I have a 2001 yamaha wr426 that won't start. It sat for about a year with fuel in it. I got it to start but it wouldn't idle and kept cutting out while I was riding it.

I pulled off the carb and cleaned it out. It still wouldn't start.

then, while I was kicking it, it started to lose compression. I could kick it thru without pulling the decomp lever.

I replaced the plug and changed the oil. The oil smelled like gas.

I put everything back together and the compression was back. I tried to start it but nothing, and the compression was gone again.

I let it sit came back and the compression was there but as soon as I started kicking it, it was gone again.

Any ideas?

First check the valve clearances. I am guessing you have a few tight valves there.

Then check the play on the decompression cable and check the routing.

after that, make sure that pilot jet is clean

Got it... I replaced the pilot jet, adjusted the pilot screw underneath and the idle screw. Idling great now. thanks