Please help, my son has a derbi gp1, it looses power after a while ( drops down to 10 mph from normal)when he stops it restarts its ok for a while then does it again, we have replaced the fuel tap cleaned the carb out, new inlet manifold, new cdi unit with plug and cap, new regulator, also put an anti-freezing agent in the fuel because its been cold recently. Could anybody help.
Thanks
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