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Repairing Moped Carburetors
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Technical Advice
A carburetor is a tube that air is sucked through into the engine. The speed  of the engine is controlled by how much air the carburetor allows through it. As the air passes through the carburetor, it goes through a venturi section that has a straw sticking up into the air stream. Fuel is sucked up the straw ( called an emulsion tube) and mixed with the air going into the engine.

If your engine is running badly, many times it is caused by a piece of sediment getting into the tiny holes that the fuel must pass through and plugging them, or old fuel will dry out and turn into a "tar like" goo that plugs the holes. Chemical cleaners do not work  well with either sediment or Goo, it is not the same type of dirt found in auto carburetors. The only way to really clean them correctly, since the holes are so small, is manually with a guitar string or piano wire.  The metering orifice (jets) in a carburetor are normally small screws with very precisely drilled holes in them to meter the fuel. The jets are numbered according to size, larger numbers meaning more fuel.
This is a image of a Spree carb, many are similar.
Notice the heavy dark deposits in the bowl on the top. That is caused by fuel drying out in the bowl, leaving the tar goo. This must all be removed when cleaning, or it will come loose later and replug the jets.

Most carbs have a pilot jet and idle air screw  for idle to 1/4 throttle and base idle, slide needle adjustment (E clip) for 1/4 to 3/4 throttle, and a main jet for 3/4 to full throttle.  The pilot and main jets  must be cleaned carefully to be sure no reside is left on the inside diameter of the hole. Best way is to floss them out with a small wire, like a very small guitar string. When done you must be able to see light through the jet.

The fuel in the bowl is controlled by the float and float needle, which works the same as a toilet flush mechanism.  The fuel fills the float bowl until the float rises up enough to force the float needle into its seat, shutting off the fuel flow. as the engine draws the air/fuel mix into itself, and the bowl empties, the float drops allowing the needle to open the way for more fuel to flow in from the gas tank.    If the float needle sticks or is dirty, the fuel bowl will overflow.

When you are running you engine, if it feels like it is running out of fuel, or is wheezing and weak, it may be running to lean (not enough fuel.)  If it stutters, and does not want to rev up all the way, it may be getting to much fuel  (rich)       When a two cycle engine is started cold, it will stutter a bit while warming up, then rev out to full speed. If the carb is set a bit too rich, you may get a long warmup period.    If your engine peaks out and runs up to speed right off the start, and then gets hot and slows a bit, it may be too lean.

The air cleaner assembly is critical for proper running. Remove the  air box, or part of it, and the engine may get so much air flow that it will just die out at after 1/2 throttle.   If you put oil on a foam element, it may reduce speed by running to rich, not allowing air to get through.
Typical Honda carburetor  has three screws
Bottom screw is the drain, very nice for getting out old fuel from storage, and water.  Screw that lines up with the carb slide is the idle adjustment. That should be set wherever it idles right. The Other screw that has a spring to hold it in plsc is the idle air screw. That is bottomed out, and backed out 1 and 1/2 turns in most cases.

How does that plastic choke thing work?

On most carburetors, the pilot jet and main jet are separate and removed by unscrewing with a small flat screwdriver. Be carefull that the tip on your screwdriver is in very good condition, sharp and flat. Pilots jets that are recessed into the carb body are difficult to remove and tight, if your screwdriver rounds out the slot in the jet you will buy a new carburetor.

The pilot jet on many Honda models is a brass tube pressed into the carb body and must be removed by carefully twisting and pulling with a pliers. It is common to crush the tube slightly on removal. The hole in the pilot jet is very small and a small stiff wire is required for cleaning.    Tap the tube back in place when done.

H-2  Kawasaki carburetor

Scooter Therapy Inc
12 North Few Street  |  Madison, WI 53703    
toll free 1-800-411-1543  |  local 1-608-255-1520    

fax 1-608-255-1587  |  E-mail

Scooter Therapy Inc    

12 N. Few St      
Madison, WI 53703 

toll free 1-800-411-1543    
local 1-608-255-1520    
fax 1-608-255-1587    

Hours: M-F, 9-6 Saturday, 10-3  CST

copyright (c) 2000
Scooter Therapy inc.

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