REPOSTED 4/4/19 WITH LOCAL FILE POSTING: PHOTOBUCKET NO LONGER WORKS!
No Problem. I think it becomes clear when one fully understands what is happening here, so to this end, let me offer this.
This is my lousy drawing of the throttle-bodies arrangement on the Stelvio.
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As you can see, there are butterfly valves inside of the throttle-bodies. As they open, more air is drawn in from the airbox to mix with the injected fuel. The blue line connecting them together, is the linkage rod so that when one turns, the other turns the same amount. The yellow arrow is the location of the fast speed throttle-body balance adjustment screw. The red arrow is the stop plate screw which is factory set and you should NEVER touch otherwise the whole thing is ruined and must be replaced! The green diamond is the TPS sensor which resides on top of the right hand side throttle body. Now let's look at real pics.
Here is a side view , pointing towards the rear of the left hand (sitting on the bike) throttle-body. The colored arrows match the previous diagram. Yellow is the adjustment screw, red is the stop plate screw you never touch.
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Here is the same side taken from a more straight on angle. Again, yellow arrow is adjustment screw, red arrow is factory set stop plate screw you do not touch, and now the white arrow shows the connecting rod which goes over to the right hand side throttle-body (in my lousy graphic, this was the blue line connecting the two throttle-bodies together).
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Here is the best view. Notice the white circle which shows the connecting rod going all the way over to the right hand throttle body, and the red circle which shows the factory set stop plate screw resting against the stop post from the throttle-body. This is why you never adjust the stop plate screw (red arrow) because this distance is a factory calibration which is linked to the fuel injection software as to it's position, approximately 4.9 degrees. Basically this distance is a constant so that the fuel injection map, which depends upon throttle POSITION (i.e. open or closed within it's range of motion) is calibrated at a ZERO point. It will make more sense in a moment.
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This is the throttle-body on the right-hand side. The green diamond is the TPS sensor. The white circle shows the connecting rod coming from the left-hand side which connects to the right-hand side. The throttle cable only attaches to the left-hand side throttle-body. All movement on the right-hand side is controlled by this linkage rod. You never adjust or change this rod. It too, is factory set.
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Now, let's look at the bleed screws. Left-hand side first. It is the blue arrow and you can make out the hex head on the inside of the screw. You can also see an excellent view of the factory set stop plate screw in the red circle.
Also look carefully at the yellow arrow (the fast speed throttle body balancing screw). See the mechanism with the spring? When you turn this screw, it allows the secondary plate (on the right side of the spring)to move INDEPENDENT from the left side of the spring. So, the plate on the left side of the spring is the left-hand throttle-body. It NEVER moves from the factory set position. The plate on the right side of the spring, is connected to the control rod which goes over to the right-hand side throttle body. Thus, when you adjust this screw, you are physically moving the butterfly valve in the right hand throttle-body! Get it? )You can see the two independent plates in the second photo) The left side never moves adjustment wise, but the right side can be adjusted in or out as needed to balance the high speed balance. Remember, the TPS sensor sits on top of the right-hand throttle-body - i.e. the one that can be MOVED. The fuel injection computer orders the injectors to do their thing based upon certain conditions which it always is evaluating, one being throttle position, and if we moved the butterfly valve on the right-hand throttle-body, the TPS will read an incorrect position (voltage) from the TPS sensor from the get go and our fuel injection map will be out of alignment with actual operating conditions.
Now this is WHY you need to be able to reset the TPS. Remember that the TPS position is linked to the fuel injection map in the ECU. In order for the throttle position and the fuel injection map to be in alignment there has to be a synchronized ZERO point. That point is the value set by the factory on the left-hand set plate screw of usually 4.9 degrees and this is why you NEVER adjust this screw. But we do reset the TPS so that the altered-position of the right-hand throttle-body, is set to match the factory baseline setting of 4.9, (this is what the left-hand side will ALWAYS BE - IT CANNOT EVER MOVE SO LONG AS YOU DON'T TOUCH THE STOP PLATE SCREW) regardless of where it actually is, so that everything is in alignment. Left-hand side physically and permanently always at 4.9 and right-hand side electronically reset to 4.9, irregardless of where it actually might be in order to properly balance the throttle-bodies. Does it now make perfect sense? Now you understand precisely WHAT is happening and when.
