Hi,
Try borrowing a tach that you know is good, lots of tachs are SLOW with the needle movement and are up to 1,000 rpm slow on a fast reving engine. I use a test set/tach/dwell meter from 25 years ago and it says my autometer tach is 400 rpm slow. Another way is to hook up a shift light with different rpm pills to see when you are shifting.
Good Luck,
Perry.
PS- try autolite #25 plugs at .040, toss the ngk's