Idle stumble

[k75_oakland]

I’ve had my 86 k75 about a month and would say it’s running very well overall. however, the last few days it has developed a new condition—after the bike is fully hot (riding on the freeway),  when I get to a stop, say at a long light, the idle seems fine for several seconds, then it falters, and then returns to smooth idle for a few seconds, then falters, etc. i wouldn’t call it a rough idle, just this rhythmic cycle of dropping in rpm (from around 1100 to 900) and then returning to the normal idle.

I did basic maintenance on it when I got it (changed all fluids, plugs). The PO had just checked valves and they were in spec.

Any thoughts about the cause of this? Thanks

[Laitch]

That's called surging and frequently is caused by excess air leaking into the fuel mixture. The usual suspects are the throttle body caps (red arrows), the throttle body manifolds (green arrows) and the Z-hose which vents crankcase gases into the air plenum. Start the engine and as its idling, carefully spray carb cleaner or propane (preferable because it won't create an inflammable pool) around the manifolds, at the vacuum caps and at the z-hose. If there is a change in the idle you've found the damaged component to replace. Z-hoses are especially prone to failure as they age. Heat opens up cracks in these rubber components.

[Laitch]

There's a vacuum hose under that spring from the #3 throttle body. Check it too.

[frankenduck]

the throttle body manifolds (green arrows)

Your green arrows are off. It's usually the manifolds below the TBs that crack and leak, not the ones above. Being bolted to the cylinder head they are subjected to more extreme heat cycles.

If you have a propane torch you can spray some UNLIT propane gas around the lower TB Manifolds/boots at idle. If the idle increases then you have a leak. Probably a good idea to do this outside.  I've never had the propane gas flame up when doing this but it doesn't hurt to be safe and you don't want propane floating around in your garage.

[k75_oakland]

Thanks for this info. I’ll check those hoses for leaks. A few other comments—I did replace the z breather hose as part of the routine maintenance (came with the tune up kit I got from a parts website). I was just on the bike again and it is very clearly only doing this as it gets hotter—at first the idle is fine; but if I’m at a light for a minute or so and it’s warming up more, it starts. Is that symptom consistent with the air leak?

[Laitch]

Your green arrows are off. It's usually the manifolds below the TBs that crack and leak, not the ones above. Being bolted to the cylinder head they are subjected to more extreme heat cycles.

All rubber ompounded parts—those I indicated and those to which you refer—are subject to deterioration depending upon how much the moto has been neglected and how it has been stored. Heat is not the only factor involved.

Lit propane gas is called a flame or a conflagration depending upon the tidiness involved. 

[frankenduck]

All rubber compounded parts—those I indicated and those to which you refer—are subject to deterioration depending upon how much the moto has been neglected and how it has been stored. Heat is not the only factor involved.

The TB adjustments (brass screws) happen BELOW where your green arrows are. Even if the upper ones are leaking you can still sync the TBs and get a consistent idle. Not so if the lower ones are leaking.

In my decades of working on numerous Ks, it's always the lower ones causing issues.

[Laitch]

In my decades of working on numerous Ks . . ..

Duly noted.

Any thoughts about the cause of this? Thanks

Got enough of those yet, k75_oakland? 

[k75_oakland]

All of this info is much appreciated!

[k75_oakland]

I finally got to checking into this: The hose clamps at the top of the lower boots (attaching to TB) were a little loose, but tightening them didn’t solve the problem. So I sprayed some carb cleaner (I had some lying around but not propane) around each lower manifold. No effect for #s 1 and 3, but with the central one, the idle rpm dropped significantly for a few seconds.

So I presume this is the issue, but interesting that the rpm dropped rather than increased. Maybe due to the pooling of liquid that was mentioned above? I can try with some propane but it definitely seems like something isn’t sealed here…Is there any hope in tightening the bolts that hold these to the engine, or will they definitely need to be replaced?

[frankenduck]

Not uncommon for those crack and leak in which case they need to be replaced.