Fabricating a TPS Driver

Written by Randy Fox (randysgym)   
Thursday, 27 August 2009

Introduction

yamaha road star, tps driver, throttle position sensor

All Road Stars built before the fuel injected, 2008 models vary their ignition timing via a small sensor attached to the carburetor. This sensor is called the Throttle Position Sensor -- or TPS. Theoretically, the more you twist your right wrist, the more demand is placed on the engine, and the less the ignition timing should be advanced. In real life riding this theory has a few holes, but in my experience the TPS still out-performs other available methods for the Road Star carburetors, including V.O.E.S. and M.A.P. (See the Installing a V.O.E.S. article in this website for more information on VOES and MAP). The TPS offers the potential for better torque at all speeds, better fuel economy, and longer engine life.

Unfortunately, few aftermarket carbs include a way to connect the TPS. Carburetors built for high performance (racing) applications have no need for variable ignition timing since it is assumed that the engine demands will always be maximum, and therefore the mechanic just sets the timing to a fixed point.

For many owners however, the temptation to upgrade from the stock carb to achieve more power, quicker response, more reliability, and easier tune-ability outweighs the benefits that the TPS provides, and so the TPS comes out with the stock carb. In an effort to have both a better carburetor and proper ignition-advance, a few ingenious owners have fabricated custom brackets for certain, popular, aftermarket carbs to adapt their stock TPS. However, most have simply ignored the engine's needs for variable ignition timing in their thirst for more peak horsepower.

My design has one moving part: an axle. The axle is rotated via an integrated pulley that accepts the throttle pull cable -- which has been slightly shortened. The end of the little axle is fastened directly into the TPS, which moves the same way as it did in the stock carb. Another, tandem pulley, integrated into my axle, accepts a very short cable that, in turn, actuates my aftermarket carb.

My project construction boiled down to just seven, simple, yet time consuming steps: (1) Fabricating the axle, (2) Fabricating a basic pulley, (3) Finishing the pulley for dual cables, (4) Fabricating the framework or plates, (5) Fabricating a mounting bracket and cable receiver, (6) Ordering new throttle cables, and (7) Fabricating a dust cover.

 

Background Info and Personal Notes

Variable ignition timing has been important in getting the most out of every gasoline vehicle's engine since the Ford Model 'T'. Early cars had a lever on the steering column to manipulate the timing manually. Modern fuel injected engines employ a computer chip to calculate and adjust the timing.

I initially installed a VOES system for my Mikuni HSR42 aftermarket carb. But over the first year, I noticed that many times the VOES was not advancing the ignition timing when I thought it should have. I never quite knew when my bike would act like a lion or a kitten. So when I upgraded to a Genesis, "Big Air", 48mm carb from PSIPower, Inc, I decided it was a good time to get my TPS back.

 

Since the Genesis did not provide any place to adapt an on-board TPS bracket, a stand-alone TPS module was my only option. PSIPower offers a stand-alone TPS module (PSI Genesis Big Air Accessories part# 05-009). And one big plus this unit offers is its ability to work with any carburetor. However, as of the date this article was published, their TPS module cost roughly $150. Plus, their unit uses an additional throttle cable -- either the 'push cable' or a 'splitter cable'.

 

Money aside, this last requirement was a deal breaker for me. I just wasn't interested in re-installing the archaeic 'push cable' that I had removed some years before.

 

In the end, I decided to build my own TPS module -- one that would incorporate the TPS from my stock carb, not require the throttle's 'push cable,' and would fit in the space under the tank. This article describes how I did this.

Warning: This project is among the most time consuming and exacting of any I have done on my bike. Considering the relatively small, seat-of-the-pants performance improvement it makes, most others would likely prefer to spent their time in other improvements. I am happy with my result, and would do it over again, but then, I enjoy this kind of thing.

 

Tools Needed

Angle Grinder --with cutoff wheel and grinding wheel

Electric drill (minimum) --or drill press, or machine lathe

Bench vise -- or other means to bend heavy gauge sheet metal

Pliers --for bending sheet metal, and general use

General hand tool set --screwdrivers, allen wrenches, file/s, punch, measure, etc.

Tin (sheet metal) snips --or nibbler or plasma cutter

Sandpaper --in the following grits: 50, 100, 220

Dremel, rotary type tools:

  • Dremel motor unit, or equivalent
  • Small diameter, straight-shaft, diamond cutter
  • Sanding drum
  • Cutoff wheel

4-40 tap --used to thread the set-screw hole in the pulley.

