Then you've got the wrong motor mount and/or clamps to fit your frame.
I know your low on bucks right now... So I was thinking you
might be able to get-by for awhile using some steel-wool or possibly even several layers of aluminum foil to help shim/tighten the fit until you can afford a 15-degree motor mount that truly fits your frame.
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Every time your chain flies off your damaging your sprocket, clutches drive-gear and chain more and more. That's bad enough, but just make
g*d dam sure your whole engine cant ever come flying off, because you could end up with the clutch drive-gear spinning 3600rpm in your lap until it ground your junk right off and turned you into a girl...
Every kart needs a chain/clutch guard, but with your funky rigged-motor mount you REALY need to buy or build one for that thing too, pronto !!!
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My karts frame has a bolt-hole mounting tab that allows me to install/tighten a long bolt up against the back of the motor mount, this simple bolt stops the motor mount from ever being able to slide back on the frame. (bottom-center of pic)
If your frame has one of these bolt-tabs, go get a long bolt that's threaded the whole way, or cut a suitable length of 'All-Thread', and use double/jam-nuts to tighten in place between your frame and the rear end of your motor mount.