Running out of ideas 2007 KX250F

Im working on a friends 2007 KX250F and Im running out of ideas. The bike has never run right since he bought it last year. it has always been hard to start especially when cold and never wanted to idle at lower rpms. It slowly got worse and it will hardly start now. The most it will do is pop and backfire or kick sometimes. If it does start it will run with the choke and die shortly after turning the choke off.

Heres what ive tried:

- Checked valve clearances, all within spec

-Checked cam timing, All good

- Dissassembled the carb and cleaned from top to bottom

- all stock jetting except for a 45 pilot

- Accelerator pump screw was tightened up too much so i reset it using the gauge I got with my QS2 (0.100 in from accuator arm to screw mount)

- Ive tried the fuel screw at different settings time and time again

- Ive tried going back to a new stock pilot jet (40)

- Ive tried moving the needle up and down with no luck.

Anything Ive tried doesnt seem to make any difference, can anyone think of anything Im missing or havent tried, would something like a head gasket do this?

Thanks in advance

Check your flywheel key it may be sheared. If the key is broken then the motor won't spark at the right time causing your backfire.

You could always make sure your valves arnt bent. I think it might be something to do with the pistom. Theese things eat top ends around every 50 hours or so if you dont have a high comp. You could check that and the sparkplug, and if its backfiring maybe the air filter is uber dirty? But it sounds like your top ends going out.

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