K100 Rough Idle

[Roadhawk]

Have an '85 K100RT that has been idling rough.

Not all the time, usually after it's been running a while. If I roll on throttle slowly I have to ease the clutch slowly or it wants to stall, then suddenly it's like someone turned on a switch and it takes off with full power.

Once I get going it appears to be running fine.

Ideas?

[SugarHillCTD]

Have you or anybody else done some service to the bike recently?

Curious (and experienced) minds want to know.........

Good luck.

John

[tomsljr]

Have you checked the condition of the crankcase ventilation hose? (I believe that is what it is called) Changing that and running a tank of Techron through solved a similar problem for me and turned it into a new bike.

[deilenberger]

Given the age of the rubber bits on the bike - the entire intake tract should be checked for intake leaks.. The crankcase vent hose (Z shaped hose to the rear of the thottle bodies) is the first thing to check. These have about a 2-3 year life before they start cracking. They can be changed without removing anything else - even if it doesn't seem like it.

The throttle-bodies mount to the head with rubber mounts - these have been known to crack/warp with age allowing unmetered air into the engine (most noticeable at idle) and the O rings on the fuel-injectors also have been known to dry out and crack with age. Less likely, but still possible is the big L shaped tube between the air-filter box and the intake plenum..

Best test I found for intake leaks was flammable carb cleaner. I won't tell you how to do it - but I'll tell you how I do it.


I take the bike outside away from any possible flammable materials.
I get my garage fire extinguisher and put it next to the bike.
I get some flammable carb cleaner - with the little red tube (if you're missing the tube, look in the darkest corner of your garage.. I'm convinced spiders take them there for some bizarre reason only they know.)
I warm the engine up and leave it idling.
I do small spritzes of the carb cleaner at any suspect points (like the base of the TB's and where the injectors go into the head.) I am very careful not to spitz on the exhaust since I don't want to burn up the bike.

ANY change in idle quality/speed indicates a leak. This is quite a sensitive test - the flammable carb cleaner turns into a vapor by being sucked into the intake and the idle speed will jump up when the flammable vapor hits the cylinders.

I try to do it so I don't burn anything down - so far I've been successful!

If you decide to do this test - and find any leaks - come back and tell us about them and perhaps we can help you in fixing them. And DO check the Z crankcase vent hose. It very likely has gone bad on you.

[Roadhawk]

No one has recently service the machine.

I will see if I can track down any leaks. I'll post my results.

Thanks
Shawn

[Roadhawk]

The bike has ran just fine since my first post. It did run rough a little bit today but I was miles from home. The only thing I can think of is when I am running slow on a warm day.

I will have some time to try it again Sunday.