Picked up a stock 1975 CB125 with low compression and a junk front end.
I decided to build a street tracker with the stock 125 engine modifed as much as safely possible.
Sourced this145CC cylinder and piston kit for a cb145, which is a euro version i guess? Bored cases to accept the bigger cylinder and it worked out perfect.
Next was the head, I used the head from an 86-87 ATC200X, milled .080″. This is too much! I had to cut a lot of material from the head and the piston for clearance.
In order to use the stock ignition system, you have to cut the end of the camshaft off. You want to cut the cam where the machined groove is,about 3/8″ from the end, cam chain side. Then you can assemble it like it was still stock.
Ordered this chinese knock off of an XR200 carb with throttle cable and K&N filter, and changed the main jet to a 75.
I ditched the stock muffler and went with an Emgo ,
type but it iwas made for a larger diameter head pipe, so I welded a piece of 1-1/4″ pipe to it so I could get the larger diameter needed.
A 4-speed manual clutch with the provisions for an oil cooler was purchased from ebay smilar to this 125cc Lifan
The ST90 engine mounts to the frame with 4 bolts, instead of the usual 3 bolts that the other Honda horizontal engines mount with (CT70,XR50).
The lower rear bolt holes can be used with a spacer, I made brackets that bolt to the engine with a hole to match the frames stock mounting hole.
Exhaust is a custom bent 1-1/4″ Stainless steel header matched to a ZZZ muffler.
Position of the oil cooler is a bit of a problem. The best and cleanest location is to mount it directly in front of the head using stand offs. We welded 7mm X 1.0 thread nuts to one side ofa 3/8″ tube, and an M6 nut to the other, and simply bolted the cooler to it.