Setting the Throttle Position Sensor and Throttle Screws
by Stephen Karlan (Dali Meeow) – firstname.lastname@example.org
Written for 1994-98 R850 and R1100 BMWs
I. The Problems.
Surging — A condition of increasing and decreasing power, cycling back and forth, when the throttle is held absolutely steady at a fixed rpm (such as 3,500 rpm while using a throttle lock, tape or rock-solid hand). The electronic control system is hunting (up and down) for the correct air/fuel mixture and the engine is not responding to throttle input but to electronic sensors.
To minimize surging, the throttle plate screws (two) and the Throttle Position Sensor (one TPS on the bike) are adjusted using a voltmeter. (There can be other factors, but an incorrectly set TPS is the 95% problem.)
Vibration — Shaking or oscillating. When the throttle is held in a fixed position, the vibration is constant and unchanging. It may vibrate more at one throttle opening (rpm) than at another; i.e. it may vibrate more at idle (or is more noticeable at idle) than at 3500 rpm.
To minimize vibration, the throttle bodies (one for each cylinder on each side of the bike) are balanced using mercury columns.
Ensure that the valves have proper clearances before starting any fine tuning and that plugs and wires are operating correctly. Instructions on performing a valve adjustment can be found in the Technical Information section at this web site under Adjusting Valves and Rocker Arms, or by clicking on the words “Adjusting Valves”.
II. The Approach.
It is quite possible that a bike has both a surging (throttle plate screws and TPS) and a vibration (throttle body) problem. With these instructions, you will address the surging and vibration problems in that order. If you do not have a surging problem, DO NOT adjust your TPS ! If you do not have a surging problem, DO NOT adjust your TPS ! If you are not sure about your problem, then do not continue with these instructions and, instead, balance the throttle bodies using “Throttle Body Balance for Dummies”. In fact, I strongly recommend that you balance the throttle bodies at least twice before you use the instructions in this article. Throttle body balancing is relatively fast and easy to accomplish and, you will need to balance the throttle bodies as part of setting the throttle plate screws and TPS.
In addition to surge and vibration, some bikes will present an idle anomaly in which the idle will be too high. The idle problem, which is nothing more than an idle in excess of 1200 rpm, occurs most frequently on GS and R models after resetting the throttle plate screws and the TPS. This problem is addressed at the end of the TPS adjustment instructions. Idle speed also is addressed in “Throttle Body Balance for Dummies” if you are only performing a throttle body balance.
The basic knowledge about adjusting the TPS and basic throttle settings to obtain optimal engine response is the work of Rob Lentini; his Zero=Zero article appeared in MOA’s Owners News in 1997 and is on the Internet BMW site.
This article does not present significant or new procedures, it has been written for newbies and simply goes into greater detail. It was designed to help you find the right part to tweak. These instructions assume that you know almost nothing about the bike, where parts are located, or how it functions. The priority here is to help you diagnose and adjust the bike, not engineering or theory. You may need detailed instructions the first time you do the adjustment; subsequent adjustment will require much less information. If you run into problems, contact the author by private eMail or at (305)255-1010. For another perspective on setting the TPS, look at Rob Lentini’s original work which has already been cited.
III. How to set the TPS on the R850 and R1100
WARNING: BMW does not approve of this procedure. BMW warns that “The sealed stop screws on the throttles must not be tampered with, or else the basic idle flow setting will have to be reset by the manufacturer.” There is a Throttle Position Sensor article by Bob Gorman and an early Throttle Position Sensor article by Rob Lentini that present ways to set the TPS without resetting the throttle stop screws. These do not violate the BMW warning and these may be used before resorting to this procedure. The author of this article assumes no liability for any damage or injury to you or your bike caused by any errors or omissions.
Read through these instructions, and also “Balancing Throttle Bodies“, before you pick up a wrench or screwdriver. Find the parts before you start. This will save work and cursing time.
