Make sure. There is some slack in the throttle cable.
Test for manifold air leaks by spraying the joints with WD40 while it's idling.
Having fixed any leaks-
While idling (upright) warm, screw the idle mixture screw in until the engine sounds blubbery. Screw it out just until it sounds steady. Adjust the idle speed with the throttle stop screw until its idling as slowly as it can and still sound steady.
Then repeat, because fiddling with the throttle stop may have upset the mixture.