Motorcycle Carburetor Maintenance. A Complete Guide

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Motorcycle Carburetor Maintenance is not an easy task. This is an introduction into the workings of a traditional motorcycle carburetor.
The name itself comes from the first carbide lamps that used a small hole to drip water on carbide rocks. This produced a gas that was fed through another small hole and was burned. The use of precise holes to deliver a burnable fuel is similar to the motorcycle carburetor, thus the name.

Motorcycle Carburetor Maintenance Can Be Frustrating

The motorcycle carburetor can be a temperamental device. It can cause all kinds of trouble if mishandled or neglected. They can be VERY expensive to have a dealer overhaul. It is well within the skill level of most garage mechanics, but you may have more success if you find an older guy, who hasn’t spent his life working on fuel injected systems.

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Two Common Types

Two basic types of carburetors are used by most companies. The Constant Velocity (CV) type and the Slide Type, and it is these I will be talking about. The CV type carb, (Constant Velocity or Constant Vacuum), as the Yamaha FZR1000 Genesis carbs above, use pressure to raise the throttle slide. The slide carburetor, like The Dellorto carbs from a Laverda 750SF2 below simply uses a cable to connect throttle to carburetor.

sf2 carbs

What A Carburetor Does

A carburetor is a device that controls both the amount of air and fuel that is allowed into the cylinders. The amount of air is controlled by a butterfly valve (CV) or a vertically moving throttle slide. The amount of fuel is controlled by the drop in pressure in the throat of the carburetor and the size of the holes that allow the fuel in.

To do any successful work there are a few things you will need:

  • clean work surface that will not be disturbed
  • carb cleaner, one can should do
  • clean rags
  • oil catch pan
  • toothbrush or similar brush
  • wood dowel rod and toothpicks
  • standard set of tools
  • compressed air source
  • typical manual such as Haynes or Clymers

Tuning Your Carbs

Carburetor tuning has a huge effect on engine performance. When a motorcycle manufacturer builds a bike, they sell the same model worldwide, so they couldn’t afford to install different jets in the carb to suit all the different climates and types of fuel. In addition to the climate and fuel, the manufacturer would also have to consider many other factors, such as the terrain and type of riding. And then there is the most important jetting consideration, the rider. They simply cant do this, so the jets they usually install in the carb may well be too lean or too rich depending on all the factors above.

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To better understand you need to understand the various “stages” of fueling involved. The following diagram will hopefully help, and below you will find a simple stage by stage explanation of the working part of the carburetor involved with each stage of throttle opening.

carb jetting stages

As you can see above, at idle, only the slow jet and air screw control fuel, but as the throttle is opened the various jets slide and overall dimensions of the carburetor come into use.

Quick Jetting Guide.

  1. IDLE: Set idle speed to correct r.p.m. by adjusting the IDLE SPEED SCREW.   Turn the AIR SCREW to achieve the highest idle speed and best response.  After this adjustment has been made adjust the IDLE SPEED again back to the correct r.p.m.
  2. OFF IDLE to 1/4 THROTTLE: The SLOW JET and the AIR SCREW are most effective in this range.  When you want a richer mixture use a larger SLOW JET or turn the AIR SCREW in (on some carburetors the “air screw” is on the engine side of the carburetor, It works the same but acts as an ENRICHER, so screws out to enrich rather than in, see below).  The opposite holds true for a leaner Mixture.
  3. 1/4 to 3/4 THROTTLE: The JET NEEDLE is the most effective component in this range.  Raising the needle by lowering the clip position at the top of the needle will richen the mixture.  Lowering the needle will lean the mixture.
  4. WIDE OPEN THROTTLE (W.O.T.): Changing the MAIN JET affects this range.  Select the size which offers the best W.O.T. performance, then install one size larger MAIN JET for ideal engine durability.
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Air Screw Or Fuel Screw?

NOTE: Not all Carbs have AIR SCREWS.  Some have FUEL SCREWS, it is important to differentiate as to which one you have.  This depends on the location of the screw on the carb. (looking at the center of the carb) If the screw is in between the cylinder and the carb body , it’s a FUEL SCREW, turn it OUT to get more fuel (richer) if the screw is in between the carb body and the air box, it’s an AIR SCREW, turn IN to richen it.  As far as I know this rule of thumb is correct.  Check with your dealer or manual if at all unsure.  Note: most air/fuel screws should be set somewhere between 0.5 to 3.0 turns from seated, if it is outside this range you should look towards your slow jet.

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Dont be nervous, be careful. If you can get hold of an old carb, strip it, clean it and become familiar with the parts. The more you do it the more comfortable you will become.

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CAUTION NOTE: 

  • Do not cross thread any screw threads.  Take your time and if the thread seems tight stop, back it out and try again.  Remember, if you strip a thread, the carburetor is either Junk or an interesting Paperweight.
  • Do not use excessive force to tighten any of the screws.  They need to be tight but not “Big Arnie” tight.  Just remember, you are not building an air raid shelter, you are working on a finely tuned instrument that needs a firm but delicate touch.
  • Do one carburetor at a time
  • Avoid mixing parts from one carburetor with another if at all possible.
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