When it won’t start, have you checked for spark? If you have no spark on both cylinders, look at the flywheel key – it could be sheared and the flywheel moving on the crankshaft when you try to start it, and after enough pulls, it works its way around to the correct position to make it run.
If you have spark, will it fire if you spray some gas/oil mix into the carb throat? If it fires on that, you have carb problems. I worked on a 1970s 4 HP this summer that wouldn’t start, but would start and run as long as I squirted fuel in. When I had cleaned the carb, I missed the fact that the bowl vent was clogged. With the vent clogged, The bowl would have difficulty getting enough fuel to make the choke pull some for starting. To spray fuel in, it wworks better if you remove the intake silencer from the carb first.
If it has spark and won’t fire on fuel spray into the carb, you may have a flooding problem, usually caused by a sticking carb float, or dirt in the float valve that keeps it from closing completely when the carb is full.
When you changed the head gasket, did you check the head for flatness with a straightedge? That model is well-known for the head warping. It could be sealed when warm and running and leaking between the cylinders when it cools off, making for hard starting. If it’s warped, it can be sanded flat with wet-or-dry sand paper laid on a perfectly flat surface – I use the steel top of an old circular saw table that is flat steel.
Dave
