11 HP Robins Engine issue

[drs1023]

I rebuilt my 350 cc engine (2002 workhorse) a couple of years ago. Bored. 020" and new pistons and new carb. It has NEVER stopped backfiring. Sounds like the timing is low and winds all day long with the choke to start every time. I tried moving the timing belt both directions and no help. 130+ Psi  compression. Good spark,??? 
All replies appreciated 
 

[Michael Eddie]

Hello drs1023,

That could be the dreaded robin valve lash predicament. To alleviate the backfiring, take the time to (precisely, time consumingly) adjust the valve lash until its unique feel good setting is found. Some people find this easier than others. Bewarned, adjusting the valve lash on an aged engine leads to sudden valve seal failure.

[rojo]

Could also be a burned or warped valve by not checking the valve lash until it started popping.