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Old 03-04-2012, 07:59 PM   #6
Navin OP
Beastly Adventurer
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Joined: Sep 2009
Oddometer: 8,480
TPS theory

Shamelessly copied from another thread;

FOR 950 CARBS - the Throttle Position Sensor, TPS, sets the ignition advance. The ignition set the advance based on Engine RPM and Throttle Position. This is called a 3D Map, 1. RPM, 2. TPS, 3. Ign Advance.

When you turn on the ignition, the small computer in the ignition looks for a 'referance voltage' to determine if the TPS is in range. The voltage should be 0.5-0.6 volts when the throttle is shut. You should always have the throttle shut when turning on the ignition.

This referance voltage is set at the factory at engine idle. BUT, if the idle is turned down during the break-in period, then the referance voltage may be wrong! You may need to reset the TPS after the engine loosens up and the idle position is different. I've seen several 2006 950 Adventures with a too low voltage of 0.38 v. These had a bad surge around 3000 rpm. Reseting the TPS usually fixed it without re-jetting.

If the ignition doesn't see this voltage during start up, it may not set the advance correctly and you'll get a surge between 2000-3800 rpm. This acts just like a lean carb condition. It can be masked by richening up the carb low speed mixture screws or low speed jets, but you'll get poor mileage and a weak low rpm throttle response. If the TPS is dirty or worn, you can get the same erattic behavior.

The workshop manuals show how to set this. I'd recomend to try a resultant voltage very close to, but below 0.6 v. Check it several times by turning off the ignition and make sure you still have the same voltage when you turn it back on. Make sure it tracks voltage smoothly as you open/close the throttle. Any jumps, dead spots or other flaky action means you need a new TPS.
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