Re: Backfire - Lean/Rich or timing? (archive)

Wow it seems that I kind of disagree with everyone on this one, hmmm. Isn't backfire caused by unburned gas getting into your exhaust system and exploding? If your running too rich, some of the gas is unburned when the exhaust valve is opened, it then hits the hot exhaust manifold and explodes causing backfire. The other option is retarded timing. The mixture is fine, but the timing is so late that there isn't enough time to burn all of the mixture and, again, it explodes in the exhaust manifold. I guess you could make a case for the lean condition as well: a lean mixture might burn so slowly that it doesn't get all burned up before the valve opens...To make matters worse all of this exploding in the exhaust manifold causes the manifold to heat up and causes even trace amounts of unburned gas to explode.

I also speak from direct experience on burping carbs. When the engine is cold and my choke is not pulled out (argh) I get those fun explosions from under the hood. The choke makes it run rich, so a lean condition is causing the carb burps.

You really have to have the mixture in the ballpark to avoid backfires, but I'm guessing that slightly richer is better than slightly lean. Lean is really not too good for the engine and there's quite a bit of leyway on the rich side. If it isn't knocking, by all means, advance it until it just knocks and then backoff a bit. You may have to live with the rough idle from too much advance or increase the idle speed.

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