I wholeheartedly agree that stage one, or 10hp should be the limit. I say that because there is no going back now.
The ready built ‘Stage One’ engine from the GX Tuning Store is actually 9+h.p. not 10 but I bet it’s close enough to make no perceptible difference.
That engine costs £390 (including VAT) and comprises the following parts:
Loncin (Clone) GX200 Engine,
Size 75 Main Jet
140 Emulsion Tube
18Lb Valve Springs
4° Advanced Timing Key
A 010" Thin Cylinder-Head Gasket
In addition, they machine the valve seats to three angled ones one a very expensive machine, lap the valves in and remove the baffles from the exhaust.
You can get all the parts separately (all off the net) and they costs:
Clone engine £160
75 Main jet £5
140 Emulsion Tube £5
18lb Valve Springs £18 (includes £10postage!)
4° Adv. Timing Key £10
Total for bits £198
The three angle valve seats give the much the same benefit as radiusing the valve seats.
Opening-up and smoothing-out the ports is a couple of hours work with a ball cutter in your drill and some emery cloth. (You only need to do the inlet really).
So, you have £200 left in your pocket!
If you buy a GX200 clone they have 68mm diameter piston, the same as the Honda original but if you buy a 212cc version (Lifan I think) they are 7h.p. and have a 70mm diameter piston. Everything else is the same. We have a few of these in the club so I would go for one of these. (Actually I did, it’s in my Renault)
Torque is directly proportional to piston area so there is around a 2% benefit in torque. Not a lot I know. I don’t feel the difference anyway.