Backfiring

[woodsguy2000]

I have a friend with a 250 4stroke. My question is it runs find until he stops for a long break and then tries to restart. It then backfires hard enough to knock the carb out of the intake boot. Any ideas would be appreciated. The bike was bought used and he has only had a week, so don't know how long the problem has existed. Also it is a Yamaha. Thanks Rob

 

[HiG4s]

My son's XR100 has been acting real stange for about a month now and backfiring was one of the symptoms, but it didn't do it all the time. Sometimes it wouldn't run at all. It seems the pilot jet was partial clogged as I replaced it this morning and it seems to be running better than ever with no back fires at all now. Not sure if this pertains but I would check.