Can't believe nobody has said this yet. Sounds like your points might have too much gap. Makes the engine fight the starter and also causes a rough idle. Worth a look. Swap in a new set of points, condensor, plugs and wires. Also make sure your timing is good. A good way to check if you have a foot starter is to crank it over without the ignition switch on. If it cranks fine then it is likely a timing or ignition / points issue. Hope this helps.
Joe.
