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Carburetor Jetting

Yamaha RD Carburetor Jetting

Correct carburetor jetting is important for proper combustion. If the carb is jetted too lean the combustion temperature could get too hot resulting in either a seized piston, or a hole melted right through the top. Just one size too lean is all it takes for possible disaster, therefore we offer you some guidelines that you want to consider as you refer to them. HVCcycle is in no way responsible for any damage to your motor as a result of applying any of our advice or tuning specs. If you need to do it yourself to save the money, just be willing to be learn and accept the consequences of a piston failure. Remember, you seize or burn-down, it's your fault! Our recommended method is play it safe; get several main jet sizes in the range you're working in, and just start a little rich & work down.

What is jetting?

Carburetor adjustments, usually involving changing of quickly-removable metering orifices called jets. Besides the jets, needle position adjustment, or changing, as well as changing slides are all part of the adjustments available.
Why the need for jetting in the first place? Any time a modification is made to the motor, such as install a different exhaust system, or air filter, you will need to re-jet the carbs to compensate for the difference in airflow. If air flow is enhanced, fuel flow must be adjusted accordingly. The optimum air fuel ratio is 15:1 for gasoline. It has to be just right. When exhaust gas temperature is optimum, 1250-1275 degrees F, the spark plug insulator will be a light tan color. Too hot & it'll burn light gray or white, too cool & it'll burn dark brown or black. Get familiar with exhaust note changes when jetting and get used to checking your plugs often. They are windows into your engine's soul. Other factors requiring jetting adjustments are changes in air density, altitude as well as types of fuel. Get a Sudco Mikuni tuning manual if you don't feel real comfortable with carbs. It's worth it's weight in gold.

Carb Jetting Guidelines - Just a ballpark range based on years of customer feedback and personal experience. Read your sparkplugs at different throttle settings, aim for a mocha, light brown color.

Application
Stock Porting

Main Jet

Pilot Jet

Needle clip position

RD250 y-boot stock pipes 120-130 25-27.5 3
RD250 y-boot  DG pipes  140-155 25-27.5 4
RD250 y-boot HVC Performance pipes 135-145 25-27.5 4
R5 350 (piston port) y-boot stock pipes 125-135 30-32.5 3
R5 350 (piston port) y-boot DG pipes 130-145 32.5-35 2
R5 350 (piston port) y-boot HVC Performance pipes 140-155 32.5-35 3
RD350 y-boot stock pipes 150-160 25 3
RD350 y-boot  DG pipes  210-220 25-27.5 4
RD350 y-boot HVC Performance pipes 230-250 25-27.5 4
RD400 y-boot stock pipes 125-135 25-27.5 3
RD400 y-boot DG pipes 180-220 25-27.5 4
RD400 y-boot HVC Performance pipes 180-220 25-27.5 4

Remember, use these specs at your own risk! Play it safe - start rich & work down. The specs above are a general range for U.S. spec RD's at sea level. Higher altitudes or humid conditions will require proportionally leaner settings. If you're new to this, it helps to practice removing your carbs and replacing your main and/or pilot jets as well as adjusting the needle clip position. It doesn't take long before you're as quick as a pro & really inspires your tuning confidence. For a great carburetor website with lots of diagrams and charts go to www.sudco.com, For information on our tuning service

Sudco/Mikuni Tuning
Manual

002-999.jpg

A must for any do-it-yourself
type. This 92 page manual
includes exploded views, parts,
listing, optional parts, and
guides you through tuning tech-
niques for the popular line of
Mikuni Carburetors. The definite
reference for Mikuni
Carburetors.
  (002-999)

Listed below are jetting specs that we use quite a bit. If you have a similar set up you can try the jetting listed.

JETTING

1975 RD350

VM28 stock carbs.

Y-boot and K&N filter

JL exhaust

TDR Reeds

Iridium spark plugs

Boost bottle

new points and high capacity coils 

mild porting

Needle Jet  175-P-0

Pilot Jet 27.5

Main jet will be between 180 and 240

Needle the same. Position, start in the center and go from there

1973 RD350

VM28 Stock Carbs

Uni Filter, Stock Air Box

Stock Carbs

Power Dynamo Ignition

Rebuilt Engine

Needle Jet 175 0-8 (stock)

Pilot jet 27.5

Main Jet 140 (stock)

Needle clip position as needed

air screw 1.5 turns out

Plugs B9HS

1976 RD400

Cylinders .5mm over (64.50mm) Freash rebuild

HVCcycle Performance Heads

JL Expansion Chambers

Stock 28mm Carbs

K&N Y-boot air filter

Pilot Jet 27.5

Needle Jet 175-P-6

Needle 5N13

Main Jet 220

Air screw 1/2 turn out