strange knocking noise

[sissonk]

I have a GS850 1981 , just got it going again , carb, valve and stupidity problems.
But now I have noticed something else when it gets hot there is a knocking noise that comes and goes at a slower pace than the RPM of motor. it has a hard loud sound and then it gets softer but runs a cycle like a harmonic
When I was setting the valve clearances I had one that was at the tight end but I thought was still within spec. Anyone know what a valve with no clearance sounds like ?

[robertbarr]

A tight valve doesn't make any noise -- it just tends to cause the valve to burn. At the extreme, it can lower your compression. But if it's within spec, it's within spec.

How many miles on the engine? When my venerable '79 850 got up to around 40,000 miles, it started making a knocking noise like rap-rap-rap-RAP-RAP-rap-rap-rap every three or four seconds, especially with the bike on the sidestand.

A dealership mechanic suggested it might be my cams moving around axially, which I suppose is as good an explanation as any. It's all ball bearings on the crank & rods, so there's plenty of room for slop with no real consequence. These things only run about 7 psi oil pressure, as I recall.

It really sounded like there were crank/rod problems at times, but it never caused any problems. I sold the bike at 54,000 miles and it still made that noise.

As long as you have clean oil (and oil pressure, of course) maybe you have nothing to worry about.

[bwringer]

A valve with no clearance doesn't make any sound, so it's not that.

It's nearly impossible to say what your sound is without hearing it. GS engines are often quite noisy in a variety of harmless ways. Or it could be something deadly.

So we need more details on when the noise happens and if/when the noise changes or goes away. Lots of details:

Does it happen when rolling? What speeds? On the brakes?

What RPMs does it happen? Does the noise change as the engine speeds up or slows down? Is there a change when you squeeze the clutch lever? Does the noise change as the engine warms up? Does it seem to be coming from one side or another?

Is the engine running and idling OK? Does it start instantly, hot or cold? It should - a GS that requires more than 0.1 second of cranking has a problem.

One of the most common and avoidable causes of odd noises is simply carbs that are out of sync. Have you personally synced the carbs and verified that they are actually synced relatively well? (You can't rely on a shop to do a good job synchronizing.)

[terbang]

It'll be in the cam department which is fairly typical. The cams run at half engine speed. If you can start a GS then you can ride it..

[bwringer]

When my venerable '79 850 got up to around 40,000 miles, it started making a knocking noise like rap-rap-rap-RAP-RAP-rap-rap-rap every three or four seconds, especially with the bike on the sidestand.

A dealership mechanic suggested it might be my cams moving around axially, which I suppose is as good an explanation as any.

Camshaft end float is an explanation I forgot about, and it is harmless.

[wizard]

I've had an 81 GS850 for the last 21 years. It made that same "knock-tic" sound since the first day I owned it. A few years ago, I had occasion to take the jugs off (re-ring it) and found a slightly seized wrist pin. I was overjoyed that I had finally found the noise. I put everything together and it still did it and still does. It's only annoying at stop lights.

[Guy]

Definitely sounds like cam end float. Harmless, but very irritating. Nothing to worry about.

[49er]

It's most likely excessive cam endfloat. When I used to pull up at traffic lights and service stations on my 850, people looked astounded as though I had just run a crankshaft bearing.
After stripping the top end, I found the cam endfloat was .006" on the exhaust and .005" on the inlet. You could slide them back and forward against the thrust rings and replicate the knocking noise heard when the engine was running. I have done some machining to reduce the endfloat and will report back when I have run it again.
You can ignore the noise, as it won't harm your engine. It may drive you nuts though.

[sissonk]

There are 20k miles on the bike , but the head has been changed from a newer model . when it starts and it starts very easy , it is very quiet which I was surprised as the old head was quit tinny and rattly sounding . I checked everything you guys sugested and the only place i can hear the noice is at idle when it is good and hot . the hotter it gets the noisier it gets . So maybe it is just this cam endfloat as U say. I will keep and ear to it thou.
Thanks for the help...

[KEITH KRAUSE]

Noises are always hard to describe and finding them can be harder.
It could be what the others say. If you have the stock cam chain tensioner, make sure it's operating smoothly. Test it per the factory procedure. If it passes, it's usually OK. The rod inside can burr up and stick. The fine outer spring around the knurled knob can lose strength and the tensioner won't advance along its entire range. If this happens you can turn the spring back one full turn to increase forward strength. Loose cam chains have a cycling/slower sound similar to what you describe.