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Understanding the VOES
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 advance equates 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.