Here's what to do to diagnose the problem:
- remove the spark plugs and make sure they are not oil fouled, or otherwise damaged, replace with correct ones if yes
-with the plugs out, hold a thumb over each hole while cranking to be sure there is compression (i.e. no burned valve or hole in piston etc.). The compression should blow your thumb off the hole. If you can get a compression tester, all the better.
- If compression is ok, check for spark. Hold the center wire from the coil to ground (e.g. a place on the engine, about 1/2 inch gap) and crank the engine (plugs out). there should be a good snap and visible spark. If not, could be bad points, defective condensor, no power to the coil, or a bad coil. Replace the points and condensor and retry. While distributor cap is removed, turn it over and look inside for black carbon tracks or cracks. Replace cap is anything is visible. make sure the rotor is installed, and rotates when the engine is cranked.
- If compression and coil spark are ok, install the plugs, etc. and pour into the carb about 1 ounce of FRESH gasoline. Reinstall the airfilter (in case of backfire) and try to start it. If it starts with the gas added to the carb, there is a fuel issue...carb or fuel pump.
- Before rebuilding the carb, make sure the fuel pump is working. Remove the fuel line from the carb and have someone crank the engine while holding the line over a small container like a plastic 1 liter soda bottle. If the pump delivers fuel (you'll see a good amount pumped) then the carb is the issue.
- Normally an old carb will be plugged up with dried particles and only need to be cleaned. Don't mess with float adjustment...they don't change by themselves. Carefully disassemble the carb and blow out all the holes with compressed air and carb cleaner from a can. Adjust the idle screws to gentle closed, then open 1 and 1/2 turns open.
There are more complicated reasons for "no start" like a jumped timing chain etc. but these steps should likely get you running.
Old gasoline will burn but not start an engine because the "light ends" of the gas that have to vaporize for the spark, are evaporated. Make sure the carb has fresh gas in it.
Hope this helps.
