Motorcycle Repair: carb, air fuel mixture, plug colors

Question
2002 suzuki 1500LC, I have installed cobra pipes and installed 126 jets with the matched metering rods. At 3500rpm the bike starts crapping out. I have pulled the plugs, they look slightly gray to white. The metering rods are adjustable so I have been backing off on the length to get more fuel flow. I have made 4 changes and now I can get to about 4 grand before it crapps out. Idle is smooth with throttle snap to 5500rpm instantly. I have one more slot on the metering rods to go and then I'm out of adjustment on the rich side. I am pretty sure I am going the right direction but itsure doesn't seem like it. HELP!

Answer
Hi Dennis,

If you need to go richer, install larger mains and lower the needles... and do as you have done.

Perform spark plug chops and read plug colors.

Respectfully,
Mark Shively





Spark Plug Chop

A plug chop simply means looking at the spark plugs to determine how you are jetted at a particular throttle stage. The only tricky part is in isolating the throttle stage. Before starting, clean your spark plugs, air filter, and warm the engine to operating temperature.

To check the main jet, which operates from 1/2 to full throttle, get the motorcycle up to a steady full throttle or near to it for a good few seconds, then simultaneously pull in the clutch and kill the engine. Don't let the motor drag you to a stop, or fall to an idle because you want to isolate the plug color at full throttle.

Get off and check the spark plugs colors. They should be light shades of brown color. If plugs are white, the air/fuel mixture is too lean. Install a larger main jet. If black or really dark brown, the air/fuel mixture is too rich and a smaller main jet is needed. If you need to change jets, change by one size and do another run. This work is time consuming, have patience.

Repeat the above procedure for each carburetion range. See the chart below for more details.


1.   IDLE: Set idle speed to proper RPM by adjusting the IDLE SPEED SCREW. Turn the AIR SCREW to achieve the highest speed and best response. After adjustment has been made reset the IDLE SPEED SCREW to the proper RPM.

2.   OFF IDLE to 1/4 THROTTLE: The SLOW JET and 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. 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 chip position at the top of the needle will richen the mixture. Lowering the needle will lean the mixture.

4.   WIDE OPEN THROTTLE: Changing the MAIN JET affects this range. Select the size which offers the best WOT performance, then install one size larger MAIN JET for ideal engine durability.


JETTING RANGE EFFECTIVENESS CHART


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