I mean really loud!! The first time it happened I almost jumped off! First a little backgroud info. The bike is a 1971 ct90 with 3900 miles all stock ,nice old bike. I have had it for 6 months and it has been running great. It just got very hot here, about 90 degrees, and I was riding the bike full speed through a nice left hand sweeping turn and all of a sudden this LOUD squealing grinding noise starts coming from the engine. I thought for sure I would look down and see a trail of oil and broken engine parts. The bike continued to run and except for the noise seemed fine , I mean it will accelerate and shift, no knocks. So I slowed down to check it out and the noise went away. The bike continued to run fine and if I did not go wide open it will run fine. I changed oil [10w40]when I got home and did not see any shavings. However the next hot day same thing happened. Today is cooler and everything seems great no noises at all. Do I have main bearings that are failing? I have seen one person online mention main bearings spinning in the engine casting as a potential cause of such a noise. If that is the case I would assume the engine is toast. Any thoughts? Thanks very much, Mike

neveradull,

I'm definitely no authority, but your bike is kind of acting like ours did when it wasn't getting enough oil to the top end and the cam would start squealing after warm-up.

miniac, any ideas?

Scottie

neveradull,

Ideas to try --
Will it squeal when in neutral, and you rev the engine?

If it does, then I would think that the problem is top-end related, ie cam being starved for oil. The cam does not use bearings and rides right on the head, so if the top end is on the edge of oil starvation (age or whatever), and you overheat, things can get loud quickly. I wouldn't think the main crank bearings would be a problem as they're normally bathed in oil, unless it's run WAY low on oil.

Otherwise --
The tranny main and countershafts each have a sleave bearing on one end and a ball bearing on the other. Later CT70s had rollers in place of the sleaves, I think.

Keep in mind I'm still an amatuer, but I've had my engine apart twice (no comments, now), I've seen inside a few others, and I've stayed at Holiday Inn before...

I'm just throwing ideas -- I defer to miniac or racerx. They are the dudes, here.

Scottie

Does not make noise in nuetral. The two times it has done this have been on very hot days ,top gear, at full throttle, and curiuosly on the same stretch of road , full throttle sweeping left-hander. The noise starts with no warning , one second quiet ,the next instant, this terrible squeal. As I would start to slow down the noise would continue even at very little throttle as I went down through the gears the noise would go up and down with the revs as I shifted until I pulled over. At idle the noise then stopped. When I drained the oil I got pretty much a full quart of oil out in good condition. I have put 15 miles of road driving on it today in cooler temps and it is quiet. Thanks very much for the replies, Mike

Check your wheel bearings. Get the bike off the ground and spin the wheel and see if it spins freely. Also rock the tire back and forth to check for loose wheel bearing adjustment. The noise you describe could be a bearing seazing up and squeeling against the race.