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I cannot say i have done this but this guy has - give it a read it might work for you...

So you want more power on your 50, but don't have the $3,000 for a bigger clutch, new transmission, and big bore kit? Not a problem. If you're not afraid to get your hands dirty, here's what you can do. Honda made a great deal of 90, 110, and 125 engines back in the 60's 70's and 80's. The beauty of these is that the mounts are Similar to the z50/ct70, and the cylinder is arranged the same way (in front of the crank, not on top.) So How can you get your hands on one? Well you have a few choices. Clearly the 110 will make more power, and since they are newer, they are more reliable too; and probably in better shape.

The majority of the 90/110 engines produced found their ways into ATC110’s, which were a 3 wheeler, and since triangles don’t turn well, they are now retired. This is probably also the only place to find a 125. An ATC110 motor is what I have in my z50. The only complaints I have about them are:

1. The Foot pegs are too long and rigid, you can lean the bike on them like they’re a kickstand, and taking sharp turns or on trails with a lot of bumps, forget it. They can be cut, but even then, I had problems with my rear brake pedal. The clearance won’t be as much of a problem on lifted bikes.
2. It has a pull starter. It works, but come on, this is a Honda z110, not a briggs 3 hp home-made white trash mobile.
3. The exhaust is tough to adapt to the z, mostly because it goes under the crankcase, and then to the right.

The other 90/110 engines ended up in the CT90/110. This was the uglier big brother of the trail 70. If You know about the z50, I’m sure you’re familiar with the ct. The beauty of this bike (what little it has) is that it is a bike, and thus has good ground clearance, narrow foot pegs that aren’t rigid, a heel-toe shifter (A MUST for the 110’s stiff shifts), kick starter, and exhaust that’s easier to throw on to a Juiced up Z. Most of the CT engines produced, unfortunately, were 90’s (up to 78, I think). If you can find a new 110 (81 or later), Jump on it. Why? The newer bikes had CDI, and it greatly increases the reliability, ease of maintenance, etc. No matter which bike/trike/motor you use, Here’s what you are going to need:

1) An entire, complete engine as mentioned above. Clearly you can bend these rules a bit. If you’re a mechanic, or mechanically inclined, you can buy it in pieces, or non running and rebuild (it will probably need it), etc.

2) Means to start that engine (the ATC pull starters needed service regularly, and CT kick starters are notorious for being lost; or the splines being stripped). Also here you need the corresponding Shifter and foot pegs. Don’t try and put CT foot pegs and shifter on your ATC motor. It can be done (I did it), but they are going to be crooked (the foot pegs), and the ATC 110 shifters, when cut shorter, become near impossible to down shift. (kick upwards). If it’s going to be a street or pit bike, and you happen to be a welder, or have a torch, you CAN do a jockey shift (hand lever) and use the CT foot pegs, after they are severely bent. The reason I refer to the ATC engine with CT controls is because most of you will end up with an ATC engine, there’s just that many of them.

3) A 428 Style chain. (the ct chains were the longest. More on this later.)

4) Carburetor with ATC intake (I’m not sure about the ct, if they curve back and to the left, it’ll work.) Also, include a custom air filter. They have a Kitaco one that will fit the carb opening. ATC air boxes are just overkill.

5) A wiring harness from your same bike. Make sure you have a voltage regulator, unless you don’t care about the lights, etc. This is a must for street legal projects, tho.

 6) Some sort of exhaust. You will have to cut and bend parts of it, so it might be a better idea to have this part done by someone professional. My exhaust is a Straight (not literally straight) pipe with a flange welded that makes it 110 Size. It was brutally loud, so I put on a 2 stroke silencer I had an extra of. Now it sounds like a mean ATV or 4 Stroke bike.

 Okay, so you have everything, now to get started. Your best bet is to strip the bike. This might sound tough, but it’s worth it. I did this and it makes the measurements a lot easier. If anything, get a parts bike so you can screw up a few times and not feel it in the back pocket. My 79 z50r has full suspension, but the rigid bikes will be easier. You will have to drill 1 hole in the back of the frame near the motor mount, so if you cry when your bike gets muddy, find another hobby (like antiquing, baking or quilting.) Here is the HARDEST, MOST IMPORTANT part of the whole process. Make sure the rear tire assembly is intact, and straight. If your bike has rear suspension, connect the swingarm, shocks, and axle, all except the brakes. Now get a magic marker, and make sure your 110 crank cases are clean. Line up the chain from the rear wheel with the sprocket on the stroker motor. The sprocket is too big, and the chain won’t go on, so don’t bother. Put the frame on top of the engine so it all lines up. Now use the magic marker and draw a line on the top and rear mounts. To be safe, do the top first, and then cut it. Loosely bolt it, and do the rear. You will have to use a chop saw on these. Careful not to cut the crank cases too deeply, just the mounts. You will have to grind a little bit too. USE TAPE MEASURE OFTEN AND WRITE EVERYTHING DOWN. Okay, the top mount lines up and is loosely bolted. Now get the rear narrow enough so it’s still tight on the mount. It will not line up. Here is where you need to drill a hole. Same size as the others, make sure it lines up well. Your only guide is that the engine is close to parallel with the frame, similar to the 50 engine. Okay, make sure it bolts up, and now you’re ready to paint, etc. If you have a front fender, you can trim it or remove it. If you remove it, and you like to ride, get good goggles. If it’s for the street or pits, the front tire will be in the air so much, it barely matters.

