This is Curtsey of the:
What it is Not Working Properly:
A lantern requires an exact fuel and air to operate. A flicker often means that air is being allowed in somewhere where it is not supposed to. In other words, you may have an air leak. It could also mean a weak fuel and air tube spring.
Take the lantern outside. Pressurize it but do not fire it up. Light a match or take your lighter and waive it under the main valve. If you have a tip cleaner lever, waive it under that lever too. If the flame all of a sudden gets much larger then you found your leak. If it doesn't, fire up the lantern and let it burn for a minute or two until it starts flickering. The do the wave-the-flame trick again. If the flame grows, or if the lantern's light becomes steady, you've found your leak-and problem. You'll need to tighten and/or replace the valve stem packing or the tip cleaner stem. If tightening doesn't work then please refer to our Rebuild Instructions to learn how to replace the packings.
The other problem may be your fuel and air tube spring is a little weak. A strong spring will force the F&A rod up into the F&A tube. If it is a little weak it may cause some "float" and the amount of fuel being picked up will vary enough for it to be seen. Should this be the cause you'll need to pull the valve out so click on the link above to get to rebuild instructions.