0.09" drill-bit --used to drill the set-screw hole in the pulley.

 

Materials Needed

2" x 2" x 1/4" mild steel plate (or nylon) --for making the pulley system.

2" segment of a 1/2" mild steel dowel --for the axle.

Brass/bronze round stock or bushings --This will be used to make axle bushings.

Lexan, polycarbonate, plexiglass sheet --At least 4.25" x 3.5" x 0.93". This will be for building the plates that hold the axle/pulleys in place.

8-24 x 3/4" Stand-offs --Quantity 4, for making the framework.

8-24 x 1/2" Screws --Quantity 8, for holding the plates together. I used stainless steel ones.

Thick gasket material --I used pieces of an old inner tube.

Sheet metal --Approximately 24 gauge, at least 6" x 3.5", for making spacers for the TPSensor mount and the dust cover.

6-32 x 2" Screws --Quantity 2, for mounting the TPSensor to the plates. I used stainless ones.

6-32 Washers --Quantity 2, for mounting the TPSensor to the plates. I used stainless ones.

6-32 Nuts --Quantity 2, for mounting the TPSensor to the plates. I used stainless nylock ones.

4-40 Screw --Quantity 1, for use as a set-screw to hold the pulley to the axle.

 

Fabricating the Axle

The axle’s job is threefold:

  1. It must pivot to allow the pulley to rotate.
  2. It must connect to the stock TPS so the pulley’s movement will create a corresponding movement in the TPS.
  3. It must have pivot shoulders to rest against its bushings. The shoulders control the side-play of the axle.

See photos and diagrams below.

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

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.

 

yamaha road star, tps driver, throttle position sensor

 

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.

 

yamaha road star, tps driver, throttle position sensor

 

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.

 

yamaha road star, tps driver, throttle position sensor

 

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.

 

yamaha road star, tps driver, throttle position sensor

Finishing the Pulley for Dual Cables

Once the basic pulley was made, I split it in half so as to make two semi-circular pulleys.

To do this, I first sanded, filed, machined, and/or ground one side of the pulley to be as flat, and parallel to the groove, as possible. Tip: It would probably have been easier to have done this step before cutting the groove.

Then, I drilled a series of small, closely spaced holes, along a diameter-line, but circumscribing the center area of the pulley, to meet the diameter-line holes. See photo below. Tip: I think it would have been much easier to have used a plasma cutter or a steel cutting coping saw.

 

yamaha road star, tps driver, throttle position sensor

 

Next, a cutoff wheel on a Dremel was used to cut through both sides along the diameter-line, between the holes. Then a small diameter, straight-shaft, diamond cutter bit was used on the Dremel to cut through the semi-circle holes and complete the pulley bisection. It took a long time. See photo below. Tip: Again, I think it would have been much easier to have used a plasma cutter or a steel cutting coping saw.

 

yamaha road star, tps driver, throttle position sensor

 

Remember, only 1/4 turn (90 degrees) is required, so considerably more pulley trimming can be done.

The next step was to sandwich the two, pre-flattened sides together, using an offset. To do this, I laid each pulley onto a protractor (degree wheel) and used a permanent marker to mark 45 and 135 degrees from one edge. Then it was easy to find the 90 degree offset. The pulleys were glued and clamped together at this offset. JB Weld was the glue used. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

After this, holes for the throttle-cable end-barrels needed to be drilled. My plan was to use two cable segments:

  • One to go from the handlebar grip to one of my TPS pulleys
  • One to go from my other TPS pulley to the carb

The holes were drilled at 112.5° for each pulley:

  • 90° rotation (1/4 turn)
  • 2° of addition distance to eliminate cable-end movement/wear
  • 20.5° of addition distance to accomodate the cable-end hole

See diagram below.

 

yamaha road star, tps driver, throttle position sensor

 

The pulleys overlap each other by 90°, which leaves 22.5° of non-overlap area at each end.

To drill the cable-end holes at the correct location, I first made a protractor template. See template below.

 

yamaha road star, tps driver, throttle position sensor

 

Then I transferred marks from the template to each pulley, at 90° and 92°, as given by the template. Tip: I used a permanent marker, but a scriber might offer better precision.

My next step was to find the center point for the cable-end hole. I did this by measuring the diameter of my cable end-barrel (the tiny cylinder at the end of the throttle cable) -- mine was 0.24" -- dividing that number in half, and then marking a dot on the pulley, 0.12" from my 92° mark -- in the direction away from 90°. I then drew a line through this mark and the pulley's pivot point. I called this my cable-end line.

