Inconsistent Idle


XS650 Enthusiast
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Hi all, I will be working on the bike in winter, I have a new coil and plug leads, and I will pull the carbs.

I took it out yesterday for a 50-mile ride and in the beginning after a difficult cold start the idle was 1000 to 1500 RPM but by the end of the ride it was racing at 3000 RPM, it's also backfiring.

1981 XS650 Special 2 SH, no modifications


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I imagine that the carb guys will check in, but, chances are the stock EPA jetting is a touch lean and your carb set needs a sync adjustment. There is the possibility that you have an air leak also. Check carb boots and vacuum lines to the petcocks.
Verify you ignition timing is correct or 2 or so degrees retarded.
Sounds like your carbs need a good going through and cleaning, maybe some new o-rings as well. The hard cold starting may be due to the choke feed jets being plugged or partially plugged. The chokes won't work if they are and that makes cold starting difficult. The choke feed jet is located off to the side at the bottom of the bowl. If there's gunk in the bowl, it will get sucked in and plug the jet when you put the choke on .....

Choke Jet.jpg

The jet is pressed in place so can't be removed for cleaning. You need to flush and blow it clear with carb cleaner and compressed air. The area below the jet can also get packed with gunk so you may need to dig that out. To check if the jet is clean and clear, do this .....


There is a tiny o-ring on the bottom of the mix screw that seals it in it's hole. If it's gone bad, the bike won't idle right .....

Mix Screw 80'-on.jpg

An old original is pictured here on the top screw. It's all flattened out and deformed, and needs replacing .....


From the factory, the mix screw hole is capped over with a metal plug .....


You'll need to remove that to get the mix screw out, and you must remove the mix screw and it's various parts (spring, washer, o-ring) to do a proper carb cleaning.