[gs1327]

Wizards of the twins I am requesting your input--
I am working on an 81 CM400C. The bike runs well, no smoke, 6200 miles, starts right up, charges, drives good, BUT
the PO had carb issues, It will idle normally sometimes, but if you give it a little throttle it goes up to about 3000 rpm and stays there.Usually. The PO cleaned the carbs to no avail. I took them apart and cleaned them out myself, no difference. I tried spraying a little ether around the carbs and intake boots when it was idling, no change. Compression reads 155 both sides.
Any suggestions?

 

[tbpmusic]

It will idle normally sometimes, but if you give it a little throttle it goes up to about 3000 rpm and stays there.Usually.

That usually means your carbs need to be synched up - you do have vacuum taps on that bike.

 

[66Sprint]

On the CM/CB 400's I would first suspect a problem with the throttle cable adjustment or routing.....There must be a little slack in the "pull" cable (preferably in both) but there MUST be some on the pull side..... The logical thing to do would be to have zero error, and tighten both cables, but it won't work that way.... Very counterintuitive.... Another possibility is the carbs are not perfectly synchronised....Same symptoms.....

 

[66Sprint]

Darn it Bill, you posted while I was still typing...AGAIN....LOL.... At least we agree on possibilities....LMAO!... Steve

 

[gs1327]

Thanks for the fast replies. I've got them synchronized real well, I even tried offsetting them a little bit either side of perfect to no avail. The cables work smooth and when the idle "runs away" I have verified that the throttle is all the way tight on the idle stop screw, the push/pull works fine. Is there an advancer on this thing? It's funny, if I put the bike in gear and ease out the clutch so the rpms get below 1500, it settles down. If I blip the throttle, no problem unless the RPMs get above 4000, then it stays there.

 

[66Sprint]

The advance on this bike is electronic (in the "brain" box)... However, there is a "fast-idle" lever that must be in adjustment as well as the "idle" screw........ Fast-idle on these bikes is normally 2000 to 3000 RPM..... The lever should be set with the idle screw backed off until the butterflys close, and then set so it has a 1.2mm (.047 " ) gap between it and the drum....... Maybe yours is "hanging-up", or set too close, bent, whatever........
Steve

 

[tbpmusic]

It's CDI ingnition, don't think it has a mechanical advancer........
I still think it's the carbs.

 

[66Sprint]

Hey Bill ......Wanna race?...LOL .......Yep, I agree it's in the carbs......I'm thinking the "fast-idle" lever is stuck or in play when it shouldn't be........

 

[gs1327]

The advance on this bike is electronic (in the "brain" box)... However, there is a "fast-idle" lever that must be in adjustment as well as the "idle" screw........ Fast-idle on these bikes is normally 2000 to 3000 RPM..... The lever should be set with the idle screw backed off until the butterflys close, and then set so it has a 1.2mm (.047 " ) gap between it and the drum....... Maybe yours is "hanging-up", or set too close, bent, whatever........
Steve

Thanks, gentlemen. I'll look in to the fast idle lever doohickey.
If the advance is built into the CDI I guess that cant be hanging up.
I can hardly see in there what the linkage looks like- I'll pull em off again and take ANOTHER look.
Thanks again.

 

[tbpmusic]

Well, basically they took a really good carb (the 450 carb) and screwed it up by adding a bunch of useless crap......

 

[gs1327]

Thanks for all your help. As it turns out, the right carb wouldn't run below 2500rpm, so the idle speed screw was turned in far enough so it would idle on 1 cylinder. When both cylinders would run, it would idle WAY up as a result. Took the right carb apart and found one passage from the air cutoff(what the heck is THAT for?) was blocked. the carbs had been cleaned by a local dealer, the previous owner, then by myself finally, and we all missed that one passage-fuel was not getting to the idle mixture screw. I appreciate the assist. Now it runs good, it's just ugly.

 

[66Sprint]

Tom...The air-cut valve is another of those useless "improvements" made to a good basic design... However, since it's there, it has to be working properly for the carb to work correctly....... Glad you got it figured out......

 

[tbpmusic]

I think it's an anti-backfire device - response to US safety regulations.