So, when we adjust the fast speed adjustment screw, we are moving the right-hand throttle body out of alignment from the permanent factory setting on the left-hand side, in essence, because the TPS sits on top of the right-hand side, we have moved the alignment of the throttle position with the fuel injection map. Bad thing - crappy running motorcycle.
So, when we RESET the TPS, we are telling the ECU that the value it is reading from the TPS sensor on the right-hand throttle body, should be the ZERO point (in actuality, the 4.9 factory setting which is what the left-hand side is), REGARDLESS of where the right-hand side has been moved to, thus aligning the throttle position and fuel injection map, while maintaining the actual true zero point which is physically set by the stop plate screw on the left-hand throttle-body at the factory.
We are commanding the ECU to recognize and program that at whatever position the right-hand throttle-body is at right at that moment (remember, we may have moved it left or right by adjusting the fast speed throttle-body balance screw), should be reset to the baseline 4.9 (in this particular model, others are different). We are setting that ZERO point to align the TPS sensor and the FI map to the actual operating conditions (the real position of the right-hand throttle-body).
Had the factory put the TPS sensor on the left-hand throttle body, we would never need to reset the TPS and could move the fast speed adjustment screw anytime we needed to without worrying about a TPS reset. However, because they put the sensor on the RIGHT-HAND throttle-body, and because this position can be changed via the fast speed adjustment screw, we must be able to tell the computer to reset the zero position (which it reads on the right-hand throttle body via the TPS sensor) after we've made changes to the fast speed balance adjustment screw. I hope this makes complete sense now. :geek:
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Let's look at the right-hand side air bleed screw. Again, you can see the hex head inside of the bleed screw. These screws are a BEAR to get to, especially the left-hand side.
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So, the general idea is this.
1. Connect your Centurion to the bike and turn the ignition switch on. Reset the TPS with the Centurion. (We are establishing a ZERO point based upon the position of the right-hand butterfly valve position).
2. Close BOTH air bleed screws. More than likely, only 1 will be open. Remember, closed, but not crushed! A light touch works really well here.
3. Place a large high-velocity fan right in front of the motorcycle, blowing towards the motorcycle to keep air flowing over it during this procedure. This is a MUST DO to avoid overheating! Bring engine to operating temperature > 60 degree Celsius (The Centurion you can buy from Todd here on the forum, will display the engine temperature from the engine temp sensor, very handy).
4. Turn off engine, attach your CarbMate, mercury stick, or whatever tool you are using to balance the throttle-bodies, to your vacuum ports on the bottom of the throttle-bodies, one on each side.
5. Turn engine on, and have an assistant raise speed to approximately 3800-4000 rpm and hold it there steady. Look at your balancer, and adjust the fast speed throttle-body balancing screw to bring the throttle-bodies into alignment. Turning the screw one way will raise the right-hand vacuum and turning it the other way will lower the right-hand vacuum. You will understand when you do it. CAUTION! It is easy to push on the screw while you are turning it, and this will raise the rpm erratically (you are pressing in the linkage and moving the connecting rod). The trick here is to ROTATE the adjustment screw WITHOUT pushing against it. It takes some getting used to. When you are doing it, you will instantly notice if you are pushing against the screw because your engine speed will change. Personally, I use a socket attached to a screwdriver-type handle to rotate the screw but you will have to scrape away some of the yellow marking paint for your socket to slide on easily. The socket allows for better rotation without pushing into the screw. When the fast speed adjustment is balanced, release the throttle back to idle. Turn off the engine and reset the TPS again. (We are resetting the zero point now that we have changed the position of the right-hand throttle-body via the fast speed balancing screw). When done. Disconnect your Centurion from the motorcycle.
6. Start engine again and blip the throttle and then release it and allow it to click back against the stop screw. Now at idle, what does your balancing tool say? More than likely it will read different values. The side that is HIGHER, is the side you want to open the air bleed screw on, thus allowing more air to enter and thereby reduce it's vacuum. (Don't touch the other side air bleed screw. It's not necessary.) Only one side is the higher side and if we open that side's air bleed screw, it's vacuum will become less, and as you adjust (open) the screw, the balancer tool you are using will show the throttle-bodies come into idling alignment. Once the alignment is the same for both throttle-bodies, by having adjusted ONLY the higher side, you are done!
7. Turn off engine, disconnect balancer tool, cap off your vacuum ports, and smile. You did it.
I sincerely hope that this explanation helps you. Good Luck!
