Backfiring and no power

[Director59]

Hi All!. I'm new to this forum and need a little guidance. I am a retired auto, truck and small engine mechanic and I am trying to fix my daughters boyfriends Suzuki '99 GZ250.

First of all this bike seems to be a Heinz 57 of a GZ250 as it does not have all the features the shop manual says it should have, i.e. missing a second throttle cable, electrical components missing and it looks like some spliced most of the wiring together.

The bike was leaking oil from the head gasket, so I purchased a rebuild kit and replaced the cylinder to block gasket, head gasket, cylinder head cover gasket, etc. Also replaced the spark plug. The bike worked fine for a while then it started to backfire. re-checked valve clearance and still had the same issue. Carb seemed to be dumping to much fuel, rebuilt carb, but still have the same issue. If this was a car or truck, I would check timing, but shop manual has no info for that.

The bike starts fine and runs great for about 5 mins and then wants to stall and if you give it throttle it backfires and has no power.

Can anyone point me in the right direction?

Thanks in advance,

Frank