if you have the correct reach spark plug carbon build up should not occur, if you use a short reach plug in a long reach setting then you will get build up and other thread damage too.
Uneven compression = lumpy idle whatever you do you - you can sort of tune it out at idle a bit with the mixtures but it's still going to be uneven driving. Ref firing order 2 lowest following 2 highest will be lumpier than high, low, high, low. Mine are all within 3psi = smooth idle is possible by tuning the carbs and it runs very smoothly too.
Please refer to en earlier thread , same question, all ready to rebuild it , I told them thrash it , a week later compression was even and it was freeeeee
The compression tester cross threading is something that is on the borderline all the time for me- usually the thread on the tester if brass isnt much harder than the cylinder head. Also my tester is now about 35 years old, the threads are not as sharp as when new. I always have to spin it backwards until I feel the thread click down and the tester adapter straighten up. And then I wind it in using the hose so if it is cross threaded it wont go. Same with plugs, feel them into the thread using finger tips, this is easier for me because I use Taylor Spiro Pro leads that need the cap screwed on the spark plug to grip it, so there is a bigger thing for my fingers to grip.
