Increasing compression will also help, especially with top end hp. Cap it off with an 18mm carb, K&N filter and a decent exhaust and you should see 50mph on the flat.Īdding spark advance, either the old mechanical type as used with the 'H' motors or CDI type will make the engine rev better and have crisper throttle response. 72cc is still small displacement, so you'll need to rev the motor fairly high to make power.

Once the ports have been improved, you'll need a hairier cam to take advantage of the increased breathing capability. Do not grind away the exposed portions of the valve guides. The intake port should be left with a relatively rough finish for fuel atomization the exhaust port can be polished smooth. This can be done on the exhaust side as well. You want to begin by removing all casting flash, then open the short-side raidus of the port just beneath the valve, then enlarging the manifold end and blending the two ends together in a smooth transition. It may be possible to go as large as 25mm, depending upon the specific head on your motor.Įven with the stock size intake valve, opening up the ports can really wake up the motor.

As I recall, the largest intake valves Honda ever used with the 72cc engine was 23mm. A competent performance engine builder should be able to figure it out for you, though the costs may add-up. It is possible to fit larger valves, but this is a can of worms as there are variations in head design over the years. The biggest power gains will come from port work.

Eastwood cylinder head porting kit