Begin with a cold engine and do not start the engine until after the TPS has been adjusted. Place bike on center stand with side stand retracted. You will need two special tools, a digital volt meter (DVM) for setting the TPS, and a method of measuring vacuum (either a set of mercury manometers, also called carb stix or mercury columns, or an electronic Twin-Max, both of which are used to correctly adjust the throttle bodies.
12 Steps to Throttle Set Screw / TPS Adjustment
- Loosen throttle cable (left side)
- Loosen throttle body crossover synch cable (right side)
- Back out the left throttle plate stop screw (underneath left side)
- Attach DVM to red-white TPS wire #1 (rear) – ignition on
- Move TPS to obtain near-zero reading (.006 volts) and lock TPS
- Move left throttle plate stop screw to obtain .370 volts and lock screw
- Large brass bypass screws in, bike on, warm engine, rough idle expected
- Turn the large brass bypass screws out in 1/4 increments if bike will not
idle; attach carb stix
- Set right throttle plate stop screw using carb stix
- Reduce TPS in increments of 0.020 if idle exceeds 1100-1200 rpm
- Reset throttle cable (left side) to .5 mm free play
- Perform the “Throttle Body Synch for Dummies”.
Each of the 12 steps above is explained below, with a detailed explanation of how to find and adjust the correct part. All of the information on Step 1 will follow the Step 1 heading.
Step 1. Loosen throttle cable (left side) Here are four (A-D) ways to find the throttle cable. Use all of these the first time you use these instructions to ensure that you have the correct part.
(A) Starting at the front edge of the seat, if you were to hang a string onto the left side of the bike, it would contact the black plastic air intake duct (2 1/4 inch diameter tube) about 11 to 13 inches down. Follow this black plastic duct or tube forward to where it is attached to a metal tube or duct that is 3 1/2 inches long with plastic and metal parts all over it. Everything related to the TPS adjustment is attached to this metal duct, or a similar metal duct in the right side. For our purposes, the metal duct has a top and bottom area, an outside and an inside area (close to the body of the bike). There is a bracket on the inside area where two cables are vertically attached. The cable closest to the metal duct (forward outside) is the throttle cable.
(B) Starting at the rear of the large finned metal engine cylinder that sticks out on the left side, the throttle cable is approximately two inches to the rear of this location and close to the engine.
(C) The throttle cable can be generally located by first locating two plug-type wires on the top of the metal duct, and from this point look down and between the metal duct and the bike.
(D) There is an easily removable (and frequently lost or missing) three-sided plastic cover with a round bottom that clips onto the top front area of the metal duct. This plastic cover comes off by pulling it upward and toward the front of the bike. Take this plastic cover off (no tools necessary) and the throttle cable is close to the location where the inside of the plastic contacted the metal duct.
Now that you have located the general area of the throttle cable, by careful inspection you will see that two cables come into the same area and mount on one metal bracket or metal plate located on the inside area of metal duct. [On 1997 models there is one cable only, the throttle cable. Disregard the references to other cables.] The cable that is the furthest from the engine is the throttle cable (still forward outside). To make certain you have the correct cable, move the throttle on the right handlebar and this should move the throttle cable that you have identified on the left side. (You may need another person to move the throttle on the right side of the bike while you watch the cable on the left side.) If the left side cable does not move, you have not located the throttle cable. Move the throttle on the handlebar and look on the left side for a moving cable that matches the information above. Now check the cable next to the throttle cable by moving the choke lever on the left handlebar and watch the choke cable move; do no loosen the choke cable. There is a third cable to the rear of these two which is the crossover synch cable; this is on its own mount and need not be loosened or adjusted in any way.
If you are not certain you have found the throttle cable, either find another BMW rider with an R bike and ask for help finding it, or go pester your local mechanic.
This is a tight area in which to work and so you will remove the three-sided plastic cover with the round bottom that mounts on the gas intake area. There are two plugs on top of the metal duct that were covered by the three-side plastic piece you have removed; these are fuel injection controls. The front plug of the two may be removed by depressing (press in) on the wire clip on the plug and then lifting the plug from its mounting. After removing both the three-sided plastic cover and the fuel injection plug, there will be more room in which to work.
