Yes this can and does occur on occassion. Remove the needle and and bend the detent spring(s) in, then carefully reinstall the needle.

Your engine is going lean in flight and folks have already given you most of the common reasons and cures. Another to ad to the list is a tank that is to low in the fuselage will give similar symptoms. The tanks center should be level with the spraybar (jet) in the carb, no matter if the engine is mounted upright, side ways or inverted the tank needs to center on the spraybar.

Here are another couple of hints. When returning an old airplane to flight If the fuel tank has been assembled then don,t use it. Replace with a new tank always. Coventional bung plug type tanks have been assembled for a long time even if they have never seen fuel will be almost impossible to reseal if you remove the plug to check clunks etc.

Also if I may the very best type tank to use in most conventional glow setups are the Hayes tanks:

http://www3.towerhobbies.com/cgi-bin...?&I=LXK829&P=7

This is the best tank possible. it does not use a rubber bung that will deform but instead a hard plastic plug with an O ring and it seals beautifully, but more important the plug can be removed countless times for years and still seal on replacement. Only the clunk line goes through the plug. Far easier to assemble and far more reliable over great periods of time.