Idle speed unexpectedly changed

[Julio A.]

This is weird and has been eating at me for sometime now.

My carburetors are properly synchronized, and idle speed is set at around 1k rpm when the engine is warm. This has been for the past 4 months now. I use the bike almost everyday and I had no problems related to this whatsoever in this time frame.

I rode the bike today for a good 300 kilometers in mixed highway and twisty roads. I'll admit I was riding it a bit hard to keep up with the guys I was riding with. Suddenly, my idle rpm shot up to 3k and wasn't going down. I stopped in a gas station got my screwdriver and tuned the idle speed by ear (left side took less than half a turn). After which the bike ran great for the remaining 120km's.

As soon as I got home, I checked the air filter, and it was clean. The cables are less than 2 years old and I just inspected them and they were fine. I pulled the plugs and there is no sign of excessive lean or rich mixture. I double checked the choke and there is no way it engaged during that time.
 
Is it actually possible that the idle screw mixture throttle stop screw can move a slight amount when the bike is in use? The throttle was constantly open during the time this happened.

[Barry]

Is it actually possible that the idle screw mixture can move a slight amount when the bike is in use? The throttle was constantly open during the time this happened.  

I can't imagine the mixture screws moving and anyway if  previously set correctly they shouldn't have had that impact on your idle speed. Just to clarify, what did you adjust to bring the idle down, the mixture screws or the throttle stop screws.

[Julio A.]

I'm sorry, I edited my first post.

It was the throttle stop screws which I fiddled to bring the idle down.

[mrclubike]

That is odd
Pull the air cleaner elbows and check to make sure your throttle flaps are not loose in the throttle shafts and moving around

[thrang]

You probably checked this but you might have trapped the throttle cable somewhere. Although its never happened to me on my R65, my wife's old 450GS Suzuki would occasionally do the same, I eventually traced it to the cable catching on an over long bolt holding the head lamp bracket when the bars were pushed over to the left.

[Barry]

t was the throttle stop screws which I fiddled to bring the idle down.  

Can't see them moving either as they are usually stiff enough not to. What else could it be though. I thought about the cables but if they were hung up somewhere, adjusting the throttle stop screws wouldn't have lowered the idle speed.

[Luca]

How about the old sticky ignition advance problem?

Did you try holding the brake and then slipping the clutch out to see if you could make the idle drop down to normal?

[nhmaf]

I am suspicious that the root of the problem was something else (possibly a briefly stuck advance mechanism) that then unstuck with some tweaking of the engine RPM, or possibly cable end pulling out of position?

[Julio A.]

I did the "forcing the idle to drop by dragging the clutch" technique. As soon as I released the clutch, the RPM shot back up again.
I just serviced the advance mechanism last February. when I opened it up, the thing was good as new inside.  

What I did notice when I adjusted the carbs on site is that the left side carburetor was leading the right side, forcing the rpm up. I only adjusted that side.

This morning, I removed the tank, inspected the cables, checked valve clearances, checked cylinder compression, removed the intake elbows and inspected the carburetors thoroughly. I really can't find anything out of order on the bike. I just got back from a short 80km run and it ran like a top. No dead spots, no flaws, no excessive vibrations.

I took my sister's clear nail polish and dabbed the idle stop screws with it. Should be tacky enough not to let it move on its own yet not strong enough to seize the whole screw.