You are now ready to loosen the throttle cable, which was the whole purpose in finding this cable. You must loosen the lock nut, the six-sided nut that sits on top of the bracket, with a small wrench (counterclockwise). There is a metal piece that is threaded into the lock nut which can now be loosed with your fingers by turning it in a clockwise direction. Loosen the lock nut as needed so the cable is loose. Check this by twisting the throttle on the handlebar; you must be able to give at least a 45-degree turn on the throttle and see that the throttle cable is loose. Now depress (squeeze) the wire clip on the fuel injection front plug and replace the plug you previously removed.
Step 2. Loosen throttle body crossover synch cable (right side) After loosening the throttle cable, it’s time to move on and to look at the right side of the bike. Go to the same general area on the right side as you were examining on the left. There are fewer controls and only one cable. Here are two ways (A, B) to find the right crossover synch cable.
(A) There is a metal 2 1/4 inch diameter tube (similar to the left side but with fewer black plastic or cable items attached to it) that also continues into the large finned cylinder (as it does on the left side). There also may be a three-sided plastic guard with a round bottom if you have an RS or R (not on the GS or RT, and not there if it has been lost). From the metal tube area, look into the side of the bike to see a single cable mounted on a metal bracket; it is the only cable in the area.
(B) The right crossover synch cable is located about two inches from the rear of the large finned cylinder and close to the engine.
First remove the three-sided plastic cover with the round bottom if there is one. Next loosen the lock nut that is located near the top of the bracket (turn counter clockwise). After the lock nut is loose you can loosen the cable by screwing (clockwise) the metal piece that is above the lock nut. Screw until the cable is loose. The loose cable can be seen below the metal bracket.
Step 3. Back out the left throttle plate stop screw (underneath left side) It’s now time to return to the left side of the bike. When the throttle on the handlebar is twisted, it not only moved the cable you have now loosened, it also moved a metal flange piece that comes in contact with a metal plate. There are two ways to find this metal flange piece and the left throttle plate area.
(A) Check instructions for Step 4 and find the TPS. If there were an imaginary line that passed through the right TPS screw and then continued through the round metal body on which the TPS is mounted, that line would intersect the left throttle plate stop screw. When you move the throttle on the handlebar you also are moving the metal flange piece that contacts the left throttle plate stop screw.
(B) The metal throttle flange is located near the inside of the metal 2 1/4-inch diameter tube and is very low and near the inside of the tube. The easiest way to see this is to put your head on the ground just forward of the rear tire and to look up. This is an awkward position but the metal flange piece is easy to touch. The screw, that limits how far it can close, is located on a metal plate above the flange. This screw determines the distance between the flange and the throttle plate. [If you see an adjustment screw on a long arm, this is related to the throttle advance (adjusts the “choke”); this is NOT the screw you are looking for.]
Now that you have found the flange and screw, you now must figure out how to adjust the screw. Loosen the silver metal clamp that secures the black plastic air intake hose to the metal throttle body and rotate the clamp for a more unobstructed working area; remove the hose if necessary. Remove (cut off) the plastic cable tie that secures a wire if it obstructs the working area. Replace the cable tie, hose and clamp when finished with the throttle plate screw adjustment. The physical manipulation that is described in the next paragraph (especially on RS models) is a challenge to your stamina and will take time, patience and any odd-ball tool that works. If you find a tool that makes this easy, please please please contact this author with your hint.
Your job is to loosen the 10 mm lock nut (counterclockwise) using a small wrench, and then loosen the 8 mm stop screw (counterclockwise) using a wrench or a screwdriver so that the metal flange piece does not contact the screw at all. If you loosen the screw a few turns you should be able to see a gap between the end of the screw and the metal flange piece. You must see a gap, which means the left throttle is closed. There must be a gap !
