It could also be an obstructed fuel line or plugged filter, or a weak ignition coil. Take the fuel line off the carb and put it in a coffee can. Have someone just bump the engine over a little at a time. When the pump kicks you should get a fairly decent sized shot of fuel in the can. Pull the center wire out of the dist cap and lean it near some metal. Have someone crank it over and look at the spark, it should be bright blue, not yellow.
