The International CBX Owners Association

That's interesting. I too have been setting my mixture screws no more than two turns out the last few years. As I mentioned elsewhere, the factory set them around one turn out, give or take.

Dave

Just a wild thought on that: could be with the very open idle jet screw setting that when snapping back to idle the quick high vacuum sucks an extra shot of fuel to be burned off. Would think that might cause some other reaction. But, ???? I run 1 1/2 turns with no issues and good idle. (82 stock)

Hi Andrew --

You have received several responses by now so have probably resolved this, but reading through the posts I didn't see a real common cause for this: A slight intake leak between one or more of the carbs and their boot. This will cause the exact problems you describe, revs and runs fine, but won't settle down to idle right away, she tends to hold a few K of revs before settling down to a normal idle speed.

Troubleshooting this is fairly easy except for the usual access issues to the inside pair of carbs. You'll need an aerosol can of WD 40 with that red 6" tube. Let the bike idle at normal speed, and slowly and carefully try to put a light bead (spray) between the carb and the boot. Do one carb at a time and make sure you get the full circumference. Do about 1/4 of the pie at a time on each carb since it can't really be done in a single circle. When you hit the air leak the motor will rev. You can wait a few minutes and repeat the process until you have identified the exact location, usually/hopefully it is only one carb with the problem.

Boots dry out and get brittle (although after a rebuild yours are probably new). It is also very easy to slightly mis-align the factory hoseclamps on the inside carbs, they can look positioned properly on the bottom and still have slipped a bit on the top enough to cause a pinch and a slight air leak.

Old dirt bike trick I used often (especially with older boots that can be real stubborn about letting you put a bank of six carbs back in) was to lightly coat the inside of the rubber intake boots with axle grease. Not only did carbs slip in much easier, instead of getting frequent intake leaks I never had the problem after using this trick. Horror stories about fouling plugs and/or the aesthetics of grease on the intake track weren't endorsed by many but sure worked for me many, many times.

Hope this helps one of our fellow X'ers,

Jon

Sorry I missed your last comment Dave. I haven't verified the advancer with a strobe light. it certainly snaps closed fast enough.
I'm currently in the process of increasing the valve clearances after Mike Nixon suggested that my low compression numbers are causing problems.
I've also run into a mis-fire above 7k that feels ignition related. It's certainly trying my patience

Jon,

Thanks for your reply too. It is definitely not an air leak at the carb boots. They are new, and I have used propane to check for leaks. I cannot being myself to spray WD-40 anywhere near the carbs since they are pristine :)

I had Preston sync the carbs and he was of the opinion its a problem with these old carbs, even though Mike Nixon totally rebuilt them.

As I mentioned, I am changing the valve clearances, making them all bigger. They were all in factory spec but the theory is that these old "soft" cylinder heads cause poor compression and this has a knock-on effect on all sorts of things.

Larry,

So far I've had my idle jets anywhere from 1 1/4 to 2 1/12 turns. Makes not one iota of difference to this issue unfortunately.

Just some more 'guessing': throttle cable tension? Easy and quick check. Cable 'pinch'? Had a somewhat similar problem, recently, with my old Suzuki. So happened that I had pinched a throttle cable when I reinstalled the fuel tank. Found the problem when I removed the tank to work on the carbs. Fortunately, just relocated the cable with a tie-wrap, reinstalled the tank and all was well. But, check your cable slack. Need some.

Funny but painful. I love the bike. Had it for 15 years. Had to park it a couple of times, family and work. After each park pulled and cleaned carbs. Always started and ran well. Last week sounded rough above 6000rpm and did carb sync. Now I've got the fast idle problem. Pulled carbs and did bench sync. Full choke and motor fires up instantly (seriously) every time but can't adjust idle (2500). Checked cable slack etc and looks good. Runs great but idle isn't adjustable. My idle mixture screws are set at 2/ 1/2 turns and I'm thinking this might be the problem but wanted to check with you guys. I've dropped the engine about 10 times now (sigh) but on the road the the bike runs well and is a blast to ride. Bought a 1981 CB900F Super Sport last Saturday. On the road tomorrow.
Jerry

After all this, it's beginning to sound to me like something is amiss with the choke fast idle 'cam'. I've done that job about 4 years ago. Don't remember, now, exactly how to adjust it. I'm thinking it will require carb removal. Hopefully, Dave will get into this one with some better words.
P.S. I'd back off those idle jet screws to 1 1/2 - 1 3/4 max.

Yes, that adjustment requires carb removal. Here is what was sent to me by Mike Nixon when i asked a while back.

"Choke relaxed/butterflies open and throttle closed/butterflies closed and idle knob completely backed out until it no longer touches anything. Suggested gap of 0.5mm"


Sasha

I never had a high idle problem until I tried syncing the carbs. So I pulled them off and out of curiosity shone a light through from the intake side and could clearly see gaps at the bottom of the butterflies. I loosened all the sync lock nuts and adjusted them until there was almost no light visible at the bottom of them. I had to go back and forth a couple of times because if one butterfly touched too soon it held the others up. I reinstalled the carbs and the fast idle was gone. Whether this was my inability to do a proper sync or not I'm not sure. ?? The bike fired right up and ran fine on a test ride.
Jerry

One little butterfly hang-up is all it takes. If it is now idling smooth as a kitty-kat purr at 800 or 900, you're all set. Hope you get one more 'run' this year. Enjoy! Sounds like another happy ending. :)

Just saw this post so you may have fixed it already but my 79 had this problem after rebuilding my carbs and after making sure everything else was spot on and the hanging idle can be a ‘lean’ running problem after speaking to a carb ‘guru’ he suggested turning the fuel mixture screws out another turn until I had no more high idle or slow to return to idle problem. I set them at 2-1/2 turns at rebuild and went another turn to adjust the idle down. Problem fixed.

Glad that worked for you, mostly it does not. When all is right, these things like no more than 2 turns out.

Dave

One other suggestion to always check -- cables. Including the free-play adjustment. Try to take out too much free-play and you get just enough drag to slow the return to idle. Don't forget the return cable.