Hello everyone. Picked up a new to me Big bear 400 the other day. The thing runs awesome for about 15 or 20 minutes but then starts sputtering at low rpm. As soon as you rev it up a little it comes out of it and runs great again. It’s really only a problem after coming to a stop or slowing down. It does have a pretty new carburetor on it. I didn’t put it on but I’m leaning away from that being the problem since it doesn’t do it when it’s cold. I’m also not thinking it’s compression related since typically compression increases as the engine warms up. Could it be a coil or spark plug issue? Any suggestions are appreciated. Thanks

 

Check the plug colour, if it isn't black and sooty your problem probably isn't carb related. Misfires when hot are usually spark related. Fit a new plug and if that doesn't fix it, the cause is usually a stator coil, but testing is a problem due to it only doing it hot, and being intermittent. Worth checking for an overcharging regulator first (no more than 14.5v at the battery when running) but CDI, coil and stator are other possibilities, however changing them will cost big money and may not fix it.