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Tuning Your Carburetor

For 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.


Learn 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 notes.

START FROM THE BEGINNING (some people will do it differently, but this is my method):

  1. Get the Main jets 116, 118, 120, 122. Start with 118.
  2. Get mixer tubes B3E and B5E. Start with B3E.
  3. Turn 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.
  4. Now back it out 1 and 1/2 turns. Pay attention! Be exact!
  5. Turn out the idle screw (slide stop on top of the carb!) counter clockwise a couple full turns.
  6. Install new spark plug. Make sure you are putting in the correct type of plug.
  7. Turn the ignition on and turn on the fuel.
  8. Kick the bike a few times until it wants to start but dies.
  9. If 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.
  10. Kick 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 work with.
  11. Once 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.
  12. Turn 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.
  13. Mark 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.
  14. Release throttle and turn out air mix screw 1/4 turn.
  15. Twist 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.
  16. 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


The VTScoot InfoBase

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The Vermont Scooterist Spring Tune-Up
Scooter Buying 101 Tools to Carry
Riding Gear 101 Buyer Beware: Modern
Buyer Beware: Vintage VT "Moped" Definition
Tuning Your Carburetor Basic Maintenance
How to Manual Shift Vespa vs Lambretta
Rules For Group Riding  



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