I agree with RedFish. Find out what you don't have. If no parts have been changed since it last ran good, things like timing are not likely to be the problem. Start by putting a timing light on each plug while it's running and see if there is an electrical miss. You may have to vary the RPM to bring out the problem. If it's good and steady, it's probably fuel. Start with making sure the choke isn't stuck. Then a flow test, by taking the fuel line loose somewhere after the pump and have someone crank over the engine (ground the coil secondary wire so it doesn't start) and see if you have a good stream of fuel coming out and into a container for inspection. OK? next would be a new filter and cut open the old one to check for debris and/or water. If it still has the problem after that, you'll probably have to go into the carb and clean it out also.