Hi Ian

A possibility is that I do not use the bike for long enough runs and don't get it hot enough. However, it does look like carbon as opposed to soot. There is a small soot deposit on the exhaust outlet, but the inlet appears to me to be carbon.

Valve stems and guides all look good.

The spark plug always looks ok, i.e. biscuit brown.

Kind regards

Pat

ps It is the correct carb (I think).

email (option): sacombsashtrees@hotmail.com

Hi Patrick..If the bike is running rich at lower to mid throttle openings it is not unusual to get 'blowback' of fuel vapour from the carb..An M20 without an air filter will often show this from the deposits on the front face of the oiltank...This excess of fuel can lead to carbon build up but I wouldn't expect that to occur very quickly. How many miles have you covered since the engine was last rebuilt?..Ian

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

A 1,000 miles maximum from last rebuild.

It does wet sump. Would burning some oil cause this?

email (option): sacombsashtrees@hotmail.com

Pat is your valve timing correct? Sounds like inlet valve is late shutting. Just a thought.