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When I got rid of my stock carb and installed a Mikuni HSR42 flat-side, I was a happy camper except for one thing. I seemed to have a bit less power in the low and mid rpm ranges.
This, I discovered, was because the Mikuni HSR carbs have no TPS. The stock, Mikuni CV carb has a built in Throttle Position Sensor (TPS) that puts out a small, variable, voltage signal. This signal increases in voltage as the throttle is rolled on. The ignition module, a black box located in front of the battery, uses this electrical signal from the carb to adjust the spark advance for best performance and long engine life.
So with my new carb, my ignition module had no such signal. The two, ready and immediate options were, to set the timing module to least-advance, or to full-advance:
- Set to least advance, the engine pinged a lot under a load. Not good for the engine. Note: least advance equates to a wide open throttle (WOT) status.
- Set to full advance, the bike had less low-end and mid-range power. It also had less gas mileage. Not good for me. Note: full advanceequates to a closed throttle (CT) status.
A much better solution was to borrow a bit of time honored technology from the Harley-Davidson world. Rather than estimate proper spark timing based on throttle position, some Harleys, and many cars, have used intake manifold vacuum pressure.
A little device called a V.O.E.S. (Vacuum Operated Electrical Switch) uses this intake vacuum to create voltage-ON and voltage-OFF signals. It’s simple, and fills the bill.
You would think that, if ON/OFF voltages would significantly improve power, gas mileage, and engine life, a full(er) range of voltages would be better yet. However, based on my experience and others', it turns out that any advantage gained for the Road Star engine, by having a wider range of voltages, isn’t dramatic. A simplistic ON/OFF switch is a fully adequate solution in my opinion.
Note: See Author's Notes section at the end of this article for an introduction to other solution options.
Furthermore, intake vacuum in a V-twin engine is extremely erratic, so any measurement of vacuum beyond the crudest, will give very erratic results. This is probably the reason why--for the stock carb--Yamaha uses a throttle position measurement, even though vacuum measurement would ideally offer better real-world accuracy for adjusting spark timing.
Deciding Which VOES to Use
Since most VOES are sold as 'fixed-trigger-point' switches, the only real decision is, figuring out which 'strength' unit to purchase. Luckily, only one variable influences the VOES: vacuum, so the decision is fairly straightforward.
The basic VOES units which are best suited to our engines come pre-set to the following pressures:
- 3" Hg*
- 4" Hg*
- 5" Hg*
Here's how to calculate which VOES you need. This information is based on input from engine experts and experienced tuners I consulted:
3" Hg*, Drag Specialty# 2101-0007. For engines with:
- Stock pistons
- High performance carburetor
- VOES vacuum attached to carb
- Stock bore (engine displacement)
- High compression pistons
- High performance carburetor
- VOES vacuum attached to carb
- Larger bore than stock (big-bore)
- High compression pistons
- High performance carburetor
- VOES vacuum attached to carb
*Note: Hg is the standard, chemical notation for mercury, which is commonly used in measuring pressure--like a blood-pressure meter or a barometer.
Parts List
- Vacuum Operated Electrical Switch (VOES), with included mounting bracket hardware. See previous section for part numbers.
- Vacuum tubing, 5/32” diameter. Quantity: 1-foot, if you will be mounting the VOES near the carb, as I did. Tip: A piece of vacuum hose from the AIS can be used, if you have removed your AIS.
- Two tiny hose clamps, optional. They are for the vacuum hose. Tip: I used ones left over from removing my AIS.
- Flat washer for a bolt with a 6mm shaft, if you will be mounting the VOES in the same location as I did.
- Dielectric (silicone) grease, to inhibit corrosion of the electrical connections.
- Electrical tape or electrical shrink-tubing.
- High Temp gasket maker, such as Permatex RTV silicone #26B, red. This is for securely mounting the VOES to the bracket supplied.
- Wire stripping tool of some kind
- Wire cutters
- Needle nose pliers, if using 'squeeze-on' type hose clamps
- Screwdriver, if using screw-on type hose clamps
- 10mm metric wrench (socket, open-end, box, or whatever)
- 5mm hex wrench for removing the gas tank
- Large screwdriver or pliers for pushing on the VOES bracket retainer, optional, if you will be using the supplied bracket for your installation
- Large phillips (X) screwdrivers for disconnecting the battery
- Wire crimper or crimping pliers, if using crimped-on wire connectors
- Soldering pencil and solder, if using soldered connections
Note: This article assumes you have already removed your stock carb and installed a Mikuni HSR carb.
Remove the seat and tank. Disconnect the positive battery cable.
Locate the TPS electrical connector near the upper motor mount, just behind the left-side coil. It is a connector left over from removing the stock carb, and has three wires to it--yellow, blue, and blue/black. We will be tapping in to these wires, so the VOES is logically mounted nearby.
Next, you must decide on a vacuum source. The Mikuni HSR carbs come with an on-board vacuum-port--very handy.
