125cc Fully Automatic Honda Clone
 

Ok I have my new engine installed and wow the difference in power from the original 49cc semi automatic is just crazy. I can't believe the entire engine is just held in place to the bike frame with 2 skinny cheap 6" bolts with no kind of reinforcement in the frame or motor mount of any kind. Oh well if its been working for Honda since the 1960's it will probably continue to work for me.

So here is my only complaint. As I feared in prior postings the fully automatic engine would be too 'torqueeeee' and that fear has come true. It would be great if this was a dirt bike but its a simple street Honda Cub clone made to ride easy. I don't know how fast it will go yet but at 35mph the engine is really screaming but still has lots of power left. So I guess I need to mess with the sprockets. The front sprocket is a real simple change out. It is currently a 14 tooth. Do I need to go up or down to make the bike not work so hard? I still need enough torque to get me away from street lights with the traffic but I want it to be able to cruise 35-40 much easier. I will have to deal with the back sprocket if changing out the front doesn't give me the desired affect. Thanks for the help.

Adjuster

You need to either increase the countershaft size or decrease the rear sprocket size to lower the revs at speed. I'd try to fit a 16 or 17 tooth sprocket on the countershaft.

What size is the rear sprocket?

It is marked 420-41T


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I presume that these sprockets are a match for your bike, but you'd need to measure to be sure: http://www.jtsprockets.com/catalogue/model/4943

You currently have a 2.93 final drive ratio, and you need to get that number lower, which is actually higher gearing. Confusing, I know.

A jump to a 17 tooth front sprocket would take you to 2.41.

I just took the sprocket cover off to confirm my front sprocket is marked:

SD420-14T


What does the SD represent?


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Probably Spline Drive. There are 2 common size countershaft sizes on the Chinese horizontal engines, 17mm and I forget if it's 20mm or 22mm. In any case you'll need to get the right inside diameter for your engine.

I would guess a 17 front, combined with your 41 rear, should do the trick.

Thanks everyone I have ordered a 16t and a 17t. And I have an original 13t on my 49cc and a 14t on my new 125cc. They are only 9$ each for the new 16 and 17. I hope I have all my bases covered. This little 125cc motor amazes me.


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I wouldn't doubt if you're able to cruise at 55mph with the right gearing. The limiting factor will be the actual space available for the countershaft sprocket. IOW, if you had the space and an 18 tooth would fit (and is available), it might be worth a try.

You're probably gonna need a new, longer chain.

The sprocket change made a huge difference. Its a joy to ride now.


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My chain has that deal where as you spin the tire it has tight spots and loose spots. I have the tension adjusted correctly using the tight spots. The problem is the loose spots now have too much play in them. Is this something I can fix or adjust or does this mean I need a new chain? The bike and chain are brand new by the way.


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That chain symptom is due to a poor quality sprocket (most likely rear).

Thanks, is the sprocket out of round? They are cheap enough on Ebay but I would like to make sure I get a good one.


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I am willing to bet that a Honda part will fit. Try looking for a CT90 sprocket too.

Yes and no. The sprocket is likely round, but the center hole is probably not actually centered. That's the problem with my quad rear sprocket. I'm thinking of cobbling together a chain tensioner to deal with it.