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Help...50cc to 70cc jug..slower

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I swapped my sons 50cc jug to a 70cc jug with the same carb but with a 62 main jet. Twin air filter and removed the baffle and put a billet cap on the pipe. I thought it would have boosted the power but it seems to be slower or at best the same. Any ideas?? I ordered a xr70 carb and hope that will help.

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Unfortunately you're not gonna see the kind of gains that you would think you would get. The low compression is the reason why. If you're using the 70 head put the 50 head back on. The valves may be smaller but so is the combustion chamber. That will help a bit. If you decide to go with the carb swap you will find that the intake won't fit. I use an intake off an ATC70.

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We went down 2 teeth on the countershaft and it helped acceleration a lot. Also lessens the big jump between 1st and 2nd.

Don't worry about losing top-end when you gear it down, the 70 can't pull the tall stock gearing anyway. I haven't tested it, but it may even go faster geared down because it's getting closer to its peak power.

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