racing idle only when hot

[nickmo]

I did the search and came up with some remedies for a racing idle, but I didn't see any specific to the problem I'm having. If I missed it please point me in the right direction.

I took my little project out for its first real ride, about 100 miles total, and everything went pretty great. The only problem I had was: after the bike gets nice and hot the idle races. It performs well under acceleration and while crusing, and doesn't race from a cold or nearly cold start. It only races at normal operating temps. Basically everything was fine while I was in motion but if I stopped for gas or at a light the idle would go up as soon as the clutch came in.

The setup is a 79 with points and 38s (stock jetting), open exhaust w torque cones, stock charging and firing system.

Any help would be appreciated.
Thanks

 

[fancyketchup]

Definitely a vacuum leak its very common to have this issue when the engine is hot. Check intake boots/vacuum barbs if you have them for cracking and spray some carb cleaner around anything that can leak vacuum and listen for a change in idle.

 

[nickmo]

Thank you. I'll check that out. I have a question that crossed my mind when you said vacuum barb. The barbs on my carb holders are tied together with a piece of tubing. Basically someone ran a piece of vacuum tube from one barb to the other. Should they be capped individually or is that acceptable?

I don't mean to pin you down, I've noticed it before but never thought to ask.

 

[I am Carbon]

the barbs being conected is ok. thats is done to make it idle a little smoother.a vacume leak is most likely. I had the same problem for a couple days and it was that i simply had the idle up to high. it ran very well until it got to running temp and it raced and made shifting harder and so forth

 

[yamaman]

could be your ATU if you have never serviced this unit, then it's time you did. lots of posts on how 2 do this here. could well be your problem. cheers

 

[nickmo]

It was advancing normally when I set the timing a while back. I will revisit that possibility if it turns out not to be a vacuum leak. I hadn't thought of that though, I appreciate it.

 

[yamaman]

just another option is all. the concern is not the ATU advancing. it is the ATU not retarding as quick as it should when 100 miles hot. the ATU shaft should turn freely, maybe if it is drx, it is nipping up a bit when it gets hot. cheers