I have a 3w-42 and right out of the box I was getting 6800 on a Mejzlik (Still can't spell it) 20x10.
That Classic prop of yours is good for Warbirds scale looks but does not do well with my 3w's.
I have about 2 gallons through the 3w-42 and it has increased to 7000. Let it run at least 5 gallons through it and also go for a different prop for a while.

http://pages.prodigy.net/gcleveland_grsmodels/

The above is an address for GRS... they make a little narrower prop that look exactly like Mejzliks, just cheaper. They are in the Southern USA.

I use their 20x10 to break my engine in then switch back.

Good Luck
Hal