So, my crazy machinist buddie wants me to tear town this half-assed project he started before he became a machinist and rebuild the engine while he brings the chassis to the shop. It's a Yamaha Moto 4 (YFM80) 80cc four stroke made between 1985-1988 however he ditched the original motor for a Yamaha RT100 100cc air cooled two stroke motor (made between 1990-1998) but I can not confirm a year (RT100 Serial # 8UL-091478). There is no suspension so while I am doing a COMPLETE overhaul on the RT100 engine he will be fabbing up the YFM chassis rear end to accept an 1985/1986 LT250R aluminum swing arm (direct shock we are not using the linkage) and will probably use an LT230 rear axle since I THINK it's shorter than the 250R, right? Also I will be drilling a stem hole in the rear side of the front rims and flipping them inverted for a few extra inches of width in the front end to help match the new rear width! You can see the RT100 oil injection reservoir/tank attached to the YFM frame, that will be ripped back off and thrown away because I am not trusting an oil injector with a heavily built motor we will be premixing all our fuel. Original plastics will be replaced...the twist throttle, brake and clutch levers are off a DRZ-250 bike. The seat is deff built for an adult so a 100cc two stroke should be more then enough for a play bike of this size (probably comparable to a 150cc four stroke give or take):

Future YFM100R:




What they looked like new:




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That is a TON of hours spent fabbing, misc. hard to find parts...etc. Would have been easier to just start with a bigger, better set up quad. just my .02

 

How do you figure? The chassis is lunch break material nothing at all to a good welder. Parts will not be hard to find since I am not using the original motor, what a wheel bearing? I don't think this will be complicated at all compared to things we have built in the past! There is no frame this size with suspension and putting a liquid cooled engine in would be a nightmare so I think this combo is perfect.

 

I pulled the motor and sent the chassis to his shop, red marks the cuts for now so I can finagle in the 250r swinger (which is wider than the frame haha):





In the following photo the red bars will be the new structure, minus any changes the engineer makes at the shop. The shock mount will be last in this process after the swinger is mounted, some sort of a small MX shock with an overflow (KX80 or similar/smaller):

 

It came back today for some mock-up with the swinger now that the rear end is open, there won't be much swing arm travel but the intention is to extend the wheel base instead of adding a wheelie bar. It will still serve as a cushion for takeoff but the chassis itself is limited by the lack of front suspension. The front tires are on a 1" board because they are balled 17" tires and I will be replacing them with new tred 18" tires (tallest I can find for a 7" rim), that leaves the rear axle 9" off the ground as you see it in the photo. The front axle as seen in photo is 10" off the ground, with 9" in the rear I can fit 18" tires back there to match the front but I will probably lean more towards wide 16" tires with more swing arm angle:



Top frame bar continuance still needs to be welded, and a shock support:



Some sort of knuckles will need to be securely welded to the frame:



Not only are we using an LT250R swing arm but also the front bumper/guard. It will be welded on and the ends will be cut off:

 

I love cool projects that are different. I'm not sure I completely understand this project, I would have started with something bigger, but I'd love to see it when it's done. What do you plan to do with it when you finish it?

 

But then it's not a "mini quad" (4 wheel play bike) I plan on using it to get hurt, I hope my friends get hurt on it too lol everyone that sheds blood has to put a blood fingerprint on the frame rail and I will clear coat them in HA! Out of all seriousness though just a weekend toy, drag a straight line or up sand hills not trying to outperform similar size quads. It is more the fact we already had this chassis and an RT100...it was a drunken idea that spiraled and now we are committed. I drove it with the RT100 engine hose clamped into this frame with no expansion chamber it was loud as **** and probably did some damage, had the RT100 sprocket macgyvered onto the axle it was a mess but we built it in a few hours (years ago) I could have ultimately started with something like an LT80 chassis...but this is more unique.


Here is a render that took me quite a while to produce of what the final product will look like. I went with a Quadzilla paint scheme and some Banshee headlights, if you look I added bar risers as well. The front fenders will be cut to replicate Quadzilla quarter fenders, I left the original engine in this rendering it would be a lot of work to put the RT100 engine in. If you look to the rear you will see I used the exact photo I took of the mockup for the swing arm (ripped sticker) the shock is of random but I colored red (the handle bars and RT100 engine will be red as well) I don't think the stock shock location will work for me I might get stuck using the linkage mount. I added an FMF Shorty since we're going 2-stroke that will most likely be our choice, I also cut the curved end of the rear fenders so they are straight. The rear wheel is accurate to where it is going to be if I did the math right, I tried to keep the whole thing as scale as possible:

 

Ha ha i love drunken ideas, they are not always practical once you sober up, though lol. If it turns out like your rendering it will be the coolest moto-4 80 I have ever seen!

 

It will, I can guarantee it. I did a similar color scheme on an LT230S last year:

 

I disassembled the top end and started on the transmission, have not removed the clutch yet. I have a very important question...does the oil pump change the ratio at different RPMs at all? With it being controlled by a throttle cable "Y" splitter you would think the throttle effects the ratio somehow. I hope not because I am ditching the pump for premix fuel:



The oil pump is driven by a plastic drive gear that runs off the crankshaft...why plastic? They do this on the LT250R too; the coolant pump is ran off a plastic gear. I may try to run some sort of lighting coil off this shaft and run the wires out the top where the oil tubes went through a grommet, because I'm pretty sure the RT100 had no lights right? If I don't do that I will be plug welding the shaft hole up to retain oil WITHOUT an oil pump plastic driven gear and drive shaft:



On the inside I will pull out the oil seal and plug weld from that side too:



Can anyone tell me what this unused "section" is in this clutch cover? It clearly holds something in another model engine, looks like a clutch release worm gear:



Backside of the unused "section" molding, also circled in red is the threaded cable hole for the oil pump (will be plug welded as well):



From the outside you can see the large plugged hole circled in red that leads into the unused "section" you can also see the threaded oil pump cable hole circled in red that needs to be plug welded: