Hi Mark,
thanks for you input regards not going down the route of having too rich a fuel intake. Having looked at the colour of the Plug would you be happy running at that colour at settings mentioned?
John B
With modern fuels, looking at the plug color is not as useful as it was years ago. You don't get a nice light brown color as you used to, even if the carburetor jetting and ignition timing is perfect. Your spark plug looks fine to me and I would not worry it is running too lean.
A better test these days is to look down the spark plug hole with a light and look at the top of the piston. If it's running too hot you will see it. If it has the normal black/brown carbon build up on the piston crown, you are OK. If you have a new piston in the bike, it will take many miles to see anything.
I forgot on an M20 looking down the spark plug hole only allows you to see the intake valve. So bad advice on my part for this particular bike! If it was overhead valve, it would work.
There is no inlet track inside the head so looking down the plug hole with a colour tune works just fine & dandy.
I do it quite often but of course you have to run the engine from an alternative tank beacuse there is not enough space between the engine & fuel tank
Other problem is the Colortune was never meant to be used on a cast iron head of an air cooled engine so don't leave it there too long unless you want a plastic coated head .