Next, I placed an aluminum straight-edge (but a credit card would work as well) down into the pulley groove. I positioned the straight-edge so that it touched the bottom of the groove at my cable-end line. Then I precisely marked the points where the straight-edge met the outside edge (along the pulley's largest diameter). Note: A line between these points will be perpendicular to my cable-end line. See photo below. The photo does not show the 92° mark.

 

yamaha road star, tps driver, throttle position sensor

 

Then I repositioned the straight-edge to the outside, and aligned it across my two edge-marks. Then I just marked a line segment to intersect my cable-end line. I thus had a 'cross-hairs' to mark just about where the center of the cable-end hole would be.

Next, I adjusted my 'cross-hairs' point by one half the thickness of the throttle cable (0.008" in my case) up -- toward the perimeter of the pulley -- and center-punched it as the center-point of the cable-end hole.

I chucked a 1/4" drill bit into my drill press, and drilled through, for the cable end-barrel. Tip: You could use a hand-held drill, as long as you're careful to get a clean, straight hole.

Repeat those steps for the opposing pulley. See result, below.

 

yamaha road star, tps driver, throttle position sensor

 

Then I just trimmed the ends of each pulley segment. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

Then it was just a matter of cutting a slot through ½ of the pulley into the cable end-barrel hole so as to allow the cable end-barrel to be inserted. Tip: I cut my cable-insert slot into the cable-end-barrel hole from the outside edge of the pulley. See photo below. This makes it very difficult to install the cables onto the pulleys. I wish I would have cut the cable-insert slot from the hole to the perimeter of the pulley. Although that would leave the pulley slightly weaker, there is very little stress on it anyway.

And finally, you could drill some holes and do some milling to lighten the overall weight, and clean it up a bit. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

Assembling the Pulley Parts

The axle and pulley get connected via a simple set screw in a hole drilled through the edge of the pulley's center. This allows me to adjust the pulleys to be in exact sync (harmony) with the TPS.

To make this hole, I Drilled a 0.09" hole into the edge of the pulley's axle hole. This hole was then tapped for a 4-40 set-screw. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

Fabricating the Framework or Plates

I built my plates from the following:

  • 0.093" thick, clear, Lexan (polycarbonate/plexiglass)
  • 10-24 x 3/4" 'stand-offs'
  • Stainless steel screws

See photos and diagram below.

 

yamaha road star, tps driver, throttle position sensor

 

 

yamaha road star, tps driver, throttle position sensor

 

There were several tricky parts in building the framework:

  • The stand-offs needed to be shortened so that the unit would be as thin as possible.
  • Two of the stand-offs needed to be filed (or milled) to accommodate the swing of the pulley so that the unit could be as narrow and short as possible. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

The plexiglass plates were drilled for the stand-offs so that the stand-offs were flush with the edges of the plates. Also, the plates were drilled for the axle pivot bushings at a point centered side-to-side.

The next step was to make the axle pivot bushings. I made mine in my mini machine lathe, but you could buy some ready-made bushings through a clock parts supplier, or perhaps an R/C model store. See photos and diagram of mine below.

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

Next, the bushings were 'glued' into the plates. The smaller bushing was secured using silicone, high-temp, RTV, gasket maker. I figured using silicone would allow me to easily replace the bushing some day, as needed. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

The bigger bushing needed stronger glue. I ended up using small amounts of JB Weld, which I hope to be able to file away, should the need arise to replace that bushing.

After the bushings were in, I needed to create a ‘well’ for the Yamaha TPS unit, since I had decided not to make any modifications (cutting, filing, or trimming) to the TPS. Tip: By keeping the Yamaha TPS unit stock, I could do a simple swap-out, in case mine failed at some point down the road.

The ‘well’ I created was built up from the following (in order, from bottom to top):

  • A piece of 26 gauge sheet metal
  • 3 plexiglass layers
  • A piece of rubber inner tube
  • Another piece of 26 gauge sheet metal

The first 4 layers were glued together, and the bottom layer (sheet metal) of the stack was glued to the plate. The top 2 layers (rubber and top sheet metal layer) were left unglued. See photos below.

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

I made the ‘well’ oval shaped for no reason in particular. Once the ‘well’ was glued up, I reduced the overall height a little, to position the Yamaha TPS to the proper depth. To do this, I just sanded off the top plexiglass layer of the ‘well’ until proper depth was achieved.

