1937 16h - too hot?

Hi Gary,

   The spark plug grade doesn't affect the temperature of the engine, the 2 main reasons for running hot are weak fuel mixture or retarded ignition. First you should check your ignition timing is correct, a good indication of this is if you pull back the lever to full advance, the bike should kick back when starting. The magneto on your bike is a tight wire advance, which in my opinion is the "Devil's Work". This needs the lever to be tightened so it is hard to move because vibration can make this work back to the retarded position if loose. When you are happy the timing is correct, check your spark plug colour. If it is running weak the plug colour will be a light brown colour. The correct colour shoul be dark brown, but a sooty black will be too rich. 

They do run hot.  But as Richard says: check that ignition.  It should be set to open at 7/16" on full advance.  You can set it to 1/2" if you want to experiment. You can always back it off a little.

At least one book says (wrongly) 7/32".  That makes it hot and sluggish.

I use SAE 50. as with all Norton singles..

A Champion D 16 plug

And always E 0 petrol.. Ethanol may weaken the mixture.

Yes, they do run a bit  hot........