I recently had a Honda Tena engine 'rebuilt' - the guy put in a new piston and did some fiddling and filing, cleaned it up, etc. I'm not sure if this is a 'rebuild', but anyway it solved the problem of the bike being weak, and only cost about 980 baht.

I did notice that he filled up the gas tank with fuel - I think it was fuel in which he had added additional two-stroke oil, I'm guessing to assist with the break-in period.

So, since then the bike's been running great with an obvious increase in compression and power, but just today I noticed a few small backfires at different times, upon deceleration. My own guess is either the extra oil in the fuel or perhaps something he might have done wrong in the re-build has fouled the plug. I'll take it in tomorrow to find out, but I wonder if any of you armchair mechanics can have guess.

...had that problem years ago with a small 4 stroke ( Triumph Tiger Cub) on deceleration ....it was due to a leaking exhaust at the manifold....

on a two stroke could also be carburation/reed valve ( unburned fuel in exhaust.. burning), leaking crankcase ( part of 2 stroke fuel induction cycle)...

++++

..so much goes on in two stroke cycle ..process of elimination really..good luck