belt throwing

Oklacarcollecto said

If all the pulleys are aligned correctly then it is most likely caused by the crank pulley shake.

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I concur with Mr. OK. Also, if the belt is not very stiff (i.e. can be easily twisted), then it is much easier for the pulleys to throw it off. Try to get a heavy duty stiff belt of the same size and make sure you get it tensioned correctly.