[iansoady]

Good result but making an assumption to start with (no compression rather than "kickstart moves too easily") made it harder.

Robert Pirsig covered this very well in Zen and the Art Of Motorcycle Maintenance.

[Steve_M90]

Just for the record:

There was nothing at all wrong with he cylinder or the piston rings....

Whilst trying to analyse the failure to start, the feel of the kick start action seemed to be inconsistent – sometimes typical and sometimes very spongy. I decided to look at the front chain and I found that on the spongy ones the chain was not moving. This indicated a problem with either the kick start shaft or the clutch.

Tony Pashley advised me to check that the clutch control had a small amount of play in it – which it didn't. I released most of the adjustment without being able to gain this play (photo) but testing the kick start revealed it to be much tighter and that the cause had been a slipping clutch all along! The proof was that the bike now starts! A bit of a journey, interesting on the way and ultimately successful.

Steve

[VicYouel]

Cox and Turner phone 01 425 652627 are extremely helpful and can supply most piston rings by return.

From the sound of it you will need to inspect the rings .... have you put some oil on the top of the piston to see if the compression rises so that you can stand on the kick start at TDC (the one on full compression of course) temporarily until the oil runs past the rings?

Vic

[iansoady]

Compression tests don't work very well on our type of engine as they need the engine to be rotated quite quickly. I would go for a leakdown test. There are some instructions for making a tester here: http://www.max-boost.co.uk/max-boost/Leakdown_tester.htm

[Steve_M90]

The symptom is that the engine does not offer sufficient back-pressure (resistance) to find tdc when being turned over by the kick-start. It has run recently but was making a "popping" noise. Now it doesn't start. I don't understand what can have changed so suddenly (unless it was previously on the verge of not starting). Topic carried over from "Removing Model 90 Cylinder Head".

Engine stripped down to allow removal and decarbonisation of the cylinder head and valves. Cylinder pressure measured before and hardly registered on the gauge. Everything came apart very much as described in the "The Book of the Sunbeam". The inside of the head was quite carbonised but the valves and seats looked ok. I decided to grind them with some fine paste anyway. All cleaned and reassembled. Tappets adjusted to ensure the valves were closing. Measured pressure and marginal improvement (4 bar). Process repeated to see if was possible to add to the marginal improvement. This time I also ground the head to barrel face.  Top piston ring and barrel inspected as best as possible and no deficiencies identified. All cleaned and reassembled. Tappets adjusted again. Measured pressure and no further improvement.

I think the inlet valve is ok as a rubber glove held over the inlet pipe doesn't inflate during the compression stroke. There is a lot of air moving in and out of the crankcase breather.

If I need to inspect the piston rings, the book says to have the piston at bdc and to lift the barrel but it doesn't seem to include any reassembly instructions. Is it possible to buy replacement piston rings?

Thoughts and suggestions very welcome...