96 XP backfiring

[Mraw]

Hey all, helping my friend with his 96 xp. New engine is in. Ski backfires through carbs, he is 100% sure the rotery valve is set correctly and says he has checked it multiple times. After running on the trailer for 5 or so seconds one spark plug looks normal and the other is completely coated. Carbs were rebuilt with genuine mikini. Pop off is great. Will pressure test carbs soon. Any other thoughts?

 

[Ckrawiec]

FWIW....google cut and paste

What is the main cause of engine backfire?
Common causes of backfire are running rich (too much fuel going into cylinders) or faulty ignition, possibly a fouled (dirty) spark plug, coil, or plug wire. A "pop-back" (inlet manifold, carburetor/throttle body, or air cleaner) of an internal combustion engine can occur when the valves are not shutting correctly.

Good luck with it.......

 

[mikidymac]

Typically only thing that will cause a backfire out of the carbs is a stuck rotary valve or incorrectly timed rotary valve or an ignition issue like a spark at the wrong time.
If it is leaking fuel it will backfire out the exhaust.

Try unplugging the red wire on the rectifier before messing wit anything else.

 

[Mraw]

Thanks, tested with red wire disconected yesterday. Going to double check rotery valve today. What could cause a ignition issue?

 

[claysimo]

I had my 96 backfire out the carbs recently as well. Took the engine out to get to the bottom of it. It looks like my rotary valve lip seal was worn out a bit and leaking oil at the small place where the circular pop clip doesn't cover the seal. Hard to explain but I will put a pic to show if needed. Crazy enough it would only backfire when I gave it some gas. It was like when the compression got up there it would then push out some oil into the RV through that tiny spot. I'm hoping there are no other seals in the bottom end that have gone bad. Was fouling plugs left and right. Didn't help that I accidentally dumped oil into the engine when the oil pump line got disconnected. Went ahead and rebuilt the top end and got out all the old oil. Hoping to test this weekend and see if I've finally fixed the backfiring. I say all this to say that you should take a close look at the rubber seal around your RV gear once you get that cover off.

 

[Mraw]

Any ideas what with the ignition could be causing it?

 

[Mraw]

Had some time to work on it today, put in another black rev limiter box, red wire disconnected, no change. Going to check timing once again aswell as a new plate but I don't think that's the issue. Ski idles fine Any other ideas? Trigger coil? Coils?