When the bike warms up, the engine revs really high

[MikeC23]

Any ideas are appreciated. The bike normally runs great. However, today out of the blue, I noticed that when I'm in neutral or squeeze the clutch after the bike has got good and warm, the RPM's seem to stick around 5K. The throttle cable isn't stuck, no air leaks, the throttle itself isn't stuck. I quite confused.

 

[pamcopete]

MikeC23,

That is referred to as "idle hover" and there are three common causes, except I can't tell you what they are because I don't know what year / model XS650 you have or a history of recent work on the engine.

 

[MikeC23]

Gotcha. It is a 79 special with stock everything. No engine work. The cam chain was adjusted, timing adjusted and valves. The only carb work that I've done is to replace one of the floats because it had a pin hole in it. That was several weeks ago. The carbs were also cleaned really well before I put them back on the bike, but again, taht was months ago. Thanks!

 

[peanut]

I know you say that you have no air leaks but excess air creating a weak idle mixture is generally the culpret especially with such a sudden and dramatic change in idle.

This is a very frustrating fault which is not always easy to cure (ask me how I know !) It only takes a minute air leak to make the idle rocket sky high

You need to check the carbs and inlets really thoroughly especially where the carbs are held in the rubber boots and clamped evenly and also the joints between the rubber boots and the cylinders. new gaskets would be worth fitting here

You could have air leaks on the choke bodies as I did so they might need new gaskets... or the rubber caps on the inlet stub spigots may have fallen off . There are two brass capping screws on some carbs for attaching vacuum guages which may have vibrated loose or fallen out .

Check that neither of the diaphrams are leaking by holding them up with a finger and covering the vent at the top of each venturi opening.They should stay up or fall very slowly if they are sealed properly.

Check that both the chokes are returning and seating fully .

With such a high idle it is very difficult to adjust your idle air mix screw

 

[MikeC23]

Nice. I will go over her this evening and see if I have any of that going on.

 

[5twins]

You made no mention of servicing the advance rod. It needs to be greased. That probably hasn't been done since it was assembled at the factory so it's long over-due. When the advance rod dries out, it sticks in the advanced position. The timing stays advanced and that causes the RPMs to hang high.

 

[MikeC23]

I'll look at that as well. I never greased that. When I left work, the bike was cold and did the same thing with the idle. If I put the choke on, it took the rpm's down to a bearable level to get me home. They would drop if I was stopped, but soon as I would give it throttle, they climbed again.

 

[peanut]

If I put the choke on, it took the rpm's down to a bearable level to get me home. They would drop if I was stopped, but soon as I would give it throttle, they climbed again.

that would suggest a weak mixture

Putting the choke on is compensating for a weak mixture caused by either to little fuel or too much air

 

[MikeC23]

Would that also explain why it won't start at all now and just backfires? This think is more persnickety than a woman!

 

[MikeC23]

Well I'm a jackass. I lubed the advance rid and put it in 180 off. Disregard last comment!

 

[pamcopete]

You did some work on the carbs. Did you sync the carbs after that? Carb sync is one of the three causes of idle hover. Binding advance rod is another and air leaks is the third.

 

[MikeC23]

I did balance them a while ago. I'll sync them again and see if that helps. The advance rod is well lubed. I'll let you guys know tomorrow how things go!

 

[RFeng]

Well I'm a jackass. I lubed the advance rid and put it in 180 off. Disregard last comment!

Anyone here who has NOT done that?

 

[MikeC23]

So get this shit... I'm not sure how I didn't this, but the po put a bolt in the apt on the second carb where the idle screw goes on the main carb. It has turned itself in while I was riding causing the throttle to stay open

 

[MikeC23]

I removed the bolt, the bike runs like a boss

 

[peanut]

So get this shit... I'm not sure how I didn't this, but the po put a bolt in the apt on the second carb where the idle screw goes on the main carb. It has turned itself in while I was riding causing the throttle to stay open

thats a weird one
glad you got it sorted