Now I have a 79cc Harbor Freight 4-stroke. Next step please?

wayne z said:

Why not simply use a larger pulley on the motor?..

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Yeah, I realized that just after I posted. But, although I only have just a bit of experience with it, I really wasn't satisfied with just a 3" roller. It just seems like the contact patch is too small, and a 33% increase would be achieved with a 4" roller.
Plus, I'm planning to move the bearings and shaft to the top of the ladder. The circle would be where a 4" would be in that configuration.
Anyway, the exhaust is next in priority.
I went to Lowe's and found a galvanized pipe 3/4" female thread flange per your suggestion, but they wanted like $8 USD. (I can afford that, but it's a lot for one simple fitting.) I didn't get it because seeing all the other fittings made me imagine different possibilities, and I didn't have the stock flanged tube (that connects to the stock muffler) with me to test. I recall one post that said 3/4" conduit fits. I'll have to check that.

wayne z said:

...I would stick with the pipe flange idea. If you find conduit that fits, you will have to figure out how to attach it and the muffler very securely. Especialy if you have a muffler hanging off at 90 degrees, there's gonna be a lot of shaking and vibration to it on this unit.
The pipe flange and fittings would be very strong in this regard. Might only need a 1/2" flange and elbow though to use a standard hotdog type mower muffler, it has 1/2" threads. The 3/4 flange's hole may be too big to seal good, or to drill the bolt pattern you need.

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Allow me to tell a little side story:
As you can read in an earlier post of mine, I balked at the price of the galvanized flange fitting. Regardless, today, I went to Lowe’s just prior to its closing and collected some items I needed including the ½” version of the flange fitting, which was a bit cheaper than the ¾” but still pricy. Two male employees were hovering by me – not helping - but trying to encourage my departure so they could finish their shifts. They didn’t step over the line directly to me verbally, but when speaking to each other, they were actually directing to me. “She wants to clear the departments. You’ll have to tell her we still have one here.” With the accompanying tone, the meaning was clear: Get out of here, you last-minute laggard. We want to go home and you're keeping us. I hurried as I could, but I wasn’t just going to buy useless fittings. Within 5 minutes, I found what I needed and went to the cashier, my escorts now gone. The girl cashier had a completely different attitude. She told me she had only been working at Lowe's for 2 weeks, which was irrelevant, but it was an effort to be friendly. I suspect she was aware of the discourtesy of her fellow employees. I didn’t touch any of the items on the counter. She scanned & bagged them herself, and she asked, “Is this what you have, the bolts and 4 items?”
“That’s right,” I truthfully replied, “4 plus the bolts. How much do you need?” I, because of the push from the other employees, was actually now rushing her. The total came up, she announced it, and I realized it was too low. She had deliberately omitted the most expensive item, the flange fitting. I paid and left. I reviewed the receipt to make sure. Yes, sure enough, 3 plus the bolts, she really did give me the flange fitting for free.

“Finished jobs are for idiots.”
It’s one of my favorite sayings. It means if you don’t have the imagination to envision something else you can add to a job, then you are an idiot. Or, in another situation, if the job is so simple that it can be complete, then it’s for an idiot.
The motor-bike still needs several improvements.
The clutch slips too much. For starters, it needs a larger roller. (BTW, while I'm talking about the roller, the pin between the hockey pucks fixing them to the shaft broke. I'll replace it with a hardened one.)
And, actually, the design of moving the whole engine with the clutch is flawed. The clutch should move independently, and the engine needs to me fixed to the frame.
I placed a bolt going sideways through the frame to hook into a hole in the clutch handle while in the disengaged position. The sideways play in the handle allows hooking and unhooking. I can start the engine disengaged, mount the bike, and then engage and go.
Also, the clutch control really needs to be on the handlebars. A rider shouldn’t have to yield the hold on the handlebars to use the clutch.
The bike is difficult to mount. When I move the roller to the top of the ladder, the resulting lowering of the rear part of the ladder should make mounting easier. This will lower the center of mass too.
The exhaust needs a better muffler. Using angle grinder with an abrasive disk, I cut the previously mentioned flange fitting to fit over the exhaust port. I screwed on a small (3” [8cm] diameter 2” [5cm] long cylinder) lawn mower muffler, which I “borrowed” form my landlord’s mower. I’ll return it before they discover it gone . I need a bigger one.
I rode the bike powered by the gas engine for a few miles and even used it for an moderately difficult errand, carrying a $25 USD box of Church’s chicken. It works, and I’m consequently ending my participation in this thread.
Thank you.

maciver02 said:

Personally, I don't like the look or feel of a motor mounted high, but if you do, and you're main concern is the thing falling over,

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When you're moving, it doesn't matter. In fact, via the upsidedown broomstick effect, balance is easy. When the bike is stationary is when the top-heavyness is a problem. I figure a project down the road is a strong kickstand.

maciver02 said:

why haven't ya'll just utilized one of the many beefy center stands out there?

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What are you referring to? The engine won't fit in the triangle of this bike, if that's what you mean by center. And the (you could call it) advantage of this arrangement is that the rack and the clutch are the same element.

Actually, after hundreds of miles of use, I'm abandoning this design. I going to do something different. And I know it pegs the geek-meter, but I plan on using this 79cc engine to turn enclosed counter-rotating air props. No clutch need, only minor bike modifications, and the brakes remain the same.
At this point, I'm stalled on what type of prop to use. It needs to be efficient and cheap to make. I'm not settled on the gear box either.