The Mikuni 28mm TM flatslide carburetor provides a better than normal response time when mixing fuel and air into a motorcycle or scooter engine compared to standard round-slide carburetors. As a result, this model carburetor tends to be in high demand by those who ride and race for recreation on small-engine, two-wheeled vehicles. Changing the performance of the carburetor is not hard, but tuning it for optimum performance takes patience and multiple changes.
Apply a socket wrench to the spark plug in the engine and loosen it. Pull the plug out. Examine the tip of the plug to determine the color. If the plug is chalky white, you can anticipate that more fuel is needed; if the plug is oily black, less fuel is needed. Compare the results against a spark plug chart if you're not sure.
Remove the carburetor from the motorcycle engine using a crescent wrench. Detach the carburetor by loosening the clamps holding it to the intake manifold. Use a screwdriver to loosen the fuel line. Pull the fuel line off. Take another crescent wrench and loosen the top of the carburetor assembly. Pull the throttle slide and cable out. Disconnect the cable.
Use a crescent wrench to open up the float bowl on the bottom of the 28mm carburetor to expose the main jet assembly. Use a screwdriver to loosen and remove the jet assembly. Based on the results in Step 1, change the main jet size depending on the spark plug color to increase fuel (bigger size) or decrease it (smaller jet size). Reinstall the jet assembly. Close up the float bowl and tighten it.
Pull the throttle slide out of the carburetor. Locate the throttle needle. Change the position of the needle in the throttle based on your spark-plug readings. Adjust the setting using needle-nose pliers and by moving the clip position that holds the needle in the slide -- move the clip up for less fuel and down to the narrow end for more fuel. (See Resources.) Reinstall the slide with the needle into the carburetor body, attach the throttle cable and close it up again. Tighten the top with the crescent wrench.
Reattach the carburetor to the intake manifold and reconnect the fuel line. Pull on the throttle assembly to confirm it is connected correctly. Start the engine and let it idle. Take the scooter or motorcycle for a ride. Come back, turn the engine off and use the socket wrench to remove the spark plug again. Examine the color.
Repeat steps 1 through 5 until the spark plug shows a chocolate color, not white or oily black. Continue the tuning process until you have the brown color even at high speeds in top gear. The tuning process for your Mikuni 28mm flatslide carburetor and engine is complete when the spark plug's brown color is confirmed.
