Engine rattle above 3500 rpm

I posted this a while ago with no answer so here goes again. While riding the bike, I get a rattle/knocking in the engine that starts around 3500 rpm, give or take a few 100 rpms, and lessens in intensity as rpms go up. The engine does not do this in netural, only when there is a load, me, on the bike going down the road. It is worse at 4000 rpm aka 60 mph, at medium throttle. From posts in this forum, I thought it might be carbs but I swapped the carbs with the set on my other bike which does not have this problem.

I am now thinking the cam chain might be streached. The bike has 40000 miles on it. The chain guides may be shot too.

Compression runs in the 130/145 range and the bike runs pretty good. The valves have been lapped and do not leak. I am wondering if I have a weak valve spring and what I hear is the valve slapping shut?

Any help in this issue is appreciated.....chris3

a chain would do it weather the bike was moving or not, engines are kind of dumb that way, have you tried to put it on the center stand and get the wheel rolling, to try and chase down the rattle?
I am thinking in the way of the clutch or drive drain area

Does it do it all the time at that rpm. or only when accelerating ??

That's the rpm where spark knock usually occurs.

It does it all the time except when I reduce the throttle it goes away, less load. One other thing, there is no rattle when the engine is cold. It only happens after the engine has warmed up. Piston slap usually happens when cold and goes away when hot. I have tried different grades of fuel with no help. I checked the plugs for lean burn and adjusted the carbs accordingly. Changed the plugs too. It seems to rattle at a constant speed/throttle position. This one has me stumped. I am thinking a rod bearing, because it does it under load. I will try the center stand run to see if I can duplicate it. I doubt if it is a starter clutch because it happens at higher rpms. I don't know what a clutch basket noise sounds like. I did replace the clutch fiber plates and springs though. Nothing looked out of place or loose when I did that.......chris3

A rod bearing knocks under no load conditions and is quiet under load, so you can eliminate rod bearings. Main bearings on the other hand, knock under load conditions, not under no load conditions.

It's real difficult to diagnose a noise over the web, perhaps you could make an audio recording and post it somewhere so we could hear it?

Might be a little hairy though at 3500-4000 rpm in 5th gear :woohoo:

Have you checked the timing, or maybe advance or retard it just a bit to see if it changes or goes away

I am not sure the pick up can be moved to adjust the timing on this bike. Not like the old points bikes.......chris3

what bike are we talking about here. i like the rod bearing idea.or wrist pin.

If timing is correct, it could still be knock if you've got a lot of carbon build up on pistons or are running the wrong temp plugs. Also, Suggest checking front sprocket and motor mounts.

Post edited by: Duck, at: 2008/03/05 15:20

This bike is a KZ1100A2 shaft drive. It has 40,000 miles on it. I bought it last spring and it had the rattle then. The thing that is throwing me is the rattle is not there when the engine is cold. I will put it on the centerstand and run it through the gears this weekend and let you all know. There is no carbon on the pistons because I did a valve job when I got the bike. I cleaned all carbon off the pistons and head. The spark plugs are new and I have adjusted the carbs so they are the correct burn color. I am wondering if there is a partial obstruction in one of the mufflers that is causing a back pressure sound? I hate remove/install of stock exhaust. I have a MEC exhaust off bike #1 which is down for a valve replacement. I just don't know. I suppose I will have to pull the engine and check the lower end. I had planned on doing a ring job in the near future. Thanks.....chris3

Well, I put the bike on the center stand and warmed it up. I went through all the gears, up to 4500 rpm, and no knocking or rattle. I then rode to the hardware store and the rattle returned around 3500 rpm. Is seems to only rattle with a load on the engine/drivetrain. I pulled the pickup off and removed the centrifigual advance unit and re lubed it. It was pretty free anyway. Pulled the plugs and checked the color, they were a nice light grey. I am wondering if I have a bad/worn cam chain guide? I guess I will just use this bike as my around the town bike until I can pull the top end off and see what things look like in there. Glad I have spare bikes........chris3

coppertales wrote:

... did replace the clutch fiber plates and springs though. Nothing looked out of place or loose when I did that.......chris3

Was the rattle present before clutch plates and springs were replaced?

Has clutch push-rod been adjusted?

Just random thoughts in hoping for a simple resolution.