It seems to me the best way to answer your questions is to find out what is wrong with your machine.
First, I recommend that you stop using aerostart. It does not do much of a job of starting engines, but does a better one of washing the oil film off the moving parts inside. Try removing the air cleaner temporarily and squirting a small amount of 2-stroke fuel into the carburetor air intake, instead. Keep clear of the carburetor when you try to start it, of course - without the air cleaner it is possible for the engine to spit back.
Since it obviously has spark and at least some compression, there are five main issues that might be making it hard to start. The first is improper starting procedure. The second is a deteriorated spark plug. The third is lack of fuel delivery to the carburetor. The fourth is a problem with the carburetor's fuel pump. The fifth is amateur interference with the carburetor's mixture adjustments.
1. If you have the machine's operator's manual, follow that precisely when you try to start it.
2. If it has had the same spark plug for years, that will be at least part of the problem.
3. The fuel tank will only allow fuel to exit if air can enter to replace it, and if the pickup device in the tank is working properly. This involves air entering the tank either by a vent in the tank cap, or a vent line into the tank, plus a fuel pickup and filter device in the tank interior, a fuel suction line from the pickup to the outside of the tank, a fitting where the fuel line exits the tank, and another fuel line from there to the carburetor. All of these can and do malfuction for various reasons.
4. The engine relies on a fuel pump which is built into the carburetor, and operated by a pulse line connected to the engine crankcase. Sometimes that line is internal to the engine and connected directly to the carburetor, and other times it is a second rubber hose connected to the outside of the carburetor. This pulse system is prone to getting crudded up with black engine goo and ceasing to transmit pulses. The fuel pump itself is even more prone to giving trouble if it is left for years with fuel in it without being used. The fuel is likely to have turned to jelly, gumming up the pump completely.
5. There is a tendency for people who are having difficulty starting a small petrol engine to fiddle with the two mixture controls (idle and high speed) on the carburetor. Once they have done this, the engine will not start or run until the adjustments are reset to where they were in the first place.
We can work through these five possibilities if you want, but we would all have to know a number of details about your machine: what kind of fuel tank and feed it has, what brand and model of carburetor, etc.
