Replaced uni filter with airbox - upjet?!

[smicsmit]

When I bought my scoot, the carb has been sitting forever and the inlet hose from the airbox was busted. I bought a new carb that came with a uni filter. Got it running and jetted well, but intended to replace the airbox when I could.

Well, now I can. Ordered a new airbox; installed it. It was acting lean,I assumed it was actually very rich because of the airbox. Down jetted from a #87 to an #80. Very obviously lean at WOT, can't understand why. So I threw in a #90 main just for shiggles. It runs surprisingly better, but top speed is lower than before I replaced uni filter with the airbox. I plan on upjetting little by little later when I have time.

My question is, has anybody had to upjet after going to an airbox instead of uni filter? I was almost positive I would need to downjet and would get better mpg. Whats going on?

[GrumpyUnk]

If the carburetor has an adjusment on the needle end that seats in the slide, it could be set to the lowest clip groove, which would tend to make the needle jet cause a leaner mixture. Upjetting can allow more fuel, but the needle actually opens as the slide moves, which allows fuel to flow through the jet. If the needle were too low, I think it would tend to be lean. If non-adjustable, compare it to the original to see if it appears to be a thicker in diameter needle. The mix of fuel and air is 'emulsified'(filled with air bubbles) in the emulsion tube directly above the main jet. That mixture is flow limited by the needle jet... If the slide is sticky or the diaphragm damaged/sticky/wrinkled it could affect the slide motion...
tom