It takes four things for an engine to run:
- a source of good ignition
- a source of good fuel
- a source of oxygen
- sufficient compression
When you're troubleshooting a problem, work on one system at a time. Otherwise you will get yourself very confused and end up spending more money than you needed to. Notice that I qualified the first two items on the list with the word "good". Fuel and ignition aren't enough if they're just there. They have to be good enough to create fire. When you put fuel in the carb, don't use gas, use something more volatile. Carb cleaner works well. Or ether. You can spray a little in the mouth of the carb, and if there's any ignition and compression at all, it should fire up. Then you can tell if it's backfiring or running really rough. Once you get it started, keep shooting a light squirt of carb cleaner every second or two in the mouth of the carb to keep it running so you can continue to diagnose the problem.
You stated that the engine ran fine last year and then gradually got worse. Have you checked the air cleaner? If it's dirty enough, it can keep an engine from running. If that's the case, pulling the wiper vacuum line (to allow more air in) should make it start. That would be a clear sign that the engine isn't getting enough air.
If you suspect a vacuum leak, spray carb spray on the rubber vacuum lines. If it has a leak, the engine will accelerate when the carb spray hits the hole in the rubber hose (or in your case, it may start finally.) A bad vacuum hose would produce the exact symptoms that you describe (gradually deteriorating idle.) Pull the hose, block the vacuum port and crank it. If it starts fine then, that's the problem. Replace the vacuum hose.
Pull a plug, touch its base to ground somewhere (engine block, etc.) and crank it. Does the spark snap? Or just kind of click? Pull the distributor cap and see if the rotor is turning when you crank the engine.
These are just a few ideas, but the point is, work on
one system until you're sure it's right. Once you've eliminated it, strike it off your list and start on the second system. If you go through all four, and you still haven't found the problem, then you've missed something in one of the four. Start over, and work each system until you're sure it's right.
