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Hi there, I own a 1981 bmw r65. I have this strange problem where once the engine gets hot it suddenly misfires and looses all of its power! Once its cooled its fine it will only last about 15 to 20 mins before the same problem occurs! I have cleaned out the carbs that didn't solve the problem, checked the diode board that was all fine. replaced the htc leads and surpressor caps that did nothing! I was told to open up the valve clearance that did nothing! the mechanic I gave it to has had the bike five times and cannot seem to fix it! Can please somebody help me!?

Sounds like your valves are out of adjustment. Have you checked the clearance?

Yeah I checked the valve clearance and that was all fine! Iam pretty stumped to what it could be!

Bad coils can do something like this, although it's usually not quite that sudden a shutdown.

The key is to get it to do it again and start testing while it's bad. Very first thing when it happens: check for spark on each side. If no spark, you can start hunting down where the current is and isn't. If you have good spark, then you know it's not electrical, and there's something weird going on with your fuel supply.

What I can't explain is why your mechanic doesn't know this....

One other area I would check is the ignition module unit. If the bike is shutting down when hot, and it is a bad ignition module, that only "acts" bad when hot, IF you can cool the module down with some freon or electronics freeze spray, the bike should continue running. IF the bike continues to run normally until the module heats up again, that's your problem. Replace the module.

Rebel wrote: .... IF the bike continues to run normally until the module heats up again, that's your problem. Replace the module.

Or you might try improving the cooling of the module by replacing the heat transfer paste.

Okay, I will get the bike hot wait for it to break down on me again and test the spark plugs. I have heard of those ignition modules packing up with heat so I will replace that if the coils seem to be okay! slowly narrowing down the problems.

Thanks everyone for the info! I will try this all out in the next few days and re post to let you know how I got on:)

If you have the oem grey twin coil, aka the crackomatic, check it for cracks.

http://www.largiader.com/articles/ignition/ wrote: 2) New models (like the G/S and ST) got a single dual-output coil with 1.5 Ohms resistance. BMW 12 13 1 243 910, Bosch 0 221 500 200, known as gray coils, are prone to failure by cracking. Some fail without any visible cracks whatsoever. It's a good idea to replace these coils wherever they appear. The later black coil, or Dyna brown, or many other coils with a 1.5 Ohm primary winding can be used.
3) Around 1991 the black coil (BMW 12 13 1 244 426, Bosch 0 221 500 203) was introduced. This coil has a 0.5~0.6 Ohm primary resistance.

I keep a new set of coils in stock against the time when I have another bout with coils that crap out when warm and work again when cool. I know they are rather pricy, but my peace of mind and freedom from aggravations is apparently worth the price for me.

(Now if I could buy 'insurance' for other things that vex me life would be even better.)

Ken