Topic: "out of the blue" High Idle

[Schuyler]

1976 cb750K 

The bike is new to me, but I have ridden it quite a bit without any problems. 

Only thing to change was that one of the mufflers came loose, so I took it off. 

I start the bike, and it revs at like 3500-4000 and will not back off.  The choke is off, and it has never done this before.  At first I thought it was a cable sticking, but they don't seem to be moving (looking at them while starting the bike)

One of the cables was all but completely broken, so I replaced both.  When I started it up, it ran great for about 10 - 15 seconds.  Idled perfectly.  Then it creeped up and stayed at 3500-4000 rpm.  Turned the bike off, started it again, immediate high idle, and would not come down.   

What would cause this "out of the blue" high revving problem?

[Duke McDukiedook]

Leaky insulator?

[Spanner 1]

Sticky carb float valve allows overfill of carb which allows high fuel level in one of the carbs, fuel gets sucked-in and increases rpm....watched it happen on a friends 750F.... prolly that IMO., unless slides are suddenly sticky (not).  The scenario described will actually lift the slides! with the increased engine rpm and vacuum... believe it or not.

[Frankenkit]

Only thing to change was that one of the mufflers came loose, so I took it off. 

I start the bike, and it revs at like 3500-4000 and will not back off. 
What would cause this "out of the blue" high revving problem?

...this is a really dumb question, but I want to make sure that that base is covered...
once that muffler came off, did you put it back on?

I don't know that missing a muffler would cause a lack of backpressure and raise the idle on two cylinders or not (assuming you have a 4-into-2) but just wondering...

[Spanner 1]

I presumed the bike had a complete exhaust system... but you never know... well?
Can you give us a complete discription of your bike's set-up?

[Duke McDukiedook]

4-2 is not a stock setup for a 750, 4-4 is stock.

You have a MAC 4-2 on yours.

Make sure you carbs are completely closing when the throttle returns and nothing is hanging it up.

You sure your friend didn't screw with your idle adjuster?

[Frankenkit]

the exhaust can sometimes be a real betch to seal... maybe make sure your headers are nice and tight?  I wonder if there's something secondary to all that that you bump or nudge just right when removing the exhaust?

[Spanner 1]

Float valve seated itself would be the actual fix IMO.

[Frankenkit]

Aww, Spanner, you're simplifying something that we could blow up into something that sounds really complicated and difficult.  Where's the fun in that?

[Spanner 1]