I'm in the middle of a valve & seat job on my M20 engine at the moment. Continued problems with carbon build-up on the exhaust valve seat that eventually holds the valve open a bit, over-heating the valve edge. Surprising how the old bus will run at all with almost no compression...
I'm looking for pointers on a suitable 'modern' valve replacement? I'm aware of the need for as much meat as possible on the edge above the ground face of the valve. Any ideas out there? I'm aware that replacement valves are not hard to obtain via the usual dealers. What & where are their sources?

Hi Mike..Carbon build up at this level indicates the bike is either running with a consistently rich fuel air mixture and/or you are getting too much oil in the head via worn bore/piston/rings/guides.
This is not a problem that usually occurs with M20s.
The after market valves are generally perfectly serviceable...a GS Valves component is the best choice if you can find one. Alpha bearings used to stock these and you could try GS direct.
Upgrading the valve is a possibility...either through a valve manufacturer such as Manley Valves in the States or by ploughing through the valve catalogues such as Edmund Walker.
I use a valve that was fitted to an Iveco turbo diesel truck from a few years back..these have the necessary stem length and head diameter to allow modification to fit the M20 and come with the highest valve steel spec. Stem is chrome plated stainless with a fusion welded Nimonic 80 head.
Valve stem diameter is 9mm which means the existing guide needs resizing to suit. In conjunction with a lead free seat it is all fairly indestructable.
Sadly I can't remember the model of truck but you should find it in the catalogues or a friendly engine shop will do that for you if you provide the M20 valve dimensions...Ian

Thanks Ian. I'll investigate further. I can see the inserted seat on the inlet side, but not on the exhaust side, even though the whole area is really cleaned up. I presume that its just damned hard to see. Must be there somewhere!
Mike

Hi Mike..The barrels did not have seat inserts as standard, the seats being cut directly into the cast iron. When barrels have done a lot of work and had the seats recut several times the valves gradually become 'recessed' below the top surface of the barrel.
On the inlet side particularly this will restrict gas flow with a consequent drop off in the already limited power.
Your barrel will have had a seat insert fitted on the inlet side to restore the valve to its correct position.
The inlet seat is unaffected by hot combustion as it is passing cold incoming gases and is closed on the combustion stroke.
It is the exhaust side that benefits from a hardened seat and an improved valve, as it is exposed to the very high temperatures of the exhaust gases...Ian