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Losing compression

I've been riding my m20 for many years without any trouble, but lately something strange happens.
When i"ve been riding for about half an our the engine stops. If i try to start it again, there is no compression at all.
After only 2 or 3 minutes the compression is back and i can start the engine again, and go on.
First i thougt the exh.valve clearence was to little, so i gave it a bit more clearence. But the same thing happend again.
Most likely the exhaust valve doesn't close anymore. But why ?
Maybe the engine is overheeting (even in wintertime ?)
Could it be the carburettor ? (never had any problemens wit that)
Or is the ignition time wrong (to late ?)
Or could it be coal on the exhaust valve ?
(the ignition is recently done completely)

Any thougts ?

Re: Losing compression

Hi Micael, Well, if you are losing compression you need to think about the possible causes for that.
1)Inlet valve/seat problem.
2)Exhaust valve seat problem.
3)Head gasket failure/partial failure.
4) Valve lifter setting/operation problem.
5)Spark plug thread/timing plug thread problem.
6)Piston/ring wear/problem.
As your bike has been running for some time and the problem seems to be temperature related the exhaust valve/seat is the most likely candidate I would think but on the basis of starting with the easiest things first I would check the valve lifter, then the head gasket, then the exhaust valve/seat...and go on from there if you don't find a problem...Ian

email (option): ian@wright52.plus.com

Re: Losing compression

If the compression returns, i would inspect tappet clearance the next time it happens. Could be a valve binding in the guide when hot.Possibly because it's running hotter than usual from being retarded. I've had slipping timing pinions before.

Re: Losing compression

It may possibly be running hotter than intended due to an air leak at the exhaust valve or head gasket. Insufficient guide clearance isn't generally a problem in high mileage engines..quite the reverse in fact...but there may be a carbon build up on the valve stem causing problems. A check on the ignition timing setting will obviously be neccessary though...Ian

email (option): ian@wright52.plus.com

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