Honda CG125 Engine tune

If time permits this engine will be run in the highly enjoyable moped racing series where 125cc single cylinder 2 valve engines are allowed to compete with 50cc 2 strokes, where the little Honda is already proving to be popular.  We have  had a 1992 model here for some time and when we had to condemn a local lads Chinese copy, due to an overdose of monkey metal, it seemed too good an excuse to let go.

The pictures feature the Chinese engine which has the same specs as the "Brazilian" Honda's:

Bore 56.5mm
Stroke 49.5mm
Capacity 124cc
Compression Ratio (CR) 9.2:1

Inlet valve 27mm, opens at TDC, closes at 30 ABDC
Exhaust valve 24mm, opens at 30 BBDC, closes at TDC

Tuning Options

Capacity Increase
The class limit is 125cc so there's little room here for a power increase!

Compression Ratio
As moped race regs state that pump fuel only to be used we won't be chasing a CR of 14:1, but we will increase this a little.

Valves
For the moment valve sizes will remain the same, however, the valves are bi-metallic and badly finished above the valve head where the stem is welded. Rather than just clean this up we will undercut the stem and regrind the ports to further aid flow. Valve guides will also be machined back in order to reduce their intrusion into the ports.

Both inlet and exhaust valves are operated by a single cam, compromising power output so we are looking to "split" the cam into 2 allowing independent timing and with new profiles to increase duration.

Head Design
Both ports have a shallow angle with very little in the way of flowing radii. Ideally on the inlet side would be a downdraught conversion, cutting a new inlet port at a steeper angle, but until some careful measuring is done we will be looking at filling and grinding the existing port to smooth the entry flow.



The combustion chamber is of a domed construction with the spark plug to one side and a narrow squish band around the perimeter leaving a relatively large volume away from the spark plug at TDC, this means a long distance for the flame front to travel upon ignition robbing us of our much craved performance. We will fill the chamber by welding on the opposite side to the spark plug in order to move the compressed charge nearer to the spark for ignition, this will in turn raise the compression ratio, unfortunately above the ratio we want, so we will need to re-introduce this volume (rather a part of) into the combustion chamber. If we do this by raising the head we can get the CR we want and also increase the valve to piston clearance, allowing for a higher valve lift if necessary.

TBC...