Not so mini bike help please???

[wfc]

Hi there, I got seriously enthusiastic a while ago about minibikes. I had never seen one before untill about two months ago. They (like gokarts) just dont seem to be relevant out here in South Africa. I know, its crazy, but sadly true! Anyway, I am in the process of building a monster of an offroad kart driven by a yz490 two stroke motor. I bought a new carb for it on ebay from the States and had some time while waiting for it to arive. I just happened to have a good honda gx240 just waiting to be used, and a friend gave me this Yamaha xt250 without an engine..... I decided not to strip the bike for its parts, but try and see whether it would work with this engine. Some day I might find the right motor and return it to the bike, but in the meantime I might as well have some fun :thumbsup: Yes, I know that this isnt a minibike at all, but I'm working with what I have available right now:wink:

I needed to install a jackshaft, as the sprocket on the bike, and the sprocket on the clutch both didnt line up, and were different in pitch. This was also my first attempt at a jackshaft. Finding the sprockets was a lot harder than I expected.

The bike runs now, and everything seems to work out quite well (mechanicaly). The problem I have at the moment is that the gearing is way out. The thing has a top speed of over 74mph! I think that may be more than it did with the Yamaha engine! That also means that the gearing is wrong for low down power, an pulling off is quite slow.

I know that I have to play around with sprocket sizes to remedy this. Currently I have 10 teeth on the clutch, 12 teeth on the driven side of the jackshaft (from the clutch), 10 teeth on the drive side of the jackshaft (to the bike sprocket) and 47 teeth on the bike sprocket.

I am trying to find a bigger bike sprocket around 60 teeth, but that isnt all that easy, the highest I can manage so far is 52 teeth. What could I change on the jackshaft sprockets to bring the gear ratio in line with good torque? Would a larger sprocket on the driven side increase my gear ratio?

I am attaching some pics of how everything is set up. Any advice would be great, so thanks for chipping in.

 

[jeep4me]

Changing the driven sprocket on the jackshaft will slow you down. The way that your setup now will eventually burn up the clutch. I think the sprocket on the rear wheel is fine and is set up right for the size tire.

Is there enough room to turn your clutch around and have the driven sprocket closer to the frame? It's kinda hard to tell from the pics. It just sticks out so far. I'd hate to see you rip up a leg if the chain breaks.

Must be a fun ride though.

 

[wfc]

Hi, the clutch unfortunately cannot be turned around. Like yourself I wouldnt like to see my leg destroyed by the chain, so I am making a chain guard that will cover it completely. I agree with you about the clutch, and if you know how much it set me back, you'll understand why I am not driving it untill I sort out the gearing. You may find this quite difficult to believe, but there seems to be just one company (found after many weeks of searching) that sells these clutches, so this also affects what they charge!! You guys across the pond are realy blessed with your parts availability and prices, not complaining, just mentioning:wink:

Do you by any chance have a sugestion as to what size sprocket to fit on the jackshaft?

Thanks for the reply

 

[Oldsalt]

Based upon the tooth count on the clutch and relative diameters of the rear wheel and its sprocket I'd use at least a 24 tooth sprocket in place of the 12 tooth currently on the left end of the jack shaft. Work the gearing so that you have, say 25-30 MPH, at 3600 RPM.

 

[wfc]

Thanks Oldsalt, that makes sense, I'll try and get hold of the 24 tooth as soon as possible. With the 24 tooth being twice as many teeth, would it effectively halve the speed then?

Thanks for your input