A friend of mine is having problems with his MVVS 1.60 gasser.
The engine is having problems with the midrange... the top end is fine and low end is fine but mid range surges .
The needle settings --the low speed is turned out about 1.75 turns and the High speed about 2.5 turns.

running 91octane fuel with a 40:1 synthetic mix.
prop is 18x8 , the exhaust is the original box muffler but with 3 outlets drilled out (instead of one).

any help would be much appreciated.