Watercooled motobicycle help

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Oct 13, 2019
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Big Island
Hi Guys!

Haven't built anything in a couple year and I finally jumped off the cliff and grabbed this:

-16204111701085811466.jpg


😁 Yay!

It's a 39cc watercooled pocket bike motor is the only specs I could find were these......

18590074251149248006.jpg


I know some of those letters and words👍

So I want to be able to hook this up to my sick bike parts jackshaft kit I have for my 2 stroke ht motor bicycle engine. I know this engine uses a 25h chain, and this is where my problems begin. I found a tritan #25 roller chain 5/8" finished bore sprocket 18t, it's the least teeth I could find that would fit the 5/8 jackshaft and #25 chain. How many teeth on the front pinion gear would I need to get any kind of useful gear ratio? Or I need to find some way to hook this 25h chain to my jackshaft kit
 
Last edited:
For the chinagirl 49cc and 66cc 2 stroke motorized bike engines, a 1:4 gear ratio (engine:wheel) generally works best if you only have one sprocket.

For yours I'd say between 1:4 and 1:5 if you only use one sprocket. That will get you a general idea. I'll let someone else answer further that may know of this specific engine.
 
Weld a 25h chain sprocket to the one that's there already and space the engine over as needed.
 
I guess I could get a plain bore sprocket and weld it to the front pinion gear, need to check the spacing between the pinion and engine case. The china girl had a 10t front sprocket that connected to a 10t tooth on the jackshaft. It's the UHD jackshaft kit from sick bike parts.com. Should I hook this engine up similarly? I found the flying horse gearbox transmission:
91H2M12XuAL._AC_SL1500_.jpg



The mounting holes are the same but the clutch bell is 1mm bigger in diameter of, but this would convert to a 410 chain. Would the 1mm make a huge difference? I was even thinking of trying to swap the bell, I am pretty sure the nuts are the same, but not 100% sure
 
So this is the front pinion gear:

H96ccd3fdc1c04d84945d8b1fe5897de5E.jpg

Not sure there is too much meat on there to be able to weld on the 410 sprocket, unless I could get a thin one with a small bore and weld it on top of this gear, although I don't know how the structural integrity would be I don't have much experience welding
 
I don't think 1mm is that big of a deal, I'd definitely try it. That spindle that came with the motor looks kind of flimsy.
 
Yeah I'm was kinda sketched out with welding the two gears together, its alot of force for a weld to hold. Yes the 25h sprockets and chains are the smallest standard size chain, very flimsy. So right now my options are use the 18t 25h sprocket to hook the jackshaft to the engine, and I guess use a 18t sprocket on the engine to make it 1:1 like how the china girl was hooked up. Or buy the 4 stroke transmission gearbox and see if the 1 mm affects perfomance. I guess if all ease fails, I can try and weld a 410 sprocket to the stock engine pinion gear. I hope this works, I want a watercooled, 4.95hp@9000rpm bike to show off😁
 
That 39cc without a tuned expansion chamber will only put out up to maybe 2 - 3 hp at 6000-7000 rpm. I could see 4.95 hp at 9000 out of that with an expansion chamber.
 
That's been another problem is finding an exhaust. I found this:

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It's the cheapest I could find @$120..... I'm hoping I can make it fit on my bicycle too or I'm gonna need another option
 
That's been another problem is finding an exhaust. I found this:

View attachment 104393

It's the cheapest I could find @$120..... I'm hoping I can make it fit on my bicycle too or I'm gonna need another option
Here's a $70 one plus free ship & returns
Jog 50cc ebay
I got one and I'm about to make it fit. It fits pretty nice already, you may need some butt weld 90⁰ elbows to weld on. Maybe some emt conduit pipe too if needed.
20201223_195452.jpg
 
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