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I have an old XJ650 and as soon as it worms up it puffs and backfires. Also when it's been running hot it will hesitate when starting in first gear, but stops doing that when I give it more gas. It backfires more when hot.
What do I need to look at.
Thanks
BR

Hey -

I had the same problem in my xj650 - it was the idle circuit in the carbs.

The biggest problem was with the Idle mix screws - there's a little fuel passage from the pilot jet to the intake. About halfway up into it, there's an air passage that intersects w/ the fuel to aerate it. This mix is controlled by a tiny screw - the idle mix screw. You have to take them out and clean this passage. It requires either a soak and compressed air, or lots of time w/ a spray can of carb cleaner.

Notes -
-After you do this you will have to resynch the carbs
-Idle mix screws are FRAGILE. If you bottom them out and keep going, they WILL strip. You need to be very sensitive to any resistance.
-That being said, you need to bottom out the screws, and count how many turns it takes to bottom out, so you know how to replace them.
-put the same screw in the carb you took it out of.

Thanks for the info. I thought it might have something to do with the carbs.