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Fabricating a Basic Pulley
Update 01/31/2010: A reader has offered a source for some ready made pulleys. They won't work for all carbs, but you can check them out at Atlantis Jet Sports. Look down the page for Throttle Cable Drum For Mikuni Carburetor.
I often tend to make things more complicated than necessary, and this was no exception. I couldn't easily find a suitable pulley, so I used a 2" x 2" piece of 1/4" steel plate to make one. As stated in the calculation section of Appendix-A at the end of this article, the throttle cable should wrap one quarter-turn of a pulley with a working (trough) diameter of about 1.4" (for my carburetor). Tip: The pulley diameter needed for your carburetor may differ. See Appendix-A for details.
It would have been simpler to make two, full pulleys, I think. That way, the two parts of the throttle cable could wrap more than 90°, and then determining the cable end-barrel hole positions in the pulleys would not be critical. Also, each pulley could be later trimmed to fit the space under the tank. However, I decided to make two half-pulleys (actually from one pulley cut in half) so as to minimize the unit's over all size. The bottom line, though: Space under the tank is definitely limited. In other words, the challenge is, to design something easy to build but as small as possible.
Tip: Ideally, my pulley's working (trough) diameter should be slightly smaller than my calculated 1.43" to allow for the thickness of the cable. In reality however, any pulley with a working (trough) diameter within +/- 5% of ideal should work fine. Just be careful of going too small, as this might result in over rotation of the TPS -- breaking it.
First, I used an angle grinder to fashion a rough circle with a 2" diameter. Note: It would have been much easier to use nylon, but I couldn’t find any (until after I completed my project!).
Next, I drilled a small hole in the center of my rough circle to fit a machine screw. I used a nut to tightly secure the screw, plus another nut to lock the first nut in place.
Then I mounted the screw in my drill-press, set the drill-press to a medium-slow speed, and began spinning my circle. I used my angle grinder against the circle's edge, to create a more perfect circle. See photo below.
Next, I secured my angle grinder to a short piece of 2" x 4" wood, using pipe hanger strapping. I then set the drill-press table height to precisely the height needed so that the cutting wheel of my grinder touched the center of the edge of my circle. See photo below.
I increased the speed of my drill-press to be moderately high, turned it on, and switched on the grinder. Then I began carefully cutting a groove into the edge of my circle.
To determine how deep to cut the groove, I made a simple caliper from a scrap of 18 gauge sheet metal. I cut the width of the jaws to 1.4". Note: The width you need may differ, depending on the cable draw of your carb. See Appendix-A at the end of this article for more information. See photo below.
So I continued cutting a groove deeper and deeper, until my homemade caliper barely slipped into the groove.
After this, I moved the drill-press table down a tiny, little bit, to grind the groove a little wider. Yours may differ, depending on cutting wheel thickness, etc. Just be sure to make the groove (the pulley's trough) wide enough to accommodate the throttle cable without binding.
Next, I ground the pulley's outside diameter further down, until it was about 1.8". The smaller the pulley, without making the groove too shallow, the smaller you can make the framework-plates, and the easier it will be to find space under the tank.
Then I used sandpaper and a small, flat, jeweler's file to smooth all the edges -- especially the inner edges and the bottom (floor) of the groove. I started with the file for the outer edges, then I moved to 50 grit sandpaper, and then to 100 grit, and then 220.
Viola’! I had made a deeply grooved throttle pulley. See photo below.