Hi Jim,

Thats a good idea idea re the muffler...

I will have to check but I think the engine came new and complete from Rotax and so the muffler was new as well. Also strangely it didn't backfire when new until the first set of sparks plugs were changed at about 25 hours.

Strangely again, the first set of plugs removed look a bit strange and different from the regular plugs. They had to sets of electrodes facing each other, which is different from the regular Denso or NGK plugs normally used. I will try and find the part number.

If it is the old muffler that was reused, is there a way to clean it up or get the deposits out?

Any other ideas regarding the plugs and why after changing the plugs to the recommended ones, the backfiring would start?

Thanks