I secured the Yamaha TPS unit to my plate via 2, long, stainless screws, nyloc nuts, and a couple of washers. I decided to embed the nuts into the ‘well’ so they would be near-flush with the inside of my plate to prevent interference with pulley movement. By fixing the nuts into the plate, I am now able to adjust the stock Yamaha TPS unit by simply loosening the screws, just like it was intended.

To embed the nuts, I first glued my ‘well’ together (as stated above) using JB Weld, being very careful to line up the big holes for the TPS. Next, I carefully drilled 2 shallow holes into the plate, large enough to accommodate the nuts, that were aligned with through-holes I had already drilled to line up with the Yamaha TPS mounting holes. Then I JB-Welded the nuts to the plate, being very careful not to get glue onto the threads or the through-holes. See photo below.

 

yamaha road star, tps driver, throttle position sensor

 

yamaha road star, tps driver, throttle position sensor

 

It may not look pretty, but it’s strong.

 

Fabricating a Mounting Bracket

Designing and building the bracket took as long as building the TPS Driver itself. As small as the unit is, I still found it challenging finding a location near the carb that was spacious enough, yet also allowed a natural flow for the throttle cables. In the end, I found just enough room above the intake manifold, just forward of the upper motor mount, and behind the front rocker cover. See photo below, TPS module highlighted in yellow.

 

yamaha road star, tps driver, throttle position sensor, bracket

 

In retrospect, it might have been a better option to mount the unit between the upper motor mount and the front rocker cover -- transverse (sitting sideways). If this option would have worked out as well it looks, it would allow a more graceful throttle cable run from the TPS to the carb; mine has no room to spare.

My bracket places the TPS module as far forward as possible, and routes the throttle cable inboard of my Nemesis aftermarket coils. See my final design pattern diagrams below. Each diagram shows the same design; I just split all the info up into 4 drawings to make it easier to digest, hopefully.

 

yamaha road star, tps driver, throttle position sensor, bracket

 

yamaha road star, tps driver, throttle position sensor, bracket

 

yamaha road star, tps driver, throttle position sensor, bracket

 

yamaha road star, tps driver, throttle position sensor, bracket

 

The diagram shows bend lines as blue dash lines. It also indicates the direction and order of each bend via the numbered, red/yellow, arrow-circles. It also shows the approximate location of holes to drill.

The dimensions and angles of your project may differ. Consider this info as a good starting point. Make some paper, and some 26 gauge sheet-metal mock-ups, before finalizing your design to heavy gauge, 16 gauge plate steel.

The basic pattern, glued onto sheet metal should look something like the following photo. Note: this photo is actually of an earlier design.

 

yamaha road star, tps driver, throttle position sensor, bracket

 

Once I cut the pattern cut out, I bent along the dotted lines, and my bracket looked like the photo below.

 

yamaha road star, tps driver, throttle position sensor, bracket

 

yamaha road star, tps driver, throttle position sensor, bracket

 

Note: The above photos show a slightly lesser evolved design from my final design.

From this point, I fit, bent, fit some more, welded, trimmed, and bent some more, until I reached my final design, as given in the diagrams earlier. My bracket looked like the photos below.

 

yamaha road star, tps driver, throttle position sensor, bracket

 

I then reinforced the cable receiver tabs of the bracket via small welds. However, JB Weld epoxy would work equally well. The reinforcing supports are shown in the photos below. (Note: The photos were taken after final paint was applied.)

 

yamaha road star, tps driver, throttle position sensor, bracket

 

yamaha road star, tps driver, throttle position sensor, bracket

 

Finally, I drilled and tapped the upper motor mount on the bike’s frame to receive the 6mm bolts I used. I also replaced the right-side coil bolt with a slightly longer one, to accommodate the thickness of my bracket, mounted at the rear of the coil.

Then I cut a piece of old inner tube to use as a gasket and vibration damper. The rubber piece was doubled over and hole punched to fit under the rear mounting points (on the upper motor mount). See photo below.

 

yamaha road star, tps driver, throttle position sensor, bracket

Ordering (or building) new throttle cables

There are two cables to order:

  • A stock cable, but three inches shorter
  • A short cable, with a 5" long outer casing (cable sheath), a 9" long inner cable, a stock-type end-barrel on one end of the inner cable, and an end-barrel with a set-screw for the other end of the inner cable

The end fittings for the casing of your short cable will vary, depending on where you mount your TPS module, and what type of carb you're using. If you are installing a Genesis IV carb, like I did, you'll want an S&S type end-fitting for the cable casing's carb-end. And if you install your TPS with a bracket like mine, you'll want the other end to have an end fitting that is as short as possible. I made my own cable end fitting from an old, stock, throttle end fitting. To do this, I just shortened (trimmed) it as much as I could, leaving just enough threads to keep it strong. See photos below.

