those of you who ride scooters with carbureted motors (Vespa, Stella,
etc), I've opted to re-print some info I found on scooterbbs which
has a much more thorough method for tuning than most of the other
resources I've read elsewhere. So if your scooter is running funny
and you want to try tuning the carb to get better results from your
ride, I hope this will be of assistance.
how to set up your scooter's carb correctly. Take the time to do
it. Be extremely patient. Do it on your day off when you have a
few hours to dial it in perfect. Do it step by step. Don't assume
anything. If you get confused or forget, don't be afraid to start
from scratch. Find a baseline of operation and go from there. Have
the right tools and use a damn torque wrench. Be methodical. Take
FROM THE BEGINNING (some people will do it differently, but this
is my method):
the Main jets 116, 118, 120, 122. Start with 118.
mixer tubes B3E and B5E. Start with B3E.
Air Mix screw all the way in until it just stops. Notice I didn't
say screw it in until it gets so tight you ruin your shit.
back it out 1 and 1/2 turns. Pay attention! Be exact!
out the idle screw (slide stop on top of the carb!) counter clockwise
a couple full turns.
new spark plug. Make sure you are putting in the correct type
the ignition on and turn on the fuel.
the bike a few times until it wants to start but dies.
it starts and then dies, you need to turn in your idle screw one
full turn. If it starts and then proceeds to a high idle your
idle screw is too far in. Back it out.
it again and get it to idle. you might have to get the bike to
idle fast so you can keep it on and give yourself something to
it is running (even shitty), slowly turn in the air mix screw
until the bike stalls and dies. Turn it a half-turn at a time.
Give it a minute or two between changes. Did you need to turn
it all the way in and it still is running. Congratulations, you
have an air leak. If you turn it in 1 full turn (after starting
at 1 and 1/2 turns out) and the scooter stumbles and slowly dies,
your jet is probably correct. If it starts to die after 1/2 turn
in, your jet is too big. Assuming you don't have any air leaks.
it back out to the 1 and 1/2 out. With the scooter running, turn
the IDLE SCREW counter clockwise until the engine dies. Turn it
in a bit and start it again. Engine dies? Turn it in another 1/4
turn. Keep it up until the engine idles relatively well or as
slow as it can without dying.
your throttle at the grip 1/4 throttle, 1/2 throttle, WOT. With
the scooter running (after running continuously for about 5 minutes)
twist the throttle slowly until you hear the exhaust note start
to break up and spit.
throttle and turn out air mix screw 1/4 turn.
the throttle slowly until you hear the exhaust note start to break
up and spit. If it is still not giving you a nice clean whine
out to WOT give another 1/4 turn out on the air mix screw and
do it again.
I generally do not like to go past 2 full turns out on the air
mix screw. If you are getting so much fuel that you need that
much air your bike could run hot. So if after 2 turns out it is
still spitting, you should go to a smaller jet - or experiment
with a different mixer tube.
Article by Kevin Montanaro