80 Psi is great for that model and it’s firing, so those two factors are covered. Surging (and or backfiring/popping) at any speed is usually a sign of lean mixture, and you say the low speed needle is 1/2 turn open which too lean. Have you tried opening the low speed needle to 3/4 turn where it should be?
And try cleaning out the check valve and screen as aquasonic suggested. And the line from the check valve to the low speed orifice may also be blocked with dirt or old varnish so it should also be cleaned out. And are you sure that fuel is getting to the high speed needle?
My wife was raised in Grand Rapids and we are in Traverse City. Give me a call by phone and maybe we can figure it out. I am a very slow typist. 231-929-7479
Jim, I strongly advise you to take up Lloyd on his offer. There is no one more knowledgeable about these motors.
Improvise-Adapt-Overcome
