The Throttle Position Sensor (TPS) works off, you guessed it, the throttle. As you open the throttle up more it opens the Butterfly flap at the base of the Carburettor throat which allows more of the fuel and air mix to enter the engine inlets. The TPS is connected to the Butterfly mechanically via a spring mechanism inside the sensor that is actually a Potentiometer. It then measures a Resistance signal that is then sent to the ICM/ECU where it basically calculates the Advance or Retard necessary on ignition timing according to the throttle position per a given RPM and fires a spark to the plugs.
In the early days vehicles used to have a lever where the driver/rider would have to manually control the advance and retard on the ignition timing depending on throttle inputs. As technology has moved along they have since gone from full manual control to mechanical and then on to fully electronic. The TPS is an in-between stage as your indirectly manually controlling it from the throttle where it is then converted to an electronic signal.
Why Adjust the Throttle Position Sensor
When a new bike leaves the factory the the Throttle Position Sensor has been adjusted for best emissions not performance. This has an effect on drivability and low speed steady state cruise. Factory Pro who have a lot of experience with jetting and tuning recommends setting the TPS to 500 Ohms with a tolerance of -10 to +10 either way. It also states in the Honda workshop manual to set it to 500 Ohms.
Quote from Factory Pro
(The people that make the jet kits amongst other bits)
http://www.factorypro.com/prod_pages/prodh77.htmlAlso - as with ALL VTR1000's - the TPS must be checked and set to 500 +/- 10 ohms - or low speed cruise smoothness is impossible to attain.
They have been seen as much as 900 ohms as delivered.
As a side effect of setting the TPS to 500 it also helps to alleviate the problem of rough idle, stumble and cutting out.
How to Adjust the Throttle Position Sensor
Before adjusting the TPS make sure that you do a carb sync and set the idle speed beforehand as this could effect the reading. To adjust the Throttle Position Sensor you will require a T-25H Security Torx bit to undo the two bolts that hold it in place on the right side of the rear Carburettor and a Multimeter to measure the Resistance.
First remove the carbs off the bike which makes it far easer to adjust at first as it may require you to modify the mounting holes. It's also very fiddly to get spot on or close to 500. You will see three electrical contacts on the side of the TPS where the connector plug attaches. You need to measure the Resistance from the top two contacts as you face it. You do this with your Multimeter set to Ohms within a 2000 ohm range. Check what you TPS reads at the factory setting by placing a probe on each contact. Be careful the probes don't touch each other otherwise you will get a false reading. You will notice yours could be set to more or less around 800 from the factory. Make a note of the factory setting in case you want to put it back to standard at some stage.
Loosen the mounting bolts with the Security Torx. You can now spin the TPS anti clockwise in the slots, turn it as far as it will go to the left (anti clockwise) and measure again. My guess is that it wont be near 500. Now you can either leave it as it is or force it to go to 500 by adjusting further.
To Adjust it further there are two schools of thought. You can either elongate the slots with a dremel or similar tool by removing the brass spacers inside and elongate the slots, replace the brass spacers and use some Araldite or similar epoxy resin to fill the gap. This then allows you to turn the TPS further anti clockwise.
Or the way I prefer is to completely remove the TPS of the side of the carb. Don't worry there are no bits that can come flying out. Once the TPS has been removed you can see two metal prongs. The lower prong needs to be bent slightly downwards towards the base of the carb. Use a pair of long nosed pliers or something but go easily and gently as you don't want to snap it off or it will be replacement carbs time. I have not had one snap on me yet but there is the possibility, so bend downwards a little at a time. You have been warned!
Put the TPS back on noting the plastic prong on the back of it which slots in-between of the two metal prongs on the carb where you just bent one. Tighten the Torx bolts up and twist all the way anti clockwise and check the reading again. If still nowhere near 500 you may have to go back and bend the prong a little more. Once your happy that you can reach 500 ohms or less tighten the Torx bolts as much as you can but not fully and twist the TPS around until you reach between 490 and 510. If you can get it spot on 500 then great but as I said earlier it is fiddly.
The reason it is fiddly is because when you tighten the Torx back up fully to lock it off, the adjustment will move a little. For example, you could check it and it will read 500, tighten the Torx fully and it will read 520 as it moves from tightening back up. So the trick is to tighten the Torx as much as you can so you can barley twist it. Then move it a bit at a time, more of a jolt really. Maybe give it a gentle tap with something, a short sharp shock! Tighten the Torx fully then check again with the multimeter. Or you could try to work out by how much, more or less, it is moving by when tightening fully and compensate for this on the original reading before tightening fully. So for example, if when tightening fully it adds or removes 20 Ohms then deduct or add this from your original reading and set it to either 480 or 520 depending which way it's going. It could take you a while of fiddling around trying to get it spot on to 500, or you could just settle for it being within spec. Persistence is the key.
In an ideal world the TPS should be set with the carbs back on the bike and throttle cables connected as the throttle cable adjustment may move the butterfly slightly which will alter the reading, but there is very limited space to get in and take a reading or move the TPS around and tighten it back up. The next best thing is to not to seat the carbs back in the carb inlet rubbers and lay them to the side slightly with the throttle cables attached. Yet again this is extremely fiddly, it's hard enough to get the TPS spot on 500 with the carbs off the bike and on a workbench, on the bike it's more tricky but is possible.