First build: Chainsaw bike

So this will be my first motorbike build... well, my first legitimate one. The REAL first one died a most glorious death. It was a 20cc weedeater engine friction drive that lasted about 4 or 5 miles before I snapped the crankshaft and sent the flywheel, crank snout, and the 21mm impact socket (roller) flying all over the place. It took out the coil and throttle cable, but luckily it missed my head!

So this one is going to be a little more safe. First off, we're starting with a Schwinn that I found at a local Goodwill for $25.



I then got a bike rack from Walmart for it. It's a Schwinn bike rack, so it matches That's where I plan on mounting the engine. It's a 42cc Craftsman chainsaw.



I chose this engine for a couple of reasons. I was first going to go with a Predator 79cc engine from HF, but there was one slight little issue. I wanted to use a centrifugal clutch, and Northern Tool eto stock them. As a matter of fact, they had EVERYTHING I would have needed to install the 79cc predator. But apparently Northern Tool stopped carrying go-kart accessories about a year ago. Just my luck. I didn't want to have to order this stuff, because it just takes so long to ship... plus you have to pay for shipping. So the only thing I could think of that had a centrifugal clutch already attached was a chainsaw. So I searched Craigslist and found that engine and bought it for $30.

Unfortunately, it gets more complicated. The clutch on the chainsaw has teeth already built in. But they're the wrong type of teeth. So I go to a local bike shop to see if they can sell me a small gear, and the guy goes back to the repair shop and tosses me an 11 tooth gear. Score! So I go home and grind off the original teeth and weld on the new bicycle gear to the clutch (after spending quite a long time getting it PERFECTLY aligned).

Here's a photo of the gear, peeking out from underneath the shield:



Now, the big issue is that the engine rotates clockwise, so mounting it in the frame with the crankshaft on the left wouldn't work. So I got to thinking and came up with a new crazy plan. I'm going to mount the engine on the rear rack, mount a jackshaft below that with a friction roller to drive the rear tire. So there will be a chain coming from the clutch to the jackshaft, which drives the friction roller.



So I get to keep the pull start (bonus!), I get to keep the centrifugal clutch (another bonus!) and best of all, it will all be so contained that I can remove it from one bike, then mount it to another bike within seconds! How awesome is that!?

I can't wait until this is all done!

-Terrence

Thanks for the inputs guys, I always take all reasonable criticism on board. To answer some questions (and some counterpoints):
-I originally wanted to go with a chain drive to the rear wheel, but since the engine spins clockwise I'd have to have a jackshaft to transfer the rotation to the left side. I'd do it, but I think the friction roller is a much simper design. I'm all about keeping it simple
-I've considered the torque on the rear rack, and I have put thought in to stabilizing it with some links to the rear axle. Maybe a couple turnbuckles could be used?
-The chainsaw's clutch is just so narrow, and so close to the body of the chainsaw that using it to directly drive the rear tire as a friction drive won't work. I've already considered that as an option, but it too failed!
-The clamping force wasn't what killed the weedeater... the real reason is that I cut the crankshaft down and welded on a larger diameter grade 8 bolt to it so that the socket would center up better. Obviously that didn't work out in my favor because the weld snapped. There wasn't much room for a decent sized weld on that crankshaft anyways. Really I had no business welding on that thing anyhow. Not that it mattered, it sucked pretty bad anyways. It wouldn't accelerate at all, and barely maintained speed. Really though, what more could I have expected out of a weedeater?

Hopefully today I'll get this finished up and try taking it out for its first run. I still have to rebuild the carburetor, figure out how to mount the jackshaft assembly, and possibly figure out a stabilization method for the rack. It's pretty solid though, so that's going to be a last priority. I think if it comes down to it though, I may just drill through the seat post and install a set screw of sorts to keep it from moving.

