Might be a stupid question?
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Please can you post items for sale or wanted in the correct For Sale section. Items / bikes for sale here will be removed without warning. Reasons for this are in the FAQ. Thanks
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thmjo1
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Might be a stupid question?
But why does my NC29 have a Throttle Position Sensor? Thought these were fitted to electonic ignition systems to adjust the air/fuel mixture based on throttle position. So what does it do on carbed type motor?
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Davez29
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Re: Might be a stupid question?
I don't really know but I assumed it was doing something similar to what you have just said. Adjusting the ecu to adjust timing maybe? who knows? Put us out of our misery techie types..
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SevenThreeSeven
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Re: Might be a stupid question?
It's about ignition timing. A basic 2-dimensional ignition advance curve is a compromise so the engineers put a throttle position sensor on the carbs to let the CDI know when the rider was rapidly opening the throttle.
Here's what happens on my 2005 Kawasaki EX-250 which was such a cheaply built bike that it didn't get a throttle position sensor.
The EX-250 is legendary for its 5,000 rpm "flat spot". It's noticeable during gentle acceleration and very obvious under hard acceleration. Owners of this bike try and fail to tune the carbs so as to eliminate this flat spot in the acceleration.
The EX-250's CDI uses a very simple advance curve: 2-dimensions, X and Y, with no compensation for load. It's pre-historic, rudimentary.
At about 4,000 or 4,200 rpm, depending on the year-model of the EX-250, the CDI has the ignition timing at full advance (38 degrees BTDC for the older engines or 42 degrees BTDC for the newer engines).
This was, as I've mentioned, a compromise that helped the bike to pass EPA emissions testing and made it fuel efficient under light loads.
In the moment when you rapidly roll-on full throttle at about 4,000 rpm you suddenly make the air/fuel ratio much richer. In a modern car or motorcycle engine the ECU knows when this happens (the throttle position sensor) and it retards the ignition a bit because richer air/fuel ratios burn faster than leaner mixtures.
Because the EX-250 CDI has no way to compensate for the sudden rich (faster burning) mixture the ignition advance stays way up there at 38 or 42 degrees and you end up with too much of the burn occurring before the cylinder reaches TDC.
So instead of instantly making a lot more power (as you're anticipating when you open the throttle wide) you make only a little bit more. And the engine slowly lifts itself out of the situation, as the RPMs increase the fixed ignition advance becomes more appropriate, by default the timing of the ignition is better and as a result the bike produces more horsepower.
Here's what happens on my 2005 Kawasaki EX-250 which was such a cheaply built bike that it didn't get a throttle position sensor.
The EX-250 is legendary for its 5,000 rpm "flat spot". It's noticeable during gentle acceleration and very obvious under hard acceleration. Owners of this bike try and fail to tune the carbs so as to eliminate this flat spot in the acceleration.
The EX-250's CDI uses a very simple advance curve: 2-dimensions, X and Y, with no compensation for load. It's pre-historic, rudimentary.
At about 4,000 or 4,200 rpm, depending on the year-model of the EX-250, the CDI has the ignition timing at full advance (38 degrees BTDC for the older engines or 42 degrees BTDC for the newer engines).
This was, as I've mentioned, a compromise that helped the bike to pass EPA emissions testing and made it fuel efficient under light loads.
In the moment when you rapidly roll-on full throttle at about 4,000 rpm you suddenly make the air/fuel ratio much richer. In a modern car or motorcycle engine the ECU knows when this happens (the throttle position sensor) and it retards the ignition a bit because richer air/fuel ratios burn faster than leaner mixtures.
Because the EX-250 CDI has no way to compensate for the sudden rich (faster burning) mixture the ignition advance stays way up there at 38 or 42 degrees and you end up with too much of the burn occurring before the cylinder reaches TDC.
So instead of instantly making a lot more power (as you're anticipating when you open the throttle wide) you make only a little bit more. And the engine slowly lifts itself out of the situation, as the RPMs increase the fixed ignition advance becomes more appropriate, by default the timing of the ignition is better and as a result the bike produces more horsepower.