Troubleshooting Carburetors

Trouble shooting a rich or lean situation By  MGD "Jeff"
Carb Diagnostics and Tuning

The instructions at the "CC&D webpage" performance section are excellent and
have been successful many times, but on occasion some fine tuning may be necessary.
The riding style, accessories and total weight are areas that may need some carb

Basic info

A rich carb setting will often exhibit the same drivability characteristics as a bike with a lean
carb setting.....the mpg will be the only indicator unless some form of testing can be done.

Pipes will change color if the bike is rich or lean.

A lean bike is easier to diagnose than a rich bike.

Choke Test

If there is a particular speed and gear operating zone, where you experience a drivability
problem (example; surge, miss, hesitation) the choke test can easily be performed.
While duplicating the drivability problem, pull the choke knob out to the second notch. This
will richen the carb circuit you are operating in. If the problem improves or disappears,
that particular carb circuit is lean. If the problem gets worse , then the carb circuit is already 
too rich.
(Do not do the choke test by leaving the choke on for an extended ride.)

Tape Test For Rak AK

The tape test is similar to the choke test, or may be used with the choke test. Tape is applied
(usually duct tape) over a portion of the inlet air supply opening. By restricting the air flow
to the carbs, you can richen the fuel mixture. With the tape,  you can cover different percentages
of the inlet opening. The same results from the choke test apply. If the bike runs better, then 
it is lean.
The tape test can be effective if; you believe the bike is lean in most of the carb circuits or
if you are working in several problem areas.
Remember, the tape test will make the bike rich in properly tuned carb circuits or will richen
lean carb circuits.

Plug Test

Personal opinion,  is a waste of time. Today's gasoline's has so many additives, that it is hard 
to get spark plugs to color quickly, unless the bike is rich.
Old spark plugs will color quicker than new spark plugs. Check the color on your old plugs,
make jetting corrections, ride the bike for 20 miles then let the bike cool down and pull the plugs.
Compare the old jetting color and the jetting color. Darker is richer, gray, brown, tan...darker
is richer.

MPG Test

The bike is running fine. The bike is running perfect. Your mpg is around 40. You are rich.
MPG is determined by dividing the miles traveled by the gallons of gas purchased at a fill up.
The Canadians and Europeans will do kilometers and liters.
A properly jetted bike should be able to achieve high forties to low fifty mpg.,unless you
ride like a maniac. 

Rev Limit
Rev limiter points 1st 30-32 mph, 2nd 50-52 mph , 3rd 70-72mph, 4th 90mph

Wide open throttle to the prescribed limit points at which time one of the cylinders will shut down – (it will sound like it’s gonna blow up… don’t worry – it won’t).  Hit the first limit point when the engine all of a sudden starts to run really poorly then shift to the next point. If you hit the rev limits at the right speeds or better, this is a good indication that the bike is jetted correctly.


Also see the Theory write-up, the Roll Off test in Webslingers Jetting info and the Tuning and Troubleshooting guide by Mikuni.