Well, crap. Educated guesses?

[wymple]

Got an 81 850G. It had obviously a rough and poorly maintained life, has 41K on the odometer. So, I worked on the wiring, which showed signs of having been overly hot. Cleaned connections, replaced some wires, added a Dyna S. Adjusted the valves, all are between .05-.08mm, none were zero clearance to begin with. Tires, brakes, all that stuff. I have put about 6K on it this summer. It took a while to get the carbs & airbox problems sorted, but run good it has. I made a 950 mile trip to Ohio, and last weekend I went to Wisconsin, 525 mile trip. On the way home the idle roughed up. Number 3 cylinder is dead at idle. Smooth as silk at speed. I have good fire, changed the plug out as well. Sprayed a touch of carb cleaner in #3 as it idled to see if the idle circuit got plugged on that carb, didn't make a difference. Played with the mixture screw for the hell of it, no help. The kicker is, that as it idles in 3 cylinders, there is an audible knock in the engine that cycles on and off every couple of seconds, very regularly. It is quite as a church mouse by the time it hits 2K on the tach. This doesn't register in my mind. I would think any mechanical problem would not disappear at RPMs. With the bucket & shim stuff, there is no rocker arm or adjustment to go haywire, a broken valve spring shouldn't work better at speed. There's not much in there to happen. A burned valve or sticky valve that doesn't close might kill the cylinder at very low speeds, but why the knock? This isn't sounding like a lifter tick, it's a knock that disappears totally off idle. I'm hoping one of you experienced GS guys can give me a hint before I tear it down. If it gets expensive I may look for an 1100G engine to stick in there.

[mmattockx]

Have you checked the clutch hub nut?


Mark

[blowerbike]

i think he is hearing sounds because he has a dead hole at idle.
as he said...the sound goes away when the cyl. picks back up.

[Missile99]

i think he is hearing sounds because he has a dead hole at idle.
as he said...the sound goes away when the cyl. picks back up.

+1. It's the sound of an engine imbalance caused by your dead cylinder at idle. My idle knock at one time sounded pretty bad. I still have it on video. It happened when the gas soured and the new (Dyna-S) timing was off a touch. The only thing you need to tear apart is the carbs.

[Steve]

Before you tear apart the carbs, check your carb sync.

It would be a bit unusual for the 'master' carb to be the only one off, but that would explain a dead cylinder at idle that wakes up with application of throttle.

.

[rwnielsen]

Switch the 2 & 3 plug wires at the plugs and see if the problem remains or changes cylinders. Do one thing at a time until you isolate the problem. If it starts missing on #2 it's ignition related. If not it's elsewhere. Then go from there.

[earlfor]

Since #3 kicks in and runs normally at higher rpm, the problem is not the coils or ignition or valves. It is either a clogged idle circuit on the #3 carb, or it is an electrical problem. At idle rpm, stator electrical output is at its lowest, so it could be a faulty plug cap, or poor plug cap to plug wire connection. The first thing I would do is remove the #3 plug, put the plug wire back on it and start the bike. Then ground the plug against the engine with the bike running at idle, and check the plug to see if I had a good strong spark. If there is, it is not an electrical/ignition problem. After that, I would check the intake boot between #3 and the cylinder head for leaks, poor fit, cracks, etc. After that, I would suspect dirt/debris clogging the idle jet passage in the #3 carb.

[wymple]

My 1st thought was the idle circuitry, I thought a shot of fuel (carb cleaner) would have made it cut in momentarily. It didn't. I will try switching the plug wires between 2-3, if nothing shows different I'll tear the carbs off. I'm certain of a decent carb sync, unless there is some way for that to suddenly alter itself. Back to the garage......!

[rustybronco]

That rattle could be a result of the miss and a clutch basket with bad damper springs.

[wymple]

So, I switched the 2-3 plug wires, and it got better. Both 2 and 3 seem weak at idle, tho, they don't make the same difference as unplugging 1 or 4 do. I had a Yamaha coil in the garage, so I swapped out the coil. It now seems to idle decent enough, but I still have that insidious, annoying rattle. The clutch basket springs were beefed up a touch with 3 new aftermarket, out of the six. I'm wondering now if it's just developed that cam walk thing I've been reading about. The rattle is off & on, regular as pie, every few seconds, gone at fast idle. It has close to 42K on the engine.

[rustybronco]

The springs I'm referring to are at the back of the basket, but yes, it sounds like it might be cam walk if it's a hard metallic knock.

[wymple]

I'm doubting the clutch being a problem. Pulling the lever makes zero difference.

[rwnielsen]

I had an engine full of rattles when I rolled my bike out of storage after 11 years. The clutch is the easiest to check but getting the idle smoothed out will definitely help.

[Nessism]

Could be a bad plug cap or coil wire. It might help if you remove the plug caps, check resistance, then snip off 1/4" of wire and reinstall the caps so the connection is refreshed. Check voltage at the coils too. There should be minimal voltage loss through the harness on the way to the coils. Relay mod may be needed, or fix the harness losses.

[rustybronco]

I'm doubting the clutch being a problem. Pulling the lever makes zero difference.

The basket I'm referring to still turns when the clutch lever is pulled. The clutch hub, steels and pressure plate do not.