 

yamaha road star, tps driver, throttle position sensor, cable

 

yamaha road star, tps driver, throttle position sensor, cable

 

yamaha road star, tps driver, throttle position sensor, cable

Fabricating a dust cover

I made a simple little dust cover from 26 gauge sheet metal. The purpose is to keep the majority of dust and water off the pulley and axle. Time will tell, if my design is successful. After one summer and one fall of city commuting and riding I disassembled the entire unit, and found it as clean as the day I installed it -- so far, so good.

I used my sheet metal snips to cut out the simple pattern. See diagram below. Then I used 'duck bill' type welding pliers to sharply bend the sides to form the box. Once I was happy with my shape, I soldered the corner joints. Tip: you could use JBWeld or other strong glue if you prefer.

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

Next, I needed to create space in the dust cover for the axle and bushing -- sticking up from the framework-plate surface. I inserted the assembled TPS module, small axle end first, into the cover-box and pressed it against the box's bottom, so as to allow the axle end to scratch the inside of the cover. Then I removed the TPS module, and used a dome punch, aligned to the scratch mark, to make a bump-out (a dent) in the dust cover to give the TPS axle room to rotate freely. Tip: If you don't have a dome punch, you could use a small ballpeen hammer (the ball end) or anything to create the dent. Just be careful not to tear through the thin sheet metal.

Then I placed my TPS module into the box, and checked the depth. I wanted the dust cover box to be just slightly shallower than the depth of the TPS module, so I used a scribe (or permanent marker) to mark the inside of the box to the TPS module's depth.

Next, I ground and filed the box just below the line, and check the fit again.

Then I drilled holes for mounting the dust cover box to the TPS module. To do this, I inserted the TPS module, small axle end first, into the box and pressed it against the dust cover, so as to allow the 4 framework-plate screws to scratch the inside of the cover. Then I removed the TPS module, and drilled holes, aligned to the scratch marks.

The only remaining step was to cut the throttle cable slots -- which I did.

Note: My dust cover wouldn't go on my TPS module while mounted on the bike. The fit was just too tight. And I couldn't put the dust cover on, with the module off the bike, because that would inhibit installation of the throttle cables... so I trimmed (cut away one corner of) the dust cover enough to be able to put it on after the TPS module was mounted, even though this compromised some of the cover's ability to keep dust out. See photos below.

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

yamaha road star, tps driver, throttle position sensor, dust cover

 

yamaha road star, tps driver, throttle position sensor, dust cover

Pulling it all Together

Now that everything is built, it's time to put it together and get everything adjusted properly. First mount the TPS module to the bracket. Tip: I'd suggest using a dab of blue thread-lock on the screws as you install them.

Remove the Front-Plate from the TPS module, and gently swing the pulley through its range of motion (90°). The left and right pulley holes for the cable-ends should swing through identical (but mirrored) arcs, with respect to the Driver. If not, loosen the pulley set-screw and adjust the pulley on its shaft until identical pulley arcs are achieved -- rotating left and rotating right. Then tighten the pulley set-screw.

Next, pull out the Pulley assembly (pulleys and axle) from the framework-plates. Install the cables as follows:

  • Remove the throttle cable from the bike, or hold the TPS driver near its install position on the bike.
  • Thread the cable wire end through the TPS Driver.
  • Loop the end-barrel back and into the pulley cable-end barrel-hole, aligning the cable with the slot. See diagram below.
  • Pull the slack out of the cable, as you guide it into its pulley groove.

 

yamaha road star, tps driver, throttle position sensor, cable

 

Now do the same thing with the other cable, from the opposite side.

Next remove the outer adjuster nut from the end of one cable. Work the cable onto the bracket cable-mount.

Rethread the adjuster nut onto the cable end.

Now do the same with the other cable.

Throttle cable: Stock length - 3"

TPS Driver to carb, cable-casing: 5"

TPS Driver to carb, inner-cable: 9" or less, depending on your carb. I recommend you use an adjustable end-barrel.

 

Appendix A - Project Math & Overview

The TPS for the 1999 - 2007 Road Stars is little more than a small, black, rotating, proprietary (I think) potentiometer (pot). It incorporates a couple of screw mounting holes, a few integrated wire leads, and a wire-harness connector plug. The rotation amount, from least advance to full advance, is 1/4 turn.

