New SBT motor backfiring...NOT RV

[marz831]

I recently purchased an 800 for my 97XP. The motor ran fine through break in and up to about 10-15 hours total. Now the motor will not idle and continuosly backfires. It is now at a local shop. They have no idea what is wrong with it. It is NOT the rotary valve. I checked it and the shop checked it again. I did rebuild my carbs before installing the motor. Like i said, it ran great through the first 10 hours or so.

[Bryan Glynn]

Is it backfiring through the exhaust or intake? Under what conditions? Did it start suddenly? Other that that, is it doing any thing funny? What happened to the original engine? Any mods on the ski?

[Bill O'Neal]

That is the symptom of a bad rectifyer, or water in the coil box..... Or, possibly needs an MPEM, or a ignition pickup, or maybe the magneto cup is coming loose from the front flywheel.
Eliminate the easier stuff first.

[John Kubiak]

Yeah, what Bill said!!!