Impending bearing failure?

[hmaskill]

Hello,
My comet starts OK and runs well for about 12 miles, i.e. until it is nicely warmed up, then it loses power, slows down, and (if it is pulling hard, e.g. slightly uphill) there is a serious knocking sound coming from the engine. Retarding the ignition (it has a manual BTH magneto) has no effect. If it is going down-hill with the throttle closed, or almost closed, there is no such noise. This problem started suddenly after I had put the engine together having had the cylinder head off. It sounds serious. If it is a bearing about to fail, is it possible to tell which one other than by completely dismantling the engine (or having it dismantled)?
Regards,
Howard.

 

[rapide049]

Did you fit a new piston ?
Greg

 

[robin]

Sounds like your cylinder liner is not clamped onto the cylinder muff by the cylinder head.
i.e. 1/ head nuts loose.
2/ Poor ground head joint.
3/ Poor fit of liner in muff.
4/ Original diecast muff.

Ronin

 

[Bill Thomas]

Hello Howard, It could be Detonation, Something glowing to fire mixture, What plug are you useing and what does it look like, Maybe very weak mixture, What carb ' have you got. Good Luck Bill.

 

[Howard]

Hello Howard, It could be Detonation, Something glowing to fire mixture, What plug are you useing and what does it look like, Maybe very weak mixture, What carb ' have you got. Good Luck Bill.

That scared me, Bill. I went straight to your post, and thought I'd got a problem I'd forgotten about. Terrible getting old!

I'd go with detonation too. If it's as bad as it sounds, surely it would be knocking cold as well if it's a bearing.

I hate these posts where "the problem sounds like........."

H

 

[davidd]

It the engine runs well when cold then I think Robin has the advice to follow.

I know I keep offering advice that is resisted on these top end issues, but a leak down test would probably show you the problem in a few minutes.

I suspect your bearings are OK. The bike will usually run surprisingly well with a bearing failure.

David

David

 

[hmaskill]

Dear Greg,
Thanks for the quick response. The answer is yes, but not just before the problem.
Regards,
H.

 

[hmaskill]

Dear David,
Thanks for the quick response. What is a "leak down test"?
Regards,
H.

 

[hmaskill]

Dear Bill,
Thanks for the quick response.
It's an Amal monobloc carb and has been for the last 40 years; I have tried several plugs, the latest being a Champion N4C (it just happened to be handy); all looked OK and the changes did not seem to make much difference.
Regards,
Howard.

 

[hmaskill]

Dear Robin,
Thanks for the quick response.
It looks as though I shall have to take the head off again to check these possibilities.
Regards,
H.