Idle speed slowly increases

Hmmm...

I have a 1977 Kawasaki KZ650B1. Carburetors are the old slide type with air screws on the side of the carb body.

Carbs have been rebuilt, bench shimmed and synched. Pod filters are used. Jetting: 105 mains, 15 pilots. Air screws are 1 3/4 turns out from seated (approximately).

Starts fine, runs really nice. But the idle slowly creeps up the first 10 or so minutes the bike is off choke. If I leave the driveway and she's idling at 1000 rpm at a stoplight at mile 5 down the road it's at 1200 rpm. So I twist the idle adjster back, then it sits at 1000 or so the rest of the trip.

What would cause that initial rise in speed? Too lean on the air screws?

I think that is just what happens when it fully warms up. The 650's are pretty cold blooded, so they take a while to warm up. If you adjust it from 1200 back down to 1000rpm and leave it there, the next time you start the bike cold it probably will seem to idle a bit low (like around 800-900rpm) until it warms all the way up, but you can leave the choke cracked a bit for the first couple minutes to compensate. Ed

You have air leaks in the intake, probably at the intake boots (carb holders)

The rising idle is the classic sign of air leaks.

What do your spark plugs look like?

The spark plug color is good. All four plugs have a nice, rich deep tan/light brown color.

I've tried the WD40 spray trick on the intake boots/carbs/all points in between and didn't get any results (engine rev climb or white smoke) so I don't think it is an air leak. Once I turned the idle back down a bit to 1000 after the first few rides it seems content to sit right at 1000 - 1100 rpm. A bit on the upper end of the scale per the manual but it seems happy there.

If it persists in jumping around I'll double-check the boots for air leaks again.

Bar, agree w/650, Your' probly there. period bikes took a while to warm. 650s were notorious for hard cold starting, and after firing required a minute or two before before idling w/out choke. usually they would pick up idle speed (on choke) after a minute or two, revs would pick up from minimal to @ 2 grand on choke.. as head reached operating temp. immediate riding would drop temp to iffy idle. sutained riding results in good operating temps. read your' plugs, use choke as little as possible. plugs which just show an indication of tan are optimum. smooth transitions and good seat of the pants are a good indication of proper jetting, luck g