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Hey all. I'm new to this karting thing. Last summer I bought a Tomberlin Crossfire 150R. I drove it around the barn a few times and took it into the shop and tore the motor/drive train off. I put the motor off my '83 RM250 on it. Great top end, terrible hole-shot (too heavy). Want to put a snowmobile/larger road bike engine on. What do you suggest? Money IS hard to come by, but time and tools (and skill) aren't a problem. My brother-in-law runs a machine shop and I was a welder in the Navy.
 
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Doc Sprocket

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There is absolutely no reason the RM250 won't push that thing with authority. You just need to gear it lower. Save yourself some effort and money- Let's calculate a more appropriate gear ratio, and away you go!!!

...Oh yeah- Welcome to the forum!
 
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I didn't forgot... I was getting there....... Really........ Believe me?

Now- how many teeth on the engine sprocket, and how many on the axle sprocket?
When you say "stock" do you mean from the bike, or the kart?
 
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Just called my brother-in-law. 15 and 50 on the tooth counts. Engine lags unless I red-line from standstill. Once I get moving I'm okay. Just heavy and hard to get going. Weight of kart without rider(s) is near 600 lbs.
 
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A couple questions before going any further-
Is the engine/clutch/tranny in good running condition?
Are your driving it like a bike (high RPM)?
 
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Clutch is fresh. Used to race motocross so I'm riding/driving it like a bike... kind of. I configured a foot clutch so it's a bit more difficult to feather.
 
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Top end is scary! Third gear is way fast enough. I'm looking for 55-60 mph tops. But will be rutting, whooping, and some sand...ing. Low end is more important to me than top speed.
 
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It looks like the donor bike used 520 chain, which is interchangeable with #50. Have a look around, and see how big a #50 sprocket you can find. Off the top of my head, a 62T would equate to a 25% gear reduction. If a sprocket that large, or larger is not feasible for any reason, you can always set up a jackshaft for compound gear reduction.
 
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That (jackshaft) would be the next step. 62 teeth would probably help, but would diminish what little ground clearance I have left. And bigger tires just compounds the problem. I have a line on a '76 Honda CB 750. If I don't use the jackshaft I'll swap engines. Will keep you posted. Thanks for the help.
 
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Glad to help. You certainly could switch engines for more power, it's really just a case of personal preferences and what you're willing to do.
 
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