How to tune a scooter's carburetor

Written by tom lutzenberger
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The tuning for a scooter carburettor is a bit of a throwback process to the days before fuel injection and modern engines of today. The carburettor is in fact the mechanism that mixes fuel and air so that a combustion engine can run. Understanding how to tune them can improve your performance and scooter enjoyment.

Skill level:
Moderately Easy

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Things you need

  • Screwdrivers--both flathead and Phillips type
  • Crescent wrenches--metric and standard size
  • Carburettor manual for your scooter
  • 1/4" fuel line, usually about 3 feet for extra supply
  • Fuel line banjos (clamps to secure the fuel line)
  • Carburettor spray cleaner (do not use WD-40)
  • Shop rags
  • Replacement gaskets, jets, bolts and hardware as needed
  • Good lighting

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Instructions

    Education and Removal

  1. 1

    Review the appropriate manual for your scooter and carburettor. Find your scooter owner's manual and check if any information is available; don't be surprised if it's very general. Check the carburettor manufacturer's website for additional detailed information if your manual is missing it. Make sure you understand how your carburettor works and where all the parts are. Secure a parts diagram if possible.

  2. 2

    Remove your carburettor. Doing this is not always necessary, but it makes tuning easier. Shut off all fuel flow if on, turn off your scooter, and cut or remove all ignition sources. One spark can ignite gasoline, even if only fumes. Use the carburettor cleaner spray to clean off all grit and dirt. If performing a complete rebuild, keep all your parts together in a clean oil drain pan or tray to avoid losing them. Clean each part with carburettor cleaner and dry with a clean shop rag.

  3. 3

    Understand the difference in carburettor jetting. Different models will have different setups, but they all follow the same logic. Identify and locate your idle and low end jet, your mid-range jet, and your upper-range jet assemblies. Also identify and locate your air/fuel mixture screw if available.

  4. 4

    After removing, change your jets if you want increased fuel flow or decreased fuel flow. Most changes are in the full throttle range, so you can probably leave your idle and mid-range alone. The principle follows that if using a larger jet, you will have more fuel flow.

  5. 5

    Reinstall your carburettor, making sure all your parts are reassembled per your carburettor guide and diagram. Make sure no loose or remaining parts exist in your parts tray unless you have replaced a part. When reinstalling, make sure to use new gaskets for your carburettor if needed. Old gaskets are compressed and will cause air leaks. Order your gaskets for your specific carburettor from your scooter dealer if you don't have new ones.

    Testing and Testing Again

  1. 1

    Test your fuel line to make sure you have fuel flow. Turn on your fuel flow from your tank and let a little bit of fuel run out into a rag. Turn off when satisfied flow works, and reattach your fuel lines into the carburettor and out to the engine if needed. Hand-tighten all banjos on the fuel line and all securing bolts for the carburettor. Do not over-tighten; many carburettors are made of aluminium and will compress or strip if over-tightened. Test your carburettor by first turning on your scooter after reinstalling and attaching your fuel line. If the engine idles normal, then you're sitting state is good. If your engine speeds up or stalls out, adjust your idle screw to get the right timing. If the engine races in idle, you have an air leak; reinstall the carburettor again.

  2. 2

    Test your scooter at mid-range speed. Take it for a spin and stop frequently to listen to the engine. If it's racing fast after driving around, you have an air leak. You will need to reinstall the carburettor. If you stall out when giving it throttle, you have fuel starvation and the fuel flow is not working; go back and reinstall.

  3. 3

    Drive the scooter around for a while if it is running well, and then do a plug chop. Cut the engine while riding and coast to a stop. Check the spark plug by removing it and looking at the colour. If it is brown, you are OK to go. If it is oily black, you are running rich and you need to lower your jets and start over. If it is flaky white, you are running too hot and need to start over.

Tips and warnings

  • Testing a tuned carburettor takes time, and you make need to rebuild the carburettor a few times to remove and try different jet configurations. A scooter used for fast riding will need different jetting than one used for daily commuting. Have patience and use your knowledge of jetting from your manuals to pick the right jet range for your scooter.
  • Running an engine too rich will cause a lot of smoke and will foul spark plugs, causing your scooter to die out while driving. By running it too hot, you could seize a piston. You want to make sure your tuning is within an acceptable range for fuel flow versus air.

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