experiential adventures learning about sustainability, balance and love.

CT-90 conversion to CT-140! yx140 engine swap.

From Machines

So here’s my 1970 Honda CT-90 (90cc) right around the first day I got it. Pretty nice bike eh? only 1000 original miles! OK, fair enough fair enough, but how fast does it go? Engine screaming like a deep water oil drill, she goes 45mph on a flat surface – drill baby, drill! Plenty of speed for the forest trails ’round these parts, but for the street?, that just didn’t satisfy me. I need MORE.

From Machines

OH NOES!!! I GOT A FLAT TIRE! Well that’s just perfect. While I’m changing out the tire I might as well replace EVERYTHING else on the bike, eh?! eh!!?? any objections!?!?!?

So back to what I was saying before the unfortunate tire accident – MORE. How much more? hmmm does 50 more cubic centimeters of metal muscle sound good? yes, ok, now we’re off to a good start.

From Machines

Nude CT-90. Ok, we’re gonna move along and give her some time to get decent.

*cue engine entrance* Meet my new friend! a YX-140 engine from… *shhhh*china*shhh*

From Machines

Ya know what else? That red color doesn’t really suit me… and there are some rust spots here and there… ooo! ooo! sandpaper party! everyone get your particulate masks on!

From Machines

Completely n00d and all sanded down by yours truely. Now she’s ready to party, I have a feeling she’s not going to remember this come morning.

From Machines

Aweeee snap. Rustoleum flat black – the blacker the berry, the sweeter the motorcycle. Motorcycle juice, mmm. (And a glass of Belgian Wit – loosens me up so I can chit chat with the paint canisters.)

From Machines

Above: The crank case vent was hitting the frame… can’t have that. So i pulled it out, and redrilled the hole at an angle (pretty roughly I must admit.) It wasn’t a perfect hole, or a very good seal, it was just plain shoddy workmanship… That’s where JB-WELD comes in. Turn that mechanical frown upside down, top dead center. I tried to have the engine tipped over on it’s side through this so that metal shaving and JB weld droppings didn’t find their way into my brand new engine… I think it was 95% successful, the other 5% is vitamins and minerals.

From Machines

Above: Now we just need to add some blinkers (in 1970 motorcycles didn’t need blinkers I guess?) So I bought a cheapie $30ish kit. Just need to drill a couple holes… and…

From Machines

Now we’re in business! That’s the money shot!

I’m still just breaking her in, so no top speed runs yet.

So here’s the run down:
New engine should get 100mpg. Hopefully I can cruise at 55mph in traffic (I will probably need to adjust the gearing.)
New engine kit cost : $300ish
Blinker kit: $30ish
ehh I don’t feel like doing a final cost run down, but it was worth it dammit! There are only about 20 parts on this entire motorcycle, and the tires are cheaper than tires for my bicycle (WTF?)

I got a lot of my parts from “DR ATV” and I got my yx-140 engine kit from T-Bolt USA which is the best deal I could find that included the carb, CDI, etc. I had good experiences ordering from both places.

Other than the engine vent needing to be moved, this engine fit PERFECTLY. The intake manifold and carb also fit PERFECTLY… well, when fully torqued the carb is just barely resting against the frame, so I may use a drill and slot the holes on the intake manifold where the manifold meets the engine. That should allow me to rotate the carb away from the frame about 1/2 a centimeter or whatever i need.

This page helped me swap things: http://www.dratv.com/ct.html as well as the Honda Clone yahoo group The honda clone group has an image folder with good (better) wiring diagrams.

This is totally my little dream machine now. It sounds really small, and it is, but it hauls ass, and is just as capable off-road as it is on the road. Insurance is DIRT cheap and I can ride 250 miles on the 2.5 gallon tank (for $7 dollars?)


13 responses

  1. Justin

    Great job! I’m a member of the yahoo group they helped me a lot as well. Please break it in and make another post on how fast it is, there is not a lot of info on that out there. I want to put this motor in my supercub!

    July 21, 2010 at 11:13 pm

    • malleabis

      Will do! Thanks for commenting!

      July 23, 2010 at 5:30 am

  2. Bub

    Hello and thanks for posting this! It is very timely as I have been thinking about doing this exact swap. I didn’t see any mention in your writeup about the clutch…if I’m not mistaken, all of these new engines have a clutch, which, of course, the original did not have. Did you have trouble hooking up a clutch lever/cable to the new engine?

