Engine speeds up after warm-up

[Easter]

My son and I are having similar issues with the engines refusing to idle down after being ridden for a while or getting completely warmed up. His is an '81 GL1100 and mine is with an '81 CB750, both with similar keihn carbs, so thought to be the same cause.

Checked for (and found) vacuum leak around intake rubber but after curing the leak, still have the problem on the 750.

Both bikes will usually eventually idle down if given some resistance from a combination of brake and clutch.

Any suggestions of other causes?

[mike in idaho]

Check the spark advance unit, it needs to retard at idle to get the rpms to drop. Lube the moving parts and check the springs.

[gltriker]

Have the carburetors been synchronized?

[BlueThunder]

Did you set idle speed after the motor warmed up? If you didn't, the idle will be too high upon warm up.

[Whiskerfish]

Incorrect play in the throttle cable or incorrect routing/ binding of the throttle cable is the # 2 cause I have found behind vacuum leaks. If the throttle does not have the 1/8th- 1/4 inch of slop as it warms up a lot of times it can keep enough tension on the bellcrank to keep it above idle.

[Easter]

Thanks all. Will try to work through these suggestions as time allows. Whiskerfish, I know my cable is tight and I will loosen. My son's seems to have sufficient play so will have to look elsewhere in his case. Special Motion Pro tool required to sync the carbs on the CB is on order, but vacuum seemed fairly close. Haven't had a chance to sync my son's GL1100 but sync only effects the low idle circuit anyway and once above 1200 or so isn't it true that sync is irrelevant. I am talking about 2500rpm or so once the engine is good and hot.

I am also trying to get Black Beauty ride-able if not pretty and my '75 (Nightmare) has a couple of weird issues as well. Lots to keep up with.

[Rat]

Most all of my old hondas have idled faster at operating temp than when cold ..... I just set the idle at operating temp and deal with a short warm up period when the idle is a low .....

Gord

[Easter]

Haven't worked on Son's bike but now that "Black Beauty" is running almost good, I had a chance to work on the CB 750 this weekend.
Synced the carbs, adjusted the choke and throttle cable lengths, located a vacuum leak in the number two runner (and fixed) readjusted the idle control and it is running great. Still a little slow to idle down but then maybe it is just different than the Wings since it is such a high reving engine. It has been so long since I road any thing else that it still feels odd to me.

Now another problem has resurfaced. Since I haven't ridden this bike for some time, I had almost forgotten that the last time I took it out, it left me stranded about a mile from home due to a dead battery. This while riding, not stopped. Same thing happened today on the test ride, fortunately it quit just as I pulled in the drive. Haven't had time to do any trouble shooting yet, but know it is a good battery as I have been starting it off and on for months without the need to recharge.

Anyway, thanks to all for the suggestions regarding run-away idle. I think it was just a combination of all those factors.

[rnelson]

What method did you use to find a vacuum leak? I've tried running propane all around the engine and intake system and never have an increase in rpms. I've done the same thing with a water sprayer to see if rpms decrease. Never have anything happen. I'm wondering if there's a better way.

[BlueThunder]

What method did you use to find a vacuum leak? I've tried running propane all around the engine and intake system and never have an increase in rpms. I've done the same thing with a water sprayer to see if rpms decrease. Never have anything happen. I'm wondering if there's a better way.

I use a very strong manifier glass and inoesct every part of the rubbers off the bike. Any imperfection the part is replaced. Adopting this method has eliminated vacuum leaks 1005 in all my bikes. You can do it on the bike but a strong light is needed.

[Easter]

In this case I sprayed the intake rubbers with carb cleaner. The small tube allows you to direct the flow. It was the number 2 carb and depending on where I sprayed I had no results but if I sprayed in just the right spot it would accelerate. Runners on these old Inline fours are very difficult to get on and off so I was lucky to be able to loosen the clamp and then re-tighten after the engine was hot.

[rnelson]

I'll try it again with carb cleaner. How about the air box to the plenum seal. Is that a typical spot for these old GW's to leak? I don't know how much that would influence the motor since that's upstream of the carb anyway.

[Easter]

I am not sure about the plenum, like you said it is upstream of the carbs and even running without the air filter doesn't seem to have a major effect while idling.

[Lucien Harpress]

I don't think the airbox to plenum seal matters as much as the intake runner boots do. Worst case is that you get a extra bit of unfiltered air into the carbs. The carburetors do their thing to mix the correct air-to-fuel ratio regardless of the amount of air run into them (as it's their job), then send it down to the cylinders through the runners. If you get a bit of extra air THERE, it messes up the stoichiometric ratio set by the carbs, and you run into issues.