Revo. Almost new 3.3 engine. Running Byrons 30%
I recently put truggy tires on it. It was only shifting into 2nd occasionally. I put a larger spur gear on it. Wasn't shifting at all.
Glow plug died, I replaced. Went on to burn up 2 more plugs. I chalked this up to running too lean. The piston and button show imbeded bits of filament.

I went back to stock settings on carb and have adjusted every which way since. The engine will only start up at full throttle and will cut out as soon as I release throttle.

I have tried an extra shim. With and without, no change.
What can I do?

The glow plug filaments probably toasted your engine. Disassemble the engine and check for damage to cylinder/sleeve. Maybe you could get it pinched, but only if the piston/sleeve is not too bad. Rebuilding is an option, probably easier just to replace it.

Time to use the traxxas exchange program.

this a easy one .....if piston & sleeve have no scratches change the button head & clutch shoes and the engine should be fine from...i what i read...2 shims should be good

I put new shoes and clutch bearing in. Needed them bad! Fired up fairly easily and ran. Still wasn't shifting. After some "tuning" it kinda stopped running again. Still havent put my other head on it.

Man, right when I was really starting to get back into my Revo, she craps out.