The TM38-18 is supplied with the following factory jetting installed:
Pilot jet 20 (VM22/210), Needle 6EJ12-55 (richer and leaner available), Needle Jet (Fixed in place), Main Jet 370 (4/042), Slide 3.5 (999-832-014)
Dimension A= 44mm, B= 76mm, C= 164mm, D= 62mm, Total Width= 76mm
Unlike many motorcycle manufacturers’ purpose built and modified pollution compliant versions of the TMX carb, these factory TMX flatslide racing carbs are regarded by the factory to be the best performing version of this carburetor ever made.
The TMX features a flat slide design in a smooth bore venturi to provide improved throttle response and wider powerband. The float system with independently moving floats, working with a main jet enclosed in a baffle chamber, allows the TMX to perform smoothly without hesitation on the roughest terrain. These carbs are jetted to suit most 250 and some later 125cc high performance motocross engines and once fitted will eliminate a lot of the hesitation, hard starting and other performance problems experienced with some OEM carbs.
It is always advised to buy a spare one size up (380) and one size down (360) main jet so as to positively verify your correct fuel mixture and optimum power at WOT. When replacing carbs with throttle position sensors and other such electrical items fitted you will need to determine if these do not need to be connected before purchasing and installing one of our carbs.
If your carb measures around 12mm or 20mm longer than the 76mm front to back measurement of the TMX carb, and if intending to still use the standard air-box you will need to purchase separately from our TMX Carbs page the corresponding silver anodized extender. When ordering an extender with your carb please advise in the order comments section if you want us to press fit it to your carb with Loctite before delivery. The extenders have an outside diameter of 60mm at the air-box end, slightly smaller than the carb itself.
The picture above with the cable shows the required length of inner cable protrusion and securing fitting required at the carb end at basically a minimum length adjustment. The back board is divided into 1 and 10mm increments. If your inner cable is to short you may consider one of our Quick Action throttle and cable assemblies.
IMPORTANT TUNING CHARACTERISTICS
The jets in this carb are spot on for most motorcycles with a few requiring a main jet changed as suggested above and once fitted and checking the pilot setting, occasionally some motors may ask for a one or two size leaner pilot where Husky’s often like a 25. The needles only ever may need a change when fitted to some multi cylinder applications. Follow the tuning sheets supplied with the carb and if requiring a change, the perfect tune is easily achieved.
The Jet Needle is the tapered rod that is positioned in the throttle valve by the ‘E’-Clip. The taper of the needle increases the clearance between the Jet Needle and the fixed Needle Jet outlet as the throttle is opened. As the air flow volume increases past the throttle slide, the fuel volume is also increased to maintain the correct air/fuel ratio.
NEEDLE ‘E’-CLIP POSITION
The position of the ‘E’-Clip in the Jet Needle is used to correct or change the air/fuel ratio between 1/4th and 3/4th throttle valve position. The ‘E’-Clip can be raised or lowered on the Jet Needle. To richen the fuel mixture the ‘E’-Clip is lowered on the Jet Needle, raising the Jet Needle. To lean the fuel mixture the ‘E’-Clip is raised on the Jet Needle.
To correct the fuel mixture at 1/8th to 1/4th throttle slide position it may be necessary to change the Jet Needle. The Jet Needle will have a series of numbers stamped on it.
Example: 6EJ 12-55. The numbers 55 indicate that the outside diameter (O.D.) of the Jet Needle is 2.55mm. A smaller O.D. number gives a richer mixture. A larger O.D. number gives a leaner mixture. Typical Jet Needles available: