(long post, I have a ton of progress pics that I will slowly add but for now just posting the nearly finished result)
Figured I'd post a project I'm almost done with on here for all you moped heads. I traditionally build cars and motorcycles, but have spent the last 3 years or so tinkering with both 2stroke and 4stroke motorized bicycles. Over a year ago a buddy with a Tomos that sat for a long time was moving and needed to get rid of it, so knowing absolutely nothing about it I picked up a complete '92 for real cheap since the motor was seized up and it didn't run. I had no idea what I was gonna do with it until about 2 months ago, when I got the idea to make a motorized "brother" for my '96 GT Dyno bicycle that I have had since new in 96 as a kid. I still have a bit to do to it, but my original goal of a fun little 60mph ripper has already been achieved and I figured some of you guys might get a kick out of it.
Started with a 1999 GT Dyno bare frame I purchased used online. Went with the 99 since it looks close enough to the 96 on the surface, but is way cheaper since the 97 and older are US made frames that are much more expensive and harder to find. Cut off the brake mounts and smoothed them down. Stretched it 2" behind the seat post and widened it 1/2" to clear a DOT moped tire, then on the front end lengthened it 1" more and cut off the stock head tube and moved the complete Tomos front end to it while adding quite a bit of rake - took it from the nearly vertical BMX head tube to 26* to keep it stable at speed and bring the trail numbers to a safer level.
Cut down the bottom bracket and welded in some motor plates, which I boxed for strength and surrounded the seat post with a notched area on the back to mount the rectifier and flasher relay. Welded on some tabs for the axle adjusters to keep the wheel from pulling forward, and a tab for a steering damper, cooked up some custom decals on my vinyl plotter, and that about wrapped up the frame mods.
(will post modded+primered frame pic here)
For the motor, the jug was rusted out and the points system was stuck so I headed over to Treats and ordered the 45mm alukit w/ single ring piston and converted the old points setup to the A55 starter+flywheel and upgraded to a Parmaket CDI to pull some timing up top since it will be a high rpm build (aiming for ~11k once I start tweaking the port map). Also picked up a Jammer 1st gear clutch, and installed it with the medium springs along with a red spring in the 2nd gear centri to hold 1st gear longer. Kind of bummed about the physical gearing limitations, but will work with what I got lol.
Intake is a custom piece that I made by cutting the mounting flange off the stock intake at the widest point, then welding a much larger 4pedal reed box setup to it with a 3d printed stuffer to keep velocity up. It would never fit on a Tomos frame, but since I was modifying the frame at the same time I was able to place it in a way that would make it work. Carb is just a simple and cheap PHBG 19mm with a K&N, currently jetted rich for break in so there is some slight 4stroking when cold. It mostly cleans up after a lap or two around the block, but once I get some miles on it I will drop 1-2 jets on both the main and the slow to start tuning for power.
For the pipe, instead of a long header jog style pipe that favors mid range, I modified a YZ85 dirtbike pipe that screams up top. The Jammer clutch slips right into grabbing around 5k, the pipe starts to hit around 6500rpm, and pulls surprisingly hard to 10k+ rpm. Currently with stock 26t front sprocket and 20t rear, it hit 56mph before I left off for an intersection. Expecting ~65mph with a 30t front once I clearance the case for it...not too bad for a stock jug IMO, might be able to squeeze 70mph out of it with port work. Complete bike ready to ride is 89lbs, so a little over 20% lighter than a Tomos assuming 115lbs(not sure if thats dry or wet weight for the Tomos since I didn't weigh it before chopping it up lol).
That's about it...any questions or suggestions if I did something stupid let me know LOL. I have a fair amount of 2stroke knowledge from dirtbikes and cheap chinese bicycle engines, but this is my first moped engine so it took a lot of research.
Here is a quick vid from last week before I installed the axle adjusters and stinger brace.