    August 17, 2010 at 2:01 am

    • malleabis

      Hey bub. Yeah it has a real clutch! The stock left hand brake lever fits the clutch cable end just fine, so a new part isn’t even needed. I needed the “extra long” lifan clutch cable. picked it up from t-bolt b/c dr atv was out at the time.

      August 21, 2010 at 5:30 pm

  3. Brian Vickery

    Hello EM, today I took the 90 engine out. On Thursday I get the YX140 arriving Fedex from NJ all the way to Californy. What’s this about needing a longer clutch cable, did you raise the handlebars?
    You say the engine almost falls into place, could a dent in that frame be better than moving that air vent; or is it plausable to put a 90 bend in its place.
    How did the harness go ,, easy?
    28degrees at the moment changing motors … heartwarming work , cheers , Brian

    January 10, 2011 at 3:52 am

    • malleabis

      Hey Brian.

      Well my original ct-90 didn’t have a clutch, and my yx140 box didn’t have a clutch cable inside, so I had to order the “long” lifan clutch cable from t-bolt usa, fits fine. I tried denting my frame with a hammer and chisel, but the main frame tube is the part that hits the vent, and it was pretty thick steel… a 90-deg bend might work, but if I remember correctly, it was the metal barb fitting coming directly off the engine that hit, so there wasn’t much I could do πŸ™‚ I haven’t read about anyone else needing to do this though, so maybe my bike and engine were a little tighter than normal?

      The foot-peg mounting kit that DR ATV sells also didn’t fit perfectly, I had to grind a little metal off the foot pegs.

      January 23, 2011 at 4:55 pm

      • malleabis

        The harness was easy. The engine basically came with it’s own harness for the CDI, coil, stator. It will start and run without a battery also, so technically you don’t even have to connect it to the rest of the harness πŸ™‚ Wiring in a kill switch for the CDI is the main work to be done, from my memory… and of course the yx140 is 12v, not 6v. You have some options there, I just replaced every bulb and the flasher unit with 12v parts and connected a 12v batt.
        ***FYI*** – The stock yx140 stator WILL NOT keep the battery charged. The flywheel is also too light for street/trail use, the chance of stalling out is very high. I recommend getting a “regular” lifan flywheel and stator, which sell on ebay for $50 or so.

        January 23, 2011 at 5:02 pm

      • Brian Vickery

        G’day. at the moment I am trying to find why nothing from the stator. Nothing from the two yellow wires. Like was said previously, need to get another stator. At the moment it will run off the battery.
        I too had fit-up problems. I had to put brackets on the lower rear mount ,I also had to cut off a 1/4″ from the rear mount to get the drive sprocket to align with the final rear wheel sprocket; I had to slot, carefully the intake manifold holes so that the carbyturned and missed that frame. Also had to make an adjuster at the carby for the throttle cable to work. I Chabged out the fullwave rectifier at the battery replaced a blown fuse. Now waiting on a stator fix.. Feb 1st.. Cheers B

        February 1, 2011 at 9:24 pm

      • Brian Vickery

        Well , it is running like a top. Great motor from Mike S at G2 Moto.
        His help and knowledge created a good work environment, and it all came together well. It sure does not feel like the same bike.
        But, like riding a strange horse for the first time.. ‘takes a little time and all will be OK. I’d like to post a pic.. if I can find a way; anyway Cheers and if anyone needs some info just mail me at

        Bye for now.. B

        February 3, 2011 at 2:20 am

      • I installed the same engine in my ’83 Passport same issue with frame. I took the pressed in vent pipe out and cut about a 1/2 inch off the end. Then I cut a grove about 1/2 from the remaining end and safety wired some clear hose to vent. I just tapped the modified pressed in pipe back in and used red locktite to keep it on. With only about 120 miles so far it seems OK. I’m shocked how much air comes through the thing.

        April 9, 2012 at 1:48 pm

  4. holden

    does it tear it up off road still?

    July 31, 2011 at 7:41 am

    • malleabis

      you know it πŸ™‚

      November 17, 2011 at 5:56 am

  5. Tell me more about the rectifier if you could. I’m having problems with charging also? I get about 15vac off the two yellow wires from the stator. How much should it be putting out?

    April 9, 2012 at 1:53 pm

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