Depends on the type of engine, but here we go...
If its a simple two stroke glow engine, you need to adjust both the needly valve and slow running jet such that you get good running at both ends and good transition from one to the other, this can take some time.

best bet is to work the main needle to a point where the top-end is god, then SLOWLY adjust the slow running until that is good. At this point you will probably find that the main needle will need another tweak to attain perfection, after which only weather, prop selection and fuel type should require that you change anything.

Other than two stroke glow I don't know much, but if you can provide more details about the engine I'd be glad to help.