Step 4. Attach DVM to red-white TPS wire #1 (rear) – ignition on The TPS is mounted on the outside of the left 2 1/4 inch metal tube you have been working around; it is a black flat plastic cover that measures 2 1/2 inches wide x 2 inches high with the word Bosch imprinted on it. This cover is attached by two allen-head screws through the cover and the allen-head screws describe a horizontal line.
There is a prong attached to the bottom of the TPS, and a rubber boot that covers the bottom of this prong. The prong can be removed easily by depressing the wire, which acts like a latch, and pulling down. Do not remove the prong. Nothing bad will occur if you do remove the prong from the body of the TPS, but for this adjustment the prong must be attached.
The rubber boot that surrounds and protects the wiring that enters the bottom of the TPS must be moved out of the way. Exert only enough force on the rubber boot to wiggle it down far enough to expose about 1/2 inch of the four wires that enter the prong. The wire furthest to the rear is coded; it is white and red and called wire #1. Insert a sewing needle, a paper clip or other sharp and thin metal (electrical conducting) object up and into the plastic housing where the white and red wire enters the prong.
Step 5. Move TPS to obtain zero reading (.006 volts) and lock TPS Attach the ground probe (lead) of your digital volt meter (DVM ) to the spring of the side stand or some other grounded metal object. Attach the other probe (lead) to the object inserted into the white and red wire. With the bike on the center stand, with side stand retracted, and with engine cut-off switch on the right handle bar indicating “on”, turn the ignition key to “on” but DO NOT start the bike.
Read the DVM and record this TPS value for possible future reference. Do not remove the TPS screws; loosen them so that the TPS can be moved (adjusted). Turning the TPS slightly in one direction will increase the voltage; turning it in the other direction will decrease the voltage. Turn the TPS until the observed voltage decreases to about .006 or .005 volts, which probably is as low as the voltage will go. If it will go no lower, that’s okay. DO NOT continue to turn the TPS after it reaches its minimum value, which is close to zero (see WARNING, two paragraphs below). Start at a high value (.300) and turn the TPS until it reaches the low value (.006 or .005) and stop. [Note: Some digital voltmeters have an autorange function; below a certain value they will read in millivolts. The .006 may read as 6 MV, which is the same as .006 volts. Don’t let the decimal places confuse you.]
Stop moving the TPS just when the voltage stops decreasing and lock it down there. At this point we say the TPS is at zero. The throttle position is also at zero. The TPS is now locked and set. This is the last TPS adjustment you will need; it is now set. You will not change the position of the TPS again !! [Author’s note: The throttle opening is at zero and the TPS value is at zero, which explains Rob Lentini’s title of “Zero-Zero” for this procedure.].
WARNING: It appears more than a few Oilhead owner’s attempting Zero=Zero have set the TPS below zero volts. This may have caused running problems and/or, in one instance, engine damage. In 1999, Lentini said, “I now think it is important to set “zero” about .004 volts above the lowest achieveable point.
Step 6. Move left throttle plate stop screw to obtain .370 volts and lock screw. With the ignition still on, turn the left throttle plate stop screw clockwise (CW). By turning the screw, the flange will be moved away from the throttle plate. Continue turning the throttle plate stop screw until the voltage starts to rise, and continue until you reach 370 millivolts (.370 volts). Stop there, and lock it down with the lock nut. Blip the throttle several times to check the accuracy and repeatability of the 370 millivolts. Readjust the throttle plate screw if necessary. Remove the DVM, remove the object inserted into wire #1, replace the rubber boot onto the TPS. There is a possibility that .370 millivolts will not be the correct adjustment value for your bike. This will be explained in Step 10.
