139QMB Issues After BBK

[jacobmb]

Sorry for the long epxlanation in advance.

Bought a generic (the original owner couldn't remember the model) Chinese 49cc scooter and rebuilt it. As far as I can tell it was a Kinroad XT50QT-2 (4-Stroke), which I disassembled, repainted the frame, and rigged with a digital dash and motorcycle headlight. I must brag it actually looks quite cool.

The Issue: The stock jets are a 32 Pilot and an 80 Main jet. I put a 47mm 70-80cc BBK on it when I rebuilt the engine with mostly new parts, new head, lightweight rollers, and performance CVT kit, etc. I am positive I installed everything right (chain on properly set at TDC), the valves were adjusted after it was built (.04 feeler guage), the compression was checked and was perfect, new spark plug, etc.

After adding a cone air filter and jetting up to a 90 Main and 38 Pilot, I used it for travel around my college campus for around two months. It would max around 6600 Rpms and 44 Mph on flat ground.

Eventually my cone air filter came off after a bump, which obviously threw off the A/F ratio. We took it back to my house (upright and secured), and I messed with the jets forever (with the same type cone filter on it) and cannot get it to maintain regular full throttle.

After alot of testing the most stable A/F (runs quite well acutally) at idle seems to be about 2200 Rpm idle on the stock Pilot jet (32) with the A/F screw three and half full rotations out from all the way screwed in.

I am operating on the assumption that screwed further in is less fuel, and screwed out is more. Testing by starting at 2 and half turns out and then adjusting direction for best response, then adjusting the idle screw.

Regardles of whether I use a 92 Main jet or a 78 (the lowest I have), the throttle dies or nearly dies between 5100-6000 Rpm (3/4 - Full Throttle). The response and ability to keep running does seem better on the 78.

The spark plug indicates it is running very rich, (black fouled after testing it just three times with a new plug). This confuses me because I dont plausibly see how I can install higher power components yet the bike seems to want a noticable downjet from stock, and it was fine before. I have tried everything I can think of at this point. As an additional note, I am fairly certain the fuel petcock is fine, and I have two identical 18mm carbs (stock and new) with the position needle and float set the same, so I am certain it isn't a stuck needle either.

I am baffled at this point, any help is appreciated.