If your engine is hunting (repeatedly increasing then decreasing in speed, with a cycle-time of about 5 seconds), the usual cause is lean mixture. That engine had a main jet mixture adjustment screw sticking out of the side of the carburetor. It is easily identified: it has a spring under the screw head to keep it from moving unintentionally. There is another screw nearby with a spring under its head, which adjusts the idle speed.
To deal with the hunting, I suggest you start the engine, get it hunting, then slowly turn the mixture screw anticlockwise a little. Most likely no more than one eighth of a turn will cure your problem, which is usually due to some grime building up in the jet.
