I've tried searching for this issue and didn't find any quite like mine, so apologies if I'm covering old ground here.
I have a '78 Puch Maxi Luxe. Stock E50 engine, stock airbox, 12mm bing carb, and a new BiTurbo pipe. I use a 50:1 gas/oil ratio with an in-line fuel filter. Chains are tightened appropriately and tires are at recommended psi. I live in the northeast at about sea level and it's summertime. I'm new to mopeds and have learned what I know from this site and a couple of nice local folks.
When I added the pipe, I upjetted from the 62 that had been in the carb. I started out high and it bogged down on everything until I put the 62 back in. It runs best (in terms of acceleration and top speed, but also in terms of "sounding right") with that jet and the little plug on the back of the airbox removed. So, duh, stick with that, right?
But when I check my plug with that configuration, it looks like I'm running lean. It darkens up if I cover the airbox hole and crimp the tube that comes out the back, but I lose speed and it feels sluggish.
So what do I do? Stick with the intuitive answer and open up the airbox or go with everything I've read, which says a clean plug means my engine's gonna seize up any second? Thanks a lot for taking a minute to weigh in.