Honda ch250 motor in a ch125/150 Frame 

I received an e-mail from Steve who had installed an Elite/Spacy ch 250 Honda motor into his CH125 scooter. Given that the original 125 had a little less than 11 horsepower and the 250 a little less than 20, the boost in power would be significant. I would imagine the performance would even be better than a stock 250 due to the lesser weight of the 125 scooter. As well as the engine transplant, Steve lengthened the wheelbase by about 6 inches.  Heres what Steve had to say:

 

"........ having read your ideas for tuning the engine of your Elite, I thought you might be interested to hear how i made my Spacy go faster, which i achieved by substituting a CH250 engine in place of the 125 motor. Don't worry the swap is easier than you think (if you want proof check the part number for the main engine pivot bolt for the CH125/150 and CH250 you'll find they're the same part number)- the 125 motor had seized solid and I couldn't replace or repair it. ........mine is a custom project which, apart from the engine swap, has a 6" chassis extension to allow a "ghetto blaster" to be mounted/played....... ..........your question on the frame lengthening, is interesting as the other reason i did this is the same as yours (I'm 6'4") and my method was to cut the main frame tube just in front of the engine mounting/rear subframe "box" area (this means you only have to cut and lengthen the floor board side rails - quite an easy form to fabricate in sheet steel) and welded into place a "spigotted" (the spigots I turned were each 1/2 the length of the extension and of a moderate interferance fit - more details soon) piece of thick wall steel tube, and light gauge steel channel section strips running from the front floorboard support brackets to the rear brackets to provide fixing and support to the piece of sheet plastic used for the floorboard. (* see note below)      As far as fitting the 250 motor in the chassis, you realise that if you do this you'll lose the centre stand (not a great hardship, if you consider your top speed will be an indicated 80 MPH - my motor is in pieces at the moment - i'm tuning it to FES 250 spec and beyond (even better top speed !!!). You'll need to use the 250 CDI unit but the rectifier is optional (mines the 125 unit and seems to work ok) and to start with use the carb from the 250 as well. I forgot to mention in my last e-mail that although this swap can be done retaining the footboard mounted rear brake  I've also converted mine to a handlebar mounted rear brake lever so as to have a completely clear floor (good for carrying 25 litre barrels of oil etc.)" I wrote Steven back to ask : ......I took a closer look at the 125 frame the other day after reading your letter. The spot where you chose to extend it makes a lot of sense as it leaves a number of spots to attach the front part of the frame( to a jig) to keep the frame aligned during the extension. I suspect that making up a jig to keep everything in alignment is going to be the real work.  ..... What did you use to jig it up or did you incorporate something into the spigot that was self-aligning? I originally thought of marking the top of the frame tube with a longtitudinal cut  before cutting and then matching the mark up with a corresponding mark running from one end of the spigot to the other........ ......I used a "rough & ready" method of using the top of the steering stem tube as my datum point and then using string took measurements to the engine swinging spindle and rear footboard mounting brackets adjusting the frame until measurements "side to side" were equal, tack welded then checked measurements again then finally finish welded, this has worked ok for me.   As far as the tuning work goes I've just fitted the **fes pistoned cylinder (i've put a layer of modelling clay on the crown to measure clearances - piston to head, valves to piston etc.) and head onto the motor to trial fit and i have noticed that the fes piston crown is not uniformally raised all over the "crown" (it's less raised where the spark plug area is on the **fes, but this is on the wrong side for the ch/cn motor - this may or may not be a problem.... note: *  modifications to the frame could affect safety and should only be attempted by professionals experienced with all aspects of modifying motorcycle frames. A friend and I built mini bikes in high school and he brazed his frame instead of welding it. It came apart while the bike was racing down a hill. Not a pretty sight but a good lesson for those contemplating a quick and dirty frame chop. While the concept is straightforward, the execution demands experience and skill.) ** fes 250 is the Honda foresight 250cc scooter

 I was extremely interested in this conversion as I'd acquired a ch250 with a bent frame but running engine and also a ch125 with a seized motor but a good frame. Bolting the two together as shown in the pictures below required a bit of work and fiddling as noted.
 
 

Nov/2001 ~ I've pried the motor out of my 1985 CH250 and started eyeballing the 1984 CH125 frame for what I have to do to make it fit. On my scooters, the engine hanging assemblys are not the same but the 125's mounting assembly should accept the 250 motor. The wiring looks like it should just plug in with a couple of exceptions. My 125 didn't come with a regulator/rectifier. The 1984 125 had a rectifier coupling unlike all later CH/CN scooters. The 1984 125 regulator had a single 6 pin connector whereas later scooters all had two separate connectors.  I do have a 1989 wiring harness that I can use to splice into mine so the rectifier from the 250 can be just "plugged in". If you have a later ch125/150 you should be OK using the stock regulator or the 250 regulator as the connectors should be identical. The rectifier, cdi unit from the 250 should fit into the same mounting as the 125. The engine won't fit into the frame with the centre kickstand. The kickstand mount has to be removed all the way back to the main frame "box". I'm still having trouble fitting the 250 shocks as the motor still doesn't drop low enough. The 125/150 shocks are shorter and fit. They don't look as robust as the 250 shocks and I only have one shock from the 125 that fits the 250 engine configuration so I'll try and think of a way to fit the 250 shocks. While removing the centre stand mount doesn't appear to affect the frames integrity, the 125 frame is not nearly as solid as the 250 frame so compromising the 125s frame further is not an option; if anything I'd like to strengthen it. This whole cut and fit method could be avoided if an engine hanger 2 inches longer than stock was fabricated.  The radiator hoses on the 250 were larger than the 125/150 so an adapter is going to have to be made to mate the two different sizes; either that or the 250 cooling system fitted to the 125. The later 150 frames had additional reinforcement around the frame uprights where the engine hanger assembly mounts.

 
 

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