Got the new thermostat in
What is the model number of that engine?..... If it's been mentioned, it's hiding.
Champion spark plugs J4C or J6C are generally recommended. Due to age, the J6C may (but not always) provide a somewhat improved performance... something to keep in mind.
If that carburetor incorporates a throttle roller..... If the roller is even slightly worn, grooved, whatever, replace it. The adjustment of the roller should be set so that the throttle butterfly just starts to open when the scribe mark of the cam is dead center with the roller... not before or after.
If the carburetor has an adjustable slow speed needle valve... adjust as follows:
(Carburetor Adjustment - Single S/S Adjustable Needle Valve)
(J. Reeves)
Initial setting is: Slow speed = seat gently, then open 1-1/2 turns.
Start engine and set the rpms to where it just stays running. In segments of 1/8 turns, start to turn the S/S needle valve in. Wait a few seconds for the engine to respond. As you turn the valve in, the rpms will increase. Lower the rpms again to where the engine will just stay running.
Eventually you'll hit the point where the engine wants to die out or it will spit back (sounds like a mild backfire). At that point, back out the valve 1/4 turn. Within that 1/4 turn, you'll find the smoothest slow speed setting.
When you have finished the above adjustment, you will have no reason to move them again unless the carburetor fouls/gums up from sitting, in which case you would be required to remove, clean, and rebuild the carburetor anyway.
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