13hp 358cc Tecumseh snowking 1969 frame Hiawatha pedal bike Dual belt chain drive 60mph max Total less than $150
A little bit of history
This started way back when I still had a chainsaw engine on here, which actually was an idea that came to me in a dream. I just woke up one day and started thinking about how easy it would be to hook up. So I started asking around for old bikes and chainsaws, and miraculously, one of my co workers had both a rusty muscle bike, and a chainsaw with a seized pull cord, which he gave to me for a grand total of $5. I replaced the seat and the front wheel, previous was flat and I had a 16" on me. The chainsaw already had a friction clutch on it, so besides mounting all I really had to do was cut off the previous chain gear from the clutch, and add a throttle, in which I used a brake lever. I threw a 2 speed sprocket on the front, to use as a jackshaft to chain drive to the back, and welded a front sprocket directly onto the back one, At this stage, it did not have brakes (not important)
The chainsaw was 60cc, comparable to that of a bike engine kit, and it seemed to accelerate about the same as one too, for about 50 feet before dying. It had gas, spark, air, compression, but after this I never got the chainsaw to run again. After a while of trying I decided to just look around for another engine and try again, preferably a 4 stroke so it doesn't die again for no concernable reason.
After a while of deliberating I found some old snowblowers in a friend's junkyard, and after giving it some new gas one of them sputtered a bit. I tore it apart, took home the engine, and tested it with a clean carb. It ran like new
After hanging a chainsaw piston from the sissy bar, and giving it new handlebars only because pointy ones look cooler, I got to work in modifying the frame to actually fit the engine, by cutting and welding on the strongest flat-topped metal I had, a stop sign post. A lot of people talk about how top heavy the bike is, but I would like you to try to come up with a way to mount the engine lower without making a completely new frame.
I was able to mount the engine, but winter and school were about to hit so I shelved it, and this would give me a lot of time to consider drivetrain possibilities
At first I was going to go with basically the same thing, an automatic friction clutch with a jackshaft, but since the output shaft was 1 inch in diameter, all clutches available in this size either engaged too low rpm and burnt out in any engine size over 5hp, or were huge and over $100. After watching a lot of videos on diy bikes such as this one, it seemed like the best idea was to run with a belt drive off the engine to a jackshaft to the stock chain, with a manual foot-operated tension clutch. It seemed like the cheapest and easiest option available, and at this point I was just wanting to make the thing go, so I went with this
During a school break, I took the time to weld this together. The clutch is the stock tensioner with a snowblower handle welded onto it, and the bottom pulley was from the snowblower, welded onto a gear sprocket that fit the jackshaft. The exhaust is also a snowblower handle, with a custom soup can muffler. Because of the top heavy nature, I welded together a custom stand.
From this point, it was a matter of testing, seeing what breaks, fixing what breaks, and adding more things. After testing it at a car meet, the most important things I had to add were a new chain (the original rusty one kept breaking) and brakes, because it started to have too much weight and speed to stop with my feet still
New additions at this point were front brakes (70s), who's cable didn't fit the lever, a speedometer that's actully pretty accurate, and a walmart chain, with a tensioner off a 5-speed to keep the chain from falling off. The rear drag slick, which original and rather rare, was beginning to get concerning, as it was cracking more and the bike was getting faster, a top speed of 45 at near-overev at this point.
After I got it to a point that I deemed satisfactory, I decided to add a second gear (in hindsight it's a good think I stuck with a belt drive, as I have no idea how I could do this otherwise.) After trying about 3 different pulleys, I found on the perfect size for a second gear that gets it up to 55 in a respectable amount of time. I added some foot pegs so my feet no longer have to float above the ground, and a second clutch on the right side for the second gear. Sadly, I had to replace the back tire, which was beginning to split on it's sides and caused a rather nastly wobble to the whole thing. That brings us to the current state, in which I was able to drive it to school and back twice now, haven't been pulled over yet!
I've employed a 2 speed "transmission" that tensions one belt when stepping on a pedal and comstatly slips the other. If you do something similar I'd reccomend getting green lawnmower belts, as they stand up to slipping a lot better than car ones, which are rather made to withstand high rpm (my bike maxes out around 3900 or so so this is not a problem). Assuming my calculations are correct, 1st and 2nd gear have a 5:1 and a 4:1 gear ratio respectively. 2nd gear previously had something theoretically faster, though the rpms at gear change were too low to get it anything past 45. The bottom pulleys are joined onto a jackshaft with a custom welded gear off a cassette onto a procket I machined larger teeth off of, this goes back to wheel through a sprocket welded onto the stock assembly. The bike is a little lighter than a snowblower, (way lighter than an actual motor bike) and with me riding it can go 0-60 in a time comparable to that of a car.
Naturally aspirated, no suspension, 20in tires with original rims (back has 2 broken spokes lol), sitting at 13hp and 358cc it's the largest snowking engine tecumseh has ever produced, 4 stroke with overhead valves. Front rim brakes only. Enough torque in first gear to pop a wheelie and enough speed in second to max out around 60mph, with the comfortable acceleration of a proper motorcycle. The govenor is disconnected for a smoother ride.
This results in just under $150, and with something with such good results that could be slapped together in under a week I'd say I'm rather satisfied. Way in the future if I ever make a better version I will create a frame from scratch and invest in a torque converter to eliminate clutches and multiple gears.
Originally, the bike looked quite nice, and courtesy of some pinterest page we can see what it would've looked like back in 1969. Quite the difference huh? All in all I learned that it's actually pretty easy to make something go rather fast