Still a no-start. It's all put together and looks pretty good, but I just wish that I could get the damn thing started! I've already rebuilt the carburetor, changed the spark plug, all fuel lines, and nothing is working. I did, however, buy it as a no-start. I got it running for a short period, but it ran like crap and died. I thought that was because it needed a carb rebuild and new fuel lines, since the old ones were sucking in air. I even pulled the cylinder and re-sealed the crank case. It won't even give me a sputter now. It has spark, it has compression, and the spark plug is getting wet with fuel, but the magic is just not happening....

Well, I got it started. I took it for a quick run and the chain popped off. I pedaled back home and adjusted the positioning a little and started it back up and took it for another run. It worked great! Except the engine was bogging down pretty bad. So I brought it back in to adjust the high speed adjustment, then lo and behold it won't start again. I can get it started if I crank the high and low adjusters all the way in, back the low out just a hair, then it dies. I try again and again, nothing. I screw the low speed back all the way in, it runs for a short time. I back it out a hair, it starts, dies. I have tried backing it out more, but it just won't run. So what I think happened is that the previous owner may have cranked those needle valves in more than he should have, thus damaging the seat (or seats). So I'm probably going to order a new carburetor.

Sounds like a plan!

That's a good point, I did not replace the crankshaft seals. I did, however, apply silicone around them before installing them again. I don't know, maybe they do need replacing. I ended up ordering a brand new carburetor today. I'm definitely not new to tuning engines, big or small. I've never had issues like this though. There's nothing I can do to tune the carb in. And I pulled the muffler apart and it had a tiny amount of carbon buildup in it, but nothing that would disrupt flow.

It has electronic ignition, the flywheel is keyed properly, but I did not check the air gap. It's definitely getting a good spark though. I'll have to find the air gap spec and check it.

So the newest update: I can get it to start, run for a second or 2, then it dies on its own. If I open the throttle, it bogs out and dies. Turning the low speed screw out makes it not start... period. Crank it back in, it starts again. Adjusting the high speed screw out has no effect on idle, and it will still die after a couple seconds, or bog out and die if I give it a little throttle.

I worked on the other bike tonight. It's a Ryobi 31cc weedeater engine, direct friction drive with a scissor clutch. It rides great, except I'm having a difficult time getting the clutch to actually disengage the wheel. So far all I have right now is a clutch that pulls the weight off the drive roller, but it won't pull it all the way up. Just a little more tweaking and I'm sure I'll have that one fully operational soon.

So I adjusted the air gap, which was pretty far off. I guess it was only compounding the problem, but it definitely did not fix it.

I got the carburetor a couple days ago, but didn't get the chance to install it until today. I still have some adjustment to make to the carb, but at least I did get to ride the bike today! First ride feels good, but I have to figure out a better way of mounting the engine to the rack. It loosens the bolts, shifts the motor, followed by the chain coming off. I also need to adjust the throttle cable, because I was only able to get about maybe half throttle before I ran out of cable pull. I think that with a little more tweaking, this will be pretty awesome!

Well, now I'm right back to a no-start again. I got the engine mount situation fixed, and it stays pretty solid now. The only issue is that I cannot get it started and running again! It starts on PB Blaster, but won't stay running very long. And this is with the new carburetor. Something's just strange here....

So as it turns out, it was the carburetor. I guess it makes sense... it ran great on pb blaster, wouldn't run worth a damn by itself. No matter how I adjusted it, it wouldn't run at all. And I have already replaced all the fuel lines and I actually re-sealed it a SECOND time, just to make ABSOLUTELY sure.

So here's what really got me! My "first" bike that exploded became a parts donor. So a friend of mine that came over and looked at it and noticed that IT had a carburetor, so why not try it? Well, I laughed at him because the chainsaw engine is more than 2x the size. I told him that it's like putting a Honda carburetor on a V8 truck. But we tried it anyways and wouldn't you know it, it bolted right up and fit great, and it also ran AMAZING! It's too dark to post pics, but there will be more to follow.

So now I have to complain to the guy that sold me the carburetor that it doesn't work worth a crap, and that I either want a refund or a working carb. The one that's on there now is probably leaving some power on the table.