GX390 Stage 1 | What RPM/HP should I expect from my engine?


New member
Just about to hit stage 1 with my engine in its project, what rpm should I turn at the maximum and any idea what horsepower I would get from it?

Loncin GX390 F-P
Governer and low oil sensor removed.
Fuel pulse pump
Robertson 'Candy Cane' Exhaust
High air flow filter
HP Lash caps
PolyChromium push rods
Billet guide plate
Reinforced rocker arms 1.1:1
stock carb and stock jetting
Dual 65lb valve springs(.375 lift)
Stock jetting will be way too lean. Put in a 100 or a 102 jet (.040 ish). Thats a lot of valve spring too. Stock springs will allow 5000 to 5200 rpm and thats about all you can get from a stock rod for any amount of time. 390s dont need or want high rpm. Get a billet rod. Stock rods will not last with mods. These engines have about 8 to 1 compression in stock form. Raising that will yield added power easiest. Shave the head and/or thinner head gasket and/or flat top piston. Port the head but dont get crazy. Get a bigger carb. Lectron or Mikuni slide. at least 33mm. 36 or 38 would be better.

As far as HP goes. A stock engine with governor removed , open air filter and header with stock carb, if everything is perfectly tuned will make just shy of 18 hp.