How long is a peice of string? We need more info!!

For starters, how about telling us what engine, fuel, plug you're using. Is the truck brand new? If so, have you run the engine in? Is the carb at the factory settings? What's the weather like?

It sounds like the main mixture is too lean. If it is, you can richen the LSN all you like and it won't make any difference. Since adjustments to the main needle effect the fuel flow through the LSN, always adjust the main needle first, get it in the right ball park, and then you can make small adjustments to the LSN to ensure smooth throttling.

Air leaks can cause over-heating, that's for sure. In addition to what's already been listed by Inferno, common causes of air leaks are:

loose screws on the backplate
loose screws on the cylinder head
loose pinch screw on the carb
loose main needle banjo fitting
broken/poor fitting/perished tank lid seal o-ring