Line up the wiring harness with the frame, and attach it however you like. If your bike has CDI, get a coil from a CT110, and try tucking it near the old coil location. This step takes a lot of creativity. A lot of these bikes have features the old one didn’t, so unless you have lights, brake lights, blinkers, and horn, you might have some un-used wires. On my bike I cut the wires shorter and re-crimped them, because I wanted it to look nice.

We’ve come so far. You have a rolling chassis of a z50, painted (maybe), and a 110 engine that bolts right into the frame. If you’re worried about the z50 bolts being loose in the 110 motor, you can get a bushing made up to eat up the space. As long as it’s tight, it’ll work fine anyways. See if the engine runs. If it does, you have little work left to do. If not, troubleshoot it now. Here, we run into a dilemma. Your front sprocket is 18 teeth, for a 428 chain. The rear is probably 37 teeth, 420 chain size. The pin spacing between the 428 and 420 chains is the same. The Width is not. If you don’t mind wearing out your rear sprocket, because of the huge torque, you can use the 428 chain. That’s what I did. Just make sure it’s the right length (use a chain break, and the master link) and make sure it’s tight. It may fall off if it’s not lined up or tight. Mine has on 1 occasion in the past 5 months, and I fixed it while on site, by lining up the rear tire again. If this is going to be a show bike (few people have one, just give me props if this is where you got the idea), then you might be able to locate a sprocket, or have one machined. I thought about grinding down the front sprocket to make it thinner, but they are hard to come by, so I chose to sacrifice the rear instead. You won’t be able to mount a ct110 sprocket on the rear hub, unless you’re an avid MIG/TIG welder. Even then, these sprockets had 44 teeth, and it will make your bike unbelievably slow (on the top end).

So you have it all lined up, it starts, has a chain, the gears work, the foot pegs are intact, the motor doesn’t shake, ETC. So now what? Exhaust. My best recommendation is bending a ct90/110 exhaust to curve above the right half engine side, then cutting off the old silencer, and replacing with a more space conservative one. My 2 stroke silencer sounds pretty mean, and it’s not TOO loud, just loud enough to let people know my z50 is one of a kind. This section is up to you. Once you get it satisfactory, you’re ready to ride. Make sure you find a way to hang the exhaust in the back, or it will mess up the flange on the front.

Okay, now for some general info, and FAQ’s.

Q: How fast does it go?

A: My bike with an 8 inch rear rim and stock tire, and 37 tooth sprocket, runs 44 mph. It feels quick on a z, but it’s not fast enough. With a 30 tooth rear sprocket, I hope for 63 mph or so. If you get an extended swing arm, 10 inch rim, and 37 tooth sprocket, my calculations say 53 mph. This makes it suitable for street (not highway) use. You will turn many heads.

Q: How much heavier is it?

A: I didn’t weigh it, but my bike probably gained 30 pounds or so with the swap. This puts its toll on the suspension; you may want to get aftermarket parts (from Kitaco).

Q: How quick is it? (Different than fast)

A: It’s not bad at all. Again, with my stock gearing, it goes 44 mph, and gets there fairly quick. You might be able to beat some small quads (300 ex’s maybe, at best), and a bunch of the 4X4 ones. For bikes, try messing with people on XR100’s, as they weigh a lot more. XR 75’s, 80’s, 50’s, all no problem. 2 cycle bikes? Don’t push more than a CR65. Anything else, you’ll be embarrassed.  As far as Wheelies go (I know you’ve been waiting), I can wheelie in 3 of 4 gears right now. 2^nd works best, 1^st has too much torque. The bike has low range. Unless you’re going to tow someone who broke down, you won’t use it. When you gear your bike up to go 65, you might.

Q: How can I make it faster?

A: Simply. They sell some parts to make the atc110 race. Since the ct is the same, they will all work. Aftermarket pistons are hard to find. First step, get a top end from an ATC 125 (some of the trx125’s might work). You will need the crank also. The only other thing you can do is port the heads, find a bigger cam (I got a big al’s stage 1, it works well in midrange), open the exhaust, and put on a bigger carb. Unfortunately, they don’t sell a CDI box for these, so if you get it capable, you’ll never hit more than 10,000 RPM, it’s rev limited. This is the only place where the takegawa xr50 kits have you. Another thing to try is shaving the head down, and the top of the cylinder for more compression. Try maybe .25-.50 mm. Check your valve clearance, and you may need a custom 77 link timing chain (it can be done).

Q: Will I beat my rich friends with their fast mini’s?

A: Most of them. The 113 superhead kit is the practical limit. The 124 cc DOHC kits do have 2 cam’s, 4 valves, rev to 14,000 rpm, and make 15.2 hp. Even with the ATC125 motor and all the parts you can find, good luck. You will save $$$$$ tho.

Q: Where can I find parts?

A: Ebay. OR junkyards are your best bet. They have some online stores that sell 50 parts, but they are all overpriced. With the money you saved, you can afford these now.

Q: How much???

A: I spent $200 on my atc 110 engine, with a rebuilt pull starter. The engine was rebuilt too, which I can easily do myself, but I didn’t feel like it on this one. Plan on spending another $150 on foot pegs, shifter, electrical, exhaust, and headaches tracking it all down. When painting an old bike save some money for that too. When you start to try and go faster, things get expensive. Also, they are VERY hard to find.

Q: How can I get more info and pictures on this project?

A: Contact me:

BigTonyGill@hotmail.com, or my Ebay name is “smallballsgill”. If you like quick typing, and youre under the age of 35 or so, you know what AIM is, and my screen name is “Gill Is The Man”.

Good Luck.

News - History - Projects & Restorations - Restore it HERE! - Parts & Info - Customs - Gallery - Links - My Mini-Trail - Email