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I'm currently riding a 1981 R100 RT that's been converted to points ignition by the previous owner using an earlier bean can.
My problem now is a severe misfire at 3,500RPM when the Tacho needle swings over to the redline.
Any ideas what it could be?
Cheers

What have you checked? Point gap would be the first thing I'd check with a normal points ignition, but I'm not beancan savvy.

Check your main harness ground wire.
On my '79 R65, this connected at the front coil mount, which (on these bikes) would invariably break off at the frame, taking the ground path with it.
This is not the starter ground wire.

I am sure it is located elsewhere on your bike, but start there. If it is compromised, it would cause the intermittent symptoms you are describing - before it fails altogether, if it has the weight of a coil behind it, anyway.

If it was converted to points by the PO did he also change to coils suitable for points ignition ? If that and everything else seems in order you could try disconnecting the Tacho to check that isn't faulty and dragging the ignition system down.

BMW issued a service bulletin about insufficient tension on the points in a can that could cause points bounce and a misfire but that was reported to be at 6500 RPM so I can't imagine that's the problem. Assuming the misfire also occurs under no load conditions it would do no harm to observe any arcing at the points when reving the engine as the tacho reading high means it's counting intermediate pulses. There was a service bulletin on that too but it just said check the points for correct dwell and gap and check for the presence of a C5 condenser on the tacho circuit board which shouldn't be there but sometimes was. That also raises the question of whether a points tach and an electronic ignition tach are identical. AFAIK they are but it's just a thought.

The points arm can get partially stuck on the pivot post and limit the rpm. I usually found it by lifting the points are off with a finger nail and letting fly back. Only by experience am I able to judge if it is "fast" or "slow". One can remove the circlip, pull the arm off, clean it up and the post, and use a drop of oil, reassemble and test.

I had to put my hand in my pocket and buy new points and condensor - and the misfire has gone - thanks for your replies.

Now, while in the dark, observe the spark at the points while you rev it up from idle. This is probably how it should look.

I would bet that your condenser was bad and you could have found it by observing the spark across the points.