This is stepping into oil territory, but if I'm not mistaken Webra specifies that only castor be used in the fuel. Webras are built with really tight pistons due to them being a high compression motor built primarily for non-nitro use. Could it be that its simply overheating with the lack of castor oil in the fuel? Or is the can muffler providing not enough exhaust gasses to escape causing too much backpressure and heat?