Step 7. Large brass bypass screws in, bike on, warm engine, rough idle expected On the right side of the bike, on the outer surface of the throttle body tube, is a large brass bypass screw with a slotted (NOT Phillips) head that faces toward the rear of the bike. If you were to place a screwdriver on the screw, the handle of the screwdriver would point toward the right rear turn signal. This large brass bypass screw is approximately one inch forward of the black plastic air intake tube. You are looking for it on the right side because it is easier to locate there; there are fewer things in the way. You can see the large brass bypass screws easily when standing near the turn signals. If you look on the left side, you will find a similar large brass bypass screw. The official BMW Repair Manual calls this a “recirculating air screw” at page 00.27.
Count and record the turns you make to lightly seat both the right and left large brass bypass screws by turning them clockwise with a flat screwdriver.
Position a fan near the front tire with its output directed at the cylinders equally to avoid overheating, or use two fans, one directed to each cylinder. If your bike has a Rider Information Display (RID), allow the oil to heat up to 5 bars, otherwise idle the bike for five minutes. It will idle rough because it is not in synch. Do not be concerned about a rough idle at this time.
Step 8. Turn the large brass bypass screws out in 1/4 increments if bike will not idle; attach carb stix
If the bike will not idle at all, back both large brass bypass screws out in increments of 1/4 turn until the bike will sustain a rough idle.
Secure the mercury manometers (carb stix) on the right side on the bike, where most of the work will take place. Use any method that secures the carb stix (and the hazardous mercury) vertically while the bike is running , such as hanging the carb stix from the ceiling.
There is one black tube attached to the under side of each throttle body. Remove both black (vapor recovery) tubes and you will expose the brass nipples that point down. Attach one of the carb stix’s flexible plastic tubes to each brass nipple. Make certain that the carb stix’s plastic tubing does not touch hot exhaust parts; the tubing will melt. There is no need to plug the black vapor recovery tubes.
Step 9. Set right throttle plate stop screw using carb stix. At this time, locate the right throttle plate screw (mirror image position of the one on the left side). The screw will have a lock nut similar to the one on the left side. If it is difficult to manipulate the lock nut and screw, you have located the correct hardware.
With the bike warm and the fan on, look at the mercury columns. If they are not at an equal level, loosen the lock nut on the right side throttle plate and adjust the screw in or out until the mercury columns are equal. If the bike will not idle, then turn both large brass bypass screws out another 1/4 turn.
Step 10. Reduce TPS in increments of 0.020 if idle exceeds 1100-1200 rpm After adjusting the right side throttle plate screw to get equal mercury columns, look at the rpm. Ideally, rpm is now 1100 to 1200. Some bikes will have an abnormally high idle; as high as 1800 rpm or more. This is most prevalent in GS and R models. To correct the high idle, return to Step 6 and place one lead of the digital volt meter (DVM) back on the red and white wire of the TPS and ground the other lead. DO NOT move the Bosch black TPS. Reset the left throttle plate screw (the left rear screw with lock nut that was difficult to adjust) and reduce the TPS voltage by 0.020 volts (from .370 to .350) by turning the throttle plate screw. Continue with the rest of the Steps in order after Step 6. If the idle is still too high, continue to reduce the TPS value by increments of 0.020 until the desired idle is reached.
Step 11. Reset throttle cable (left side) to .5 mm free play Reset the left side throttle cable for about .5 mm of free play. Check this by twisting and releasing the throttle several times and then rechecking the free play.
Step 12. Perform the procedure described in “Balancing Throttle Bodies“, which also is part of the R1100 Maintenance Guide and is reproduced on the Internet BMW web site.
The author has done his best to produce accurate information. However no responsibility can be accepted for any damage or injury caused by any errors or omissions in this article. Make certain that you understand what is described and why it is being done. Use at your own risk.
“In remembrance. My thanks to Rob Lentini who graciously gave of his time and ideas. He had great patience in reviewing this work and his corrections were eased by his manner and laughter. He was too young to leave us and I have missed him. If this work helps you, we both owe him a debt. ”
Comments, corrections and questions may be directed to
Stephen Karlan (Dali Meeow) at (305) 255-1010
or via email at email@example.com.
Updated November 23, 2004.