MY first truck,bought off e-bay,ran sweet,NO issues,thru 4 tanks...,now it seems I have to progressivly richen both needles a tick every other tank to maintain the "sweet spot",I'm now up to 3 1/2 turns on both needles,and just made my first adj. on the idle screw,when doing adj. I insure tank is full to prevent the "half tank lean" scenario from throwin me off,also,befor the engine is primed for initial start-up, the OWB on the pull start slips,but after priming, no problemo.....could an air leak from this area cause these issues........or is it just the atmospheric blues.........[:-]

ok. Most likly you have an air leak starting up somewhere. It could be the backplate or the carb. Also could be your fuel tank. (it needs to be sealed tight) I would stop running it until you fix the issue.
First thing i would check is the fuel tank. Simple to remove. make sure the fuel lines, and the tank lid are sealing completely. You can take all the fuel out and plug one line, and blow on the other. it should be completly air tite.
If that checks out. I would remove your engine. take the rotostart off, and remove the backplate. use some high temp RTV sealent (local autoparts store) and seal it around its matting surface.
Then remove your carb, inspect for damange. (possible tare down, clean, and rebuild). and reinstall with some RTV around the matting surface.
Those are the most common places to have an air leak, Also check your head/cooling head bolts and make sure they are tight.

As for your one way. Its going out. No worries they do that from time to time. Keep an extra one on hand.

I agree. Sound like an air leak [link=http://www.rcracersedmonton.com/rcracers/NitroEngine101.aspx]Read this[/link]

thanks guys,found it.......bad seal on the fuel tank lid........[:-]