So all I had to do was fabricate a separate little box that would convert the maximum distance my throttle cable moves into 1/4 turn of a tiny axle. In other words, turn linear motion into circular motion.

Since my Genesis carb requires about 1 1/8" (1.125") of cable draw, from closed throttle (CT) to wide open throttle (WOT), I had all the info I needed to compute my rotation needs.

Tip: Here are the cable draw requirements for other, common carbs:

  • 11/16" (0.6875"), for the Road Star's stock, 40mm, Mikuni, CV carb
  • 13/16" (0.8125"), for the Mikuni, HSR 42mm , flatside carb (the HSR 45mm, should be roughly 15/16", though I have not measured it.)
  • 1 1/16" (1.0625"), for the S&S, Super-G carb

First, I applied the following formula from basic geometry:

Circumference = pi times diameter

Circumference, is the distance around the outside of a circle (like a wheel or gear). In my case, I knew I wanted a circle (a pulley) with a circumference four times bigger than my throttle draw (remember, I only wanted 1/4 rotation).

Pi is simply the number 3.14...

Diameter, is exactly what I needed to know. But in order to find the pulley diameter I'd need, the above formula needed to be rewritten as follows:

Diameter = Circumference / pi

My circumference is (4 x 1.125"). Which is: 4.5" .

So my circumference divided by pi is: (4.5" / 3.14)... which is: 1.43".

So the pulley diameter I wanted should be about 1.43"... or a little less than an inch and a half (e.g. as measured down in the trough of a pulley, or if I was going to use a gear, it would be the outside of a gear).

Tip: If you find math frustrating or challenging, here are the TPS pulley, 'trough' diameters for other, common carbs:

  • 0.875", for the Road Star's stock, 40mm, Mikuni, CV carb
  • 1.035", for the Mikuni, HRS 42mm , flatside carb
  • 1.353", for the S&S, Super-G carb

 

 

Appendix B - My Rack and Pinion Failure

In my first build attempt, I went with a pinion gear for my rotation, and a rack for my linear motion. Vex Robotics of Greenville, TX had what I needed in their Advanced Gear Kit. They make plastic, 2.25" long, gear-racks, and matching, 1.5" diameter, pinion gears.

I decided that sturdy plastic, made for robotics, would be durable enough especially since the torque demands of the TPS were nearly zero.

In the end, however, the rack-gear made the design just too big. I could find no easy way to make it small enough to gracefully fit under the fuel tank. I eventually abandoned the idea, and started over, using the pulley system described in this article.

The pulley system described in this article is smaller and simpler. While the design, admittedly, introduces some force into the bearings of the little axle that drives the TPS, the amount of that force is small. The brass bushings I made for axle-bearings will likely out-last the bike.

I have included my original 'rack and pinion' efforts here below, in case the method somehow works best for your application. As the photos below show, I didn't get too far. See diagram and photos below.

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

yamaha road star, tps driver, throttle position sensor, rack and pinion

 

 

 



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DISCLAIMER: This information and procedure is provided as a courtesy and is for informational purposes only.  Neither the publishers nor the authors accept any responsibility for the accuracy, applicability, or suitability of this procedure.  You assume all risks associated with the use of this information.  NEITHER THE PUBLISHERs NOR THE AUTHORs SHALL IN ANY EVENT BE LIABLE FOR ANY DIRECT, INDIRECT, PUNITIVE, SPECIAL, INCIDENTAL, OR CONSEQUENTIAL DAMAGES, OF ANY NATURE ARISING OUT OF OR IN ANY WAY CONNECTED WITH THE USE OR MISUSE OF THIS INFORMATION OR LACK OF INFORMATION.  Any type of modification or service work on your motorcycle should always be performed by a professional mechanic. If performed incorrectly, this procedure may endanger the safety of you and others on your motorcycle and possibly invalidate your manufacturer’s warranty.


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  Comments (2)
Full advance at idle?
Written by ChilliWillee, on 10-28-2013 21:31
Most engines advance timing as revs increase, not decrease; so full advance is at speed and base advance is at idle. Stock Road Star (2004) timing is 10BTDC @ idle advancing to 40BTDC @ 4,000. If you were to have full timing at idle and decrease as revs climb, you would have a problem with pinging at low rpms and sluggish response at high rpms, not to mention hot running around town and crappy gas mileage.
What?!
Written by jamesrrt, on 10-30-2011 20:00
Fuel injected models came before 2008. I have a 2004 Warrior with Fuel Injection.

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