Hi Pat, The Tufnol spacer won't have any effect on the mixture. If the mixture is too rich you first need to establish that the carb is the correct one for the bike and has the correct slide cutaway, jets, needle etc. fitted. If that is the case then check that the float and choke slide, if fitted, are functioning correctly and not causing the richness of the mixture. Then work your way through the carb settings. Bear in mind that the carb works in stages (with a slight overlap between those stages) and each adjustment and/or specification of the various parts affects operation in that stage. For example..The pilot air screw controls mixture strength from zero to 1/8 throttle approx.
If the carb is incorrectly set up, to cure richness you will either have to adjust the pilot air screw, alter the needle position, fit a larger throttle cut away or reduce the main jet size.
If you have just built the bike and are basing your observations on running up the bike in the workshop what you are seeing isn't really a carbon build up, rather a soot deposit. Could you describe in more detail the effect and the circumstances?....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.
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