Some owners, however, feel that the vacuum source should be as close to the heads as possible, to increase the accuracy of vacuum readings. The stock, and stock-type ported, intake manifolds have an integrated vacuum nipple that can be used as a source, but I have a Nemesis Racing aluminum manifold, which does not include any vacuum access. To remedy this, I could drill and tap a hole, and then mount a nipple in my manifold. After discussion with some others that have a similar setup, I decided that the benefit of doing all this would likely be minor, so I just used the carb's vacuum port.
If you will be using the carb's vacuum-port, locate it now. It's the rather long nipple that sticks straight up, at the rear of the carb. See photo below. Note: you may have to remove a plug-cap from the nipple if the carb is not new.
Take the vacuum hose you purchased (or found) for this project, and push it all the way down onto this vacuum nipple (or onto the manifold nipple, if you prefer). Then put a clamp over the hose--optional. Leave the other end of the hose free, for now. We'll finish it in a later step.
The VOES unit can be mounted anywhere, in any orientation. I used the mounting bracket that came with the unit, and attached it to my upper motor-mount bracket. However, others have used wire-ties or home-fabricated brackets just as effectively.
I found that the bracket retainer didn’t fit very snuggly, so I used some silicone gasket-maker to help create a more solid mount. To do the mount my way, first put a bead of high-heat gasket-maker (I used Permatex RTV #26B, red) around the large hole of the VOES mounting bracket. It's the big hole with the little key-slot in it. Put a moderate amount of gasket-maker on both sides of the bracket surrounding this hole.
Next, slide the bracket on to the VOES, with its key tab fitting into the key-slot of the bracket. See photo above for reference.
Now slide on the supplied retainer washer--cupped side down. This is a one-way retainer, so be sure to get the bracket, VOES, and retainer oriented correctly, before putting on the retainer. Though none of the photos show this retainer, use the photo above as a reference.
Push the retainer firmly on to the VOES, against the bracket. Use pliers or a screwdriver if you want, but be careful not to break the retainer or the VOES.
It is likely there will still be a little slop in the bracket, as the retainer doesn't fit down fully--at least mine didn't. The gasket-maker will take up this slack, once cured.
Set the bracketed VOES aside to let the gasket-maker dry.
After the gasket-maker has fully cured, mount the VOES in the bike. I mounted mine by using one of the little bolts that mounts the stock fuel-pump bracket to the upper motor-mount, between the cylinders. It is invisible when the tank is mounted. See photo below.
To do this, remove the outer-most bolt that mounts the stock fuel-pump bracket to the upper motor-mount. Slip a little washer on to the bolt; then mount the VOES at this location, and secure it using the bolt and washer.
Connecting the Wires
The VOES will be connected to the yellow and the blue/black TPS wires. It does not matter which VOES wire is connected to which TPS wire.
Since I thought I might want to try VOES units with other trip settings, I just used some wire taps. To do this, I simply clamped male connectors on both (yellow and blue/black) stock TPS wires. Then I trimmed the two, dark-grey, VOES wires to a reasonable length, and crimped female connectors onto the wires. Finally, I just smeared some dielectric grease on the contact pieces of the connectors to inhibit corrosion, and plugged the connectors together. See photo below.
You can use any method you wish, to attach the VOES to the TPS wires. Others have stolen the TPS connector from their stock carb, connected the connector-pins for the yellow and blue/black wires to the VOES, and then just plugged the connectors together.
Finishing Up
The only remaining VOES related step, is to attach the vacuum hose that you installed on the carb or manifold during an earlier step. To do this, slide a hose-clamp onto the hose, work the hose onto the nipple of the VOES, and then secure the clamp. See photo below. Note: The hose-clamp is actually optional.
Now just reinstall the gas tank, reconnect the battery, and put the seat back on. That's all there is to it.
The following are unlikely, but they are possible problems this installation might present:
- An air leak in the vacuum hose
- An air leak at the vacuum fittings
- An electrical short.
Start her up and enjoy greater low/mid range power and better gas mileage, as well as full high-RPM power. Author's Notes
Instead of a VOES, a few riders have installed a MAP (Manifold Air Pressure) sensor, while a few others have adapted the stock TPS to the HSR carburetor.
MAP Sensors: I, personally, have no experience with MAP units. However, as I understand, MAP units supply variable voltage to the ignition system, based on intake vacuum. Sounds good. The difficulty though, is that no MAP unit has been found that will offer voltage outputs on a similar power curve to the stock TPS. Also, there's the issue of the erratic intake vacuum inherent in V-twin engines.
TPS Adapting: Since TPS-adapting requires machining on the carb, not many owners have been willing to take such drastic measures.
A Delphi Road Star Riders forum member (Ken "The Mucker" Sexton), on the East Coast has begun offering a TPS adaptation service. According to The Mucker: Just ship him your HSR carb and the TPS adapter from your stock carb. He will perform his magic, and return your carb with the TPS installed and properly calibrated. The cost of this adaptation--as of this writing--is $100 plus one-way shipping. You can post a message to him on that forum, for further information.
As of this writing, I have found no performance data for these options. In addition, removing the TPS from your stock carb may reduce its resale value, if you decide to sell it.
Unfortunately, I must leave it to the reader to perform a cost/benefit analysis on the real value of VOES vs MAP vs TPS-adaptation. I made my choice…now it’s your turn.
Ride on.
