It is probably one of four things:
1) You may have a clogged air filter. I know, you probably have already checked this, but it is the easiest thing to check.
2) Your spark plugs may need changed. The do lose the gap over time, and can become covered in carbon deposits and glaze. Check to see if they need replaced or regapped.
3) You may have varnish in your carburetor. This will mean taking the carburetor off and rebuilding it. Take it apart, soak the metal parts in a carb cleaner bath for 24 hours, replace all the seals and reassemble. You can try some octane boost to try to get the motor to start and fuel to flow, which will remove some of the varnish, but eventually you will need to rebuild if this is the problem.
4) You may be losing compression on the cylinder or cylinders. This problem is only solved ** an engine rebuild. It can be checked with a compression gauge available at most auto parts stores.
You could also have a timing issue, but most of the pull-start type motors have a fixed timing that should never have to be changed or adjusted.
Hope this helps.
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