backfiring problem

[NASAphotog]

My 4 stroke 49.5cc manual trans backfires when I release the throttle. I'll shift normally through to 4th gear, then when I need to slow down, I release or back off the throttle and WHACK! a nice big fat pop. No other backfiring except when I back off the throttle. The only way to avoid it seems to be if I engage the clutch immediately as I'm backing off the throttle. I've played with the mixture a bit, but that doesn't seem to help. Any advice???
Gary

 

[Eoin70]

have u changed any things?
for example an after market exhaust?

 

[NASAphotog]

Nope, everything is stock. I have 3700 miles on it. I'm not much of a mechanic, but I can find my way around the bike. It's a baby-softail, looks like Harley.

 

[pr0ve]

Is the sparkplug old?

 

[NASAphotog]

No, checked that. It is the stock one though. I guess it couldn't hurt to throw a new on in there.

 

[Big B NRG]

Maybe your ignition slipped off it's place, so that you have a different ignition timing.