to best-lean-idle, and compare.

Another possibility is that the headers are leaky, mixing air with the exhaust gases. The hot exhaust gas will ignite and pop if the mixture is rich enough.

Pretty sure this is a side effect of the header installation though, combined with some existing condition, like you mentioned.

 

The popping is just annoying - won't really hurt the engine. My VW burbles when it's cold, and I'm decellerating (like on the downhill parking garage ramp). If it comes from an exhaust manifold (or header flange) leak, that's bad though. An exhaust leak will eventually cause a burnt exhaust valve.

 

Backfiring is a bad thing, It will cause such things as a stretched timing set, damage to small circuit components in the carb. I dont think freeing up the exhaust a little would cause this by itself. I do agree that the headers will lean it out a little, but not to the extent to make a perfectly good running motor start backfiring.I would start with the obvious, check firing order on the cylinder end and the distribuitor end of the plug wires. If everything is O.K. Then start checking timing and proper plug specs. Let us know what you find.

 

I'm betting that the firing order is still wrong...........engine ran fine before header and spark plug change.........check firing order again.

JB

 

Well, alright you convinced me. I'll check the order. Would be great if it was something that simple. Of course, I don't know what the order is supposed to be. I'm not even sure what year the engine is. I know it is at least a 1978. It was put in in 1995, I think. Is there an easy way to tell what year an engine is? I know it's a Buick 3.8, and that is about it.