I recently had to work on a friend's 1250. took the carb off to put Heli coils in the casting where the back of the air filter bolts to, had to Heli coil 2 of the 3 holes. Of course was out of Helicoils, so had to wait for Fastenal to get some in. so I temporarily put a #30 from a K341 onto his K301 so he could use it in the meantime. . It adjusted out ok, ran great, had good power but backfired on shutdown every time. whether from full throttle or from idle, if it had been allowed to run for more than 30 seconds. Only ever did this on shut down not while under load.
A week later I put the original #26 back on after I got the Helicoils, and no more backfire.
Now that said, I had an M12 on a Woods ZTR once that came with a #30 (IDK if it was original or a PO mod) and it ran great and never backfired on shutdown.... weird.
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