PHBN Carb. Tuning - HELP

The Colonel

Well-Known Member
I am having a really hard time with tuning a PHBN 17.5 carb. Not the main jet, but the low end. I can't get it NOT to stumble from idle to 1/8th throttle.
Scooter is a 2001 Aprilia Scarabeo. Cylinder is a Corsa, airbox is stock, but desnorkeled. Main jet is a 96, pilot is a 34 - stock reeds. Nice brown plugs.

It can't leave the driveway with the stock PHBN 17.5 (A-2) needle set on lean. It fouls out. I tried an A-8 needle. Runs too rich on it's leanest setting...but doesn't foul out. Then, for grins, I tried the stock needle from the stock 12mm carb. I think it's an A-11. Almost no taper. It's like a brick. Anyway...it runs OK on the leanest setting. Everything I have read says that a 34 pilot is too lean for that cylinder...let alone the stock needle set leaner than the stock 50cc setup! I don't trust it. There isn't any dead spots or bogs. There is a rattle when I close the throttle down to 1/8th, but if I add the choke, it stalls out. Sounds lean, acts rich. Any ideas??
 
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put the stock snorkel back on, it's throwing the carburetor out of whack. i run a tuned Corsa kit & a lot more on my Aerox with stock airbox with snorkel still intact.
 
you should get at least a 17mm to 19mm snorkel for your airbox & invest on some pilot & main jets, sounds like you are running too small pilot jet, that rattle you hear @ 1/8 is detonation, pilot jet controls fuel flow at 1/8 to 1/4 opening, after 1/4 to 3/4 it's the needle thick ness & taper. pilot screw is for idle to 1/8 opening.
 
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Well, you were mostly correct. It was your suggestion along with weak reeds. I put new carbon fiber reeds in and the hesitation is gone. Added a 38 pilot and it's running great!
 
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