SailorHarry

Hello again, I wanted to let you know how this turned out. Unfortunately I must report I didn't wind up going with the Yamaha 175 motor. The one at the junkyard was in fact a version with points but the cylinder head was missing as were all sorts of other parts. Really all the Yamaha motors were just bits and pieces and none of them looked very promising. They did have a complete 1980 Honda CR80R however which I took home. The engine needs work but at least it came complete bike so no worries about tracking down missing parts. It might be better for my application as the rear mount locates the swing arm and looks pretty beefy. I was wanting something a little bigger but 80cc should be "enough." Thanks for the advice though.

I do appreciate your opinion. After looking at the frame and a little more thought I have come up with a plan to build a braced framework to support the engine from beneath. It still won't be using all of the engine mounts but I think it will work. Unless someone can say they've tried that and it didn't work, I'm going for it. After all if I took everyone's advice I wouldn't have a motorcycle at all. Next week sometime I"ll be picking up the engine and will no doubt need more advice about freshening it up. Stay tuned for the opportunity to say "I told you so."

That had occurred to me and is definitely a concern. Your engine is definitely the same kind I was looking at. The upper rear mount doesn't look any less stout than the one on the CRF50 style engine which is designed to hang from only the two rear mounts. Supporting the weight of the engine is not the concern as the 175 is lighter if anything. My thinking about the forces involved in acceleration: the reaction to the engine pulling the chain forward will be the upper rear mount being pushed back against the frame while the lower rear mount is under tension as the engine wants to rotate "upward". Thus the likely failure point would be the lower rear mount potentially being torn off. Perhaps a steel brace could be ran underneath the engine tying the lower rear mount to the lower forward mount while also providing me a place to attach the foot pegs. The weight of the engine would still be supported mostly by the upper mount but both lower mounts would be keeping the engine from twisting itself loose under acceleration. Thoughts? Does my analysis of the forces involved seem correct? I suppose I could even calculate the potential load on the chain and the torque the engine would need to resist but without knowing the breaking strength of the engine mount what's the point. Of the engines I saw at the junkyard this was the one that seemed to be the best suited for me. To find something closer would require going with an engine from the smaller 50cc-85cc class which are typically more horizontal like the CRF50. A complete 1978 Suzuki JR 50 just appeared on craigslist locally, but I don't think those even have a proper transmission. Thanks for the pictures, it looks exactly like the engines I was checking out at the junk yard...except much prettier of course. But that settles it, I'll look for a VM24SS. My only demand as far as power goes is the ability to cruise at 70mph for the occasional short trip on the highway. That seems achievable with the 15-18hp I've seen quoted for the 175 provided the gearing is right. Yes that would work great, my only problem being the lack of a place to hide an oil tank. I could go one of two ways with it: either mount a bottle of some sort on the frame or modify the frame to make the seat support tube into the oil tank. But that's getting ahead of myself. Thanks for the cynical eye.

Thanks for all the great information. I'll go through the issues one by one... First, the carburetor. Those ID numbers are good to know but I don't expect to find the exact correct one. I am expecting to spend some time on jetting and tuning so no worries there. In my research online I found one source saying the 175 came with a VM30 and another saying a HSR 40 which didn't sound right at all. Since I have the luxury of picking which I want, is there any advantage to going a little larger such as a VM26? I'm not concerned with all-out power; what I'm after is solid mid-range driveability. But power is addictive and if I want more down the road it would be good to be ready. Now about the transmission. Again the web sources I found were contradictory but it does sound like the 175 is the most likely to have six speeds. Most of the engines were missing the cover with the shift pattern but one 125 that had it showed a five speed arrangement. About the oil pump, I plan on running premix initially but it would be nice to have the option of going to autoinjection down the road. There was a fairly wide variety of engines that were all clearly related but slightly different. Some had provisions for the oil pump, some didn't, one of the 125's had two spark plugs, etc. If the engine I select can have an oil pump I'll probably have to take it from another engine. Finally, that's an interesting idea about getting an engine with points. I had planned on avoiding anything with points because I didn't want the bother, but then finding the CDI may be a bother as well. How large of a difference does it make in performance and drivability? Are there inexpensive points replacement units available for these engines? In case you are curious about the bike this will be going in, here's a link to a picture. I will only be using two of the engine mounts and I may have to use a bit of a down-angle to make room for the carburetor. Harry

Hello all, I just joined seeking help with a little project I'm getting myself into. See, I just bought a little motorcycle called a Sachs MadAss. It came with a broken 50cc engine and my plan is to swap something different into it. They come with a Chinese copy of the old Honda "monkey bike" engine, sometimes called the CRF50, in either 50 or 125cc. There are a number of hot engines available from various Chinese engine suppliers ranging from 140cc to 160cc. But I'm not interested in a cheap little Chinese engine. I've always wanted a two-stroke motorcycle and this bike is just so small and light it would be like a dirt bike for the street. With the engine but no fluids it only weights 212 pounds. Today I went to a local motorcycle salvage yard looking into my options. The engine should be two stroke, air cooled, from 100cc to 200cc. I brought a cardboard template of the space available and measurements of the engine mounts. There was a good number of smaller Yamaha engines that seemed to fit my needs well and might just fit. There were a number of engines stamped 123cc and one stamped 171cc that looked to be from the same "family." Fitting one into my frame will take some fabrication and creative thinking but that's the whole point of this project. After coming home and doing some research I believe the 171cc is from a 1980ish DT 175, as they were only sold here in the US around that year. It should have an extra gear and some more power over the 125's and that's always a good thing. I also really like the fact that the engine was produced from the 1970s into the 2000s. There are a number of web articles that speak very highly of the engine as a long-lived, durable workhorse that is little known in the US but very popular around the world. This should make for good parts availability, aftermarket support, and hopefully some wise advice from knowledgable folks such as yourselves. So here are the questions. The salvage yard is not terribly organized. There's basically a pile of engines over here, a pile of carbs over there, a pile of wiring harnesses on the floor, and the guy who runs it claims to know nothing about dirt bikes. I get to mix and match whatever I can find and take it all home for $125. The 171cc engine is basically complete but I might have to take a side cover and other parts off one of the 123cc engines. There is a complete Yamaha air-cooled 250 dirt bike but that engine is too large for my purposes. Would any parts from it work with the 175 engine? Otherwise what carburetor should I look for? How do I pick the ignition module out of a pile? Anything else I absolutely have to grab while I have a warehouse full of parts to draw from? Thanks a lot of any help and I look forward to sharing my project with the group.