SM 125 back fires through carb/airbox

[Jackypops]

Engine rebuilt because was full with metal fragments something got in the cylinder and mashed up the head picture is after being cleaned up and skimmed. Shims were accidently mixed up ("FUCK") measured all shims 2.70, 2.65, 2.60, 2.55 (where would I start?) Piston cleaned and deburred ring was stiff so I sanded it down now fits and compresses in the groove just like the other 2. Holds 110 psi from the sparkplug hole. New intake manifold from grahams motorcycles (flange attaches to carb, rubber pipe from flange to intake) no airbox on the bike. The bike started for a few seconds at idle then died no life again just continuous backfire through the carb a loud popping noise. It has a bit of a spark problem, strong spark when plug removed then grounded to valve cover (green colour spark). A quick spark test using a spare plug then ground to valve cover leaving a plug screwed into the head. (White, less responsive). All wires, connections are fine no breaks. Forgot to mention side stand switch is bridged with one wire.

New wiring harness so no corroded old wires.
New relays

[DAVID THOMPSON]

there was a sticker under seat on mine had shim info on it...it was an rt125 from early 2002

[Jackypops]

Hi mate, that sticker doesn't exist on mine. The engine has lost a lot of compression only holds 25 psi now. The engine back fired through the carb and caused a small fire it was easily put out but not sure about the inside. Inside of the piston was burnt, the Conrod looked a bit dark like a naked flame had contact with it or something. I've measured the intake valve clearances won't fit anything bigger than 0.003