Start by replacing the spark plugs. (I had a similar problem on a 535 and every time this started to happen, replacing the spark plugs resolved the problem. Unfortunately that only temporarily resolved the problem as after around 250miles of daily commuting the problem returned. The second thing I would do is to put the battery on charge to ensure it is fully charged (normally meant to be the first check on a tune-up). If the problem returns, then I suspect it might be carb related (or even a leak on inlet manifold) that is causing irregular fuel mixture that might be causing your spark plugs to get coked up. There are several things that could cause this on a carb, If you are not experienced or have the proper tools for looking at these it is probably best to have them looked at by a professional.
Four things that you need for an engine to run.. Battery power for starting, Proper spark at spark plugs, Fuel and Air at the right mixture. Always worth inspecting problems in that order. After that ensure oil, water and other items are in order or start thinking of doing compression test.
