Intro» Brendan Lutes // Photos» Garth Milan
Required Tools: 3 and 4mm Allen wrenches, Phillips screwdriver, needle-nose pliers, 8, 10, and 12mm T-handles, 4 and 14mm box wrenches.
Required Parts: Diaphragm and o-rings, throttle drum and springs, linkage lever, pushrod.
For most people, general maintenance doesn¿t go much farther than changing the air filter, sparkplug, and oil, with the occasional top-end or cam job. What most people¿including us¿forget to do is pay attention to the small things, like the carburetor, for example. Inside this integral yet overlooked part of the motorcycle are quite a few small intricate pieces which work together to make the bike come to life when you stab the throttle. And like any part of a dirt bike, these pieces inside the carburetor can wear out leading to less than crisp throttle response.
According to Team Honda¿s Lars Lindstrom, many people with older¿¿02 or `03¿450s might not even know that they need to replace these parts. In fact, they might not have even heard of them. So with this in mind, we decided to team up with Jeremy McGrath¿s wrench to find out just what he does to keep the throttle crisp enough and the carb clean enough for the Champ himself.
Step One: First things first; after taking the carburetor off the bike, the covers must be removed. First remove the cap from on top of the carburetor with the two 3mm Allen bolts. Then remove the side cover that protects the throttle cable roller. When both of the covers are removed, take out the small screw that holds the throttle drum to the throttle shaft arm. Now it is time to move on to removing the worn-out parts.
Step Two: Pull out the throttle drum¿which is found on the side of the carb¿and spring that is attached. You do this by twisting it to the left. This gets it out of the way of the linkage lever and enables you to pull it straight out. Carefully pull out the throttle drum. Once the throttle drum is taken out, it is time to remove the diaphragm cover on the bottom of the carb with the three Phillips screws.
Step Three: While removing the diaphragm, it is important not to lose any of the o-rings that are sitting inside of the cover, as you will need them when putting the carb back together; it is important to be careful with these two o-rings. Below the cover there is also a spring and the diaphragm itself. It is important to pay attention to the way the diaphragm is facing under the cover, since it will need to be put back in that way when you reinstall the new components.
Step Four: You need to pull out the pushrod from the linkage. To do this, press your thumb against the linkage-lever until it stops, which means it is bottomed out on the carburetor, then take a pair of pliers and pull the pushrod out of the plastic coupling it is attached to¿you will feel it snap out. The snap will let you know that the pushrod is free to be fully removed from the carb. After that, the linkage lever can be removed. All you have to do is remove the circlip and the linkage will come right off.
Step Five: These are the worn-out parts: the throttle drum and linkage lever. Once they are replaced, your bike will feel like a million dollars. Okay¿ not actually a million dollars, but pretty close.
Step Six: This is where the reassembly begins. To put the new parts together¿after the new linkage-lever is installed with the circlip snapped back on¿push the pushrod back into the carburetor while bottoming out the linkage-lever. With the linkage-lever bottomed out, snap the pushrod back in place. Take the diaphragm and install it back in the same direction that it was removed with the spring on top of that. While you are doing this, make sure not to drop the o-rings. Once everything is in place, put the o–rings back. After that, it is time to install the new throttle drum.
Step Seven: Take the new throttle drum and begin pushing it into the carburetor. While doing this, it is important to pay attention to how the spring on the drum is sitting. You also need to push the linkage lever down to get it out of the way. Once that is done, begin pushing the drum through the carburetor and into position. It should slip all the way back into the throttle-shaft arm. Once the drum is all the way in, turn it counterclockwise until there is tension on it and lock it in place on the throttle shaft arm.
Step Eight: One of the most important things is to use red Loc-tite on the screw that holds the throttle shaft arm in place. This essentially holds the entire mechanism together, and if it comes loose, the throttle and carb will not work properly. Once you have done that, all you have to do is put the covers back on and go ride.