Engines that backfire - what do you do?

[Go Go Diablo]

Hi all,

My 502 with the Hirth 300 in it backfires. I've adjusting the timing and it's better, but still pops badly. Any suggestions?

Bruce

[cdnprairie]

Hi Bruce,

I am certainly not the go to person with respect to carbs. However I was on the ACSCC web site the other day and on thier garage page a guy was asking a similar questions with respect to backfiring and his tilly carb. It was suggested a lean condition as the culprit. That really throws a screw into things eh. Lean?...Rich?...Timing?....cause one to pull his hair out wouldn't it.

We know a lean condition doesn't always mean a carb problem......pressure test the motor....bad seal maybe??? Not much help but throw it out there.

Ken

[Go Go Diablo]

Good thoughts guys. This motor was completely apart with all new seals and gaskets. I was certainly thinking ignition of some sort, but I could be way off. It starts and can rev up, but then pow - up through the carb. Most often keeps running, but certainly not right.

I'll keep trying.

[cdnprairie]

Bruce,

Seeing that the motor was apart and all new seals etc, and if you don't believe it to be necessarily a carb issue....then what about a bad spark plug wire or plug wire cap breaking down?

Ken

[Barrie2777]

My guess is backfiring means gas igniting in the exhaust stroke, therefore the gas is probably too rich. I can't agree that a lean mix would do this. Sometimes when a engine is first started in a machine, excess gas is in the cylinder and motor base from us trying to get the initial start.

[Go Go Diablo]

Ok - more good thoughts. Thanks a bunch.

Spark plug wire and cap are new. I'll fiddle more with carburation and see what happens. It sure does seem rich cause when it backfires, it spits fuel out the carb in a volcano type shot. Damn, again with the carb issues.

Bruce

[Barrie2777]

The HUS-SKI with the huge windshield was in my shop as I prepared it to ride. I would start it everyday to test its starting ability. It would start on the first pull then I would shut it off. This was done mnay times. Eventually I determined it was a reliable starter and continued with the mechanical parts. When I was ready to take itoutside, it would not start. It turned out, (as I figure it) all the short run times did not completely burn off the gas resulting in a buildup of gas in the cylinder. The spark plug was soaked when I removed it. Through five cylces of removing the plug, heating it up, reinstalling, starting, running until it stalled. every time the plug was removed it was soaked. Now the machine runs and starts fine with a nice brown burn on the plug.

[cdrsavage]

try changing the capacitor on the ignition assembly. Its job is to keep the points from arcing. If it fails for some reason, the engine will backfire like crazy. My Diablo quit starting and would only run for a short time and backfire a lot. I changed the plug and that took care of that.

[Go Go Diablo]

You hit it on the head Ken. I found out this past weekend that the plug was slightly fouled and that was causing the backfiring. Change the plug, ran real good. I figured that if the engine was running no matter how good or bad, the plug must be firing. Guess not.

Good to go. Thanks everyone for your suggestions.