Oilhead Tech Pages


Balancing Throttle Bodies

Stephen Karlan (Dali Meeow)  -  dali@netrox.net


Written for 1994 through 1998 - R850 & R1100 BMWs

An out-of-synch condition develops (R850 and R1100) because the throttle body cables stretch or because they lose their adjustment. When this happens, and it will happen over time, the left and right throttle bodies receive different signals. The result is vibration, sometimes unreal vibration. The good news is that, over time, the cables will stretch and the cables will require less frequent adjustments. [I have done only minor tinkering during the past 24 months. When new, I adjusted the cables every month or so.]

The following procedure, which I call the Gorman/Diaz Balance, was developed by