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.
Some things to note. The vinyl on this seat was pretty old and brittle in some places. In most cases as soon as I put the Gojo on it somewhat seamed to soften. If the material had already hardened it didnt seam to soften it much.
With the engine configured to use the factory governor system, it is necessary to implement a manual choke. The stock system has the throttle in the wide open posistion at start, then the engine speed pushes it closed once it starts. Because of this, there isnt a strong enough vacuum signal across the main jet to pull fuel into the carb body, so a choke is required.
I purchased a push mower universal throttle cable kit. The kit I used comes with several different levers to use, I modifed one of them to use as the termination at the carburetor.
Simply cut the cable to length and bend a “Z” at the end using 2 needle nose pliers. I put a small hole into the stock choke lever and inserted the Z into that.
For the fuel line:
Geoff at Vegas Carts recomends using the spacer between the carb and head to source the vacuum for the fuel pump. I tapped it to 1/8″ npt, and used a 1/8″ npt x 3/16″ barbed tube elbow. Using this location will generate too much pressure from the pump, so a “relief” line needs to be installed. I didnt want it to relieve all the fuel pressure so I used a reducing tee to reduce the 3/16″ tube down to 1/8″ for the return. I then used a bored thru tube compression fitting and installed the relief tube into the tank.
I do not like the location that I put the relief line in the tank. If i was to do it again I would put it lower in the tank to keep air out of the line.
Take the engines old throttle lever off, take the first section of the golf carts throttle cable, the one that ran from switch box to the rearends speed control, and connect it to the engines governor spring. Use the engines existing cable clamp to hold the golf cart throttle cable down. This is in the weaker part of the cable so use care when tightening.
This worked perfect! You can be easy on the throttle and just creep around, or nail it and it takes off wide open, and then the governor kicks in like normal.
The engines choke will need to be utilized for smooth starting. You could possibly use the factory airbox and “choke”, I havent decided which route to head.
I cut out a flange of 3/16″ plate and custom bent the 1-1/4″ tube.
We bent it 180 degrees to send it back under the engine
Then a couple 45 degree bends to send it out the rear
Muffler/silencer and tail pipe are still in the works
Started with a 440CC GX390 clone from Vegas Carts, and the tapered adapter to retain the factory clutch. The engine comes with SST valves and stiffer valve springs, 20HP out of the box for $399 to the door. It arrives without the head bolted on, and the standard steel valves and weaker springs in place. The stainless valves and stiffer springs are shipped seperate. I saved the stainless steel valves for my GX390 alcohol burning race kart engine.
After trimming the factory crankshaft for use with the adapter, to retain the factory clutch and variator, next is a mount to adapt to the engine to cradle.
The Vegas 440cc does not have the same base bolt pattern as a Honda GX390 or clone. All you need to do is raise the engine 2″ to get the crankshaft to the original position. I did this by welding 2 pieces of 2″x2″x3/16″ angle with a plate between them. I put slots in the plate (new engine side) running parrallel with the crankshaft. The cradle has slots in it from the factory running the oter direction, so you can just put holes in the lower side of the mount, this will give you plenty of adjustment. For this model cart you want 9-5/8″ from crank centerline, to the centerline of drven pulley on the axle.
Next are the brackets to mount the factory starter generator. The GX engine typically has a fuel tank mounted on top, Vegas engine is a little different and made mounting much easier. We used some flat plate bent with a hammer in a vice. We were able to use the factory adjustment plate with the slot so that made it that much easier.