Dave, I have resurrected a YS60 two-stroke that I last ran in 1987! I took it apart and thoroughly cleaned it. I ran it today on a test bench but had some problems with throttling. I did let it build up revs over several tanks to let it all bed in again.

Top end power is still phenomenal, I got 11,500rpm on a 12x10 with no nitro, on the original hattori pipe and OS8 plug

But it will not pick up from idle, and min idle is about 6000rpm, anything lower and it quits almost instantly. At low revs it is spraying fuel up out of the carb. If I go to full power the butterfly is wide open but it doesn't pick up one rpm. If I pinch the fuel pipe to lean it it jumps and hits full power. I ended up with the regulator screwed all the way in and it still behaves like it is super rich at mid to low throttle.

As hard as I can blow I can't blow fuel through the fuel inlet through the regulator which indicates the valve is seating ok and not leaking.

When I got home I dismantled the regulator and carb completely but there is no trace of a speck of dirt and I can blow air and fuel freely through every passageway. The one way valve is clean and working, and the fuel tank and plumbing hold pressure for as long as I leave it.

The regulator is a bit different to the usual, I have attached a photo that shows it has a knurled knob outside of the housing and can't be set to a default flush with the housing which is the usual advice.

have you any ideas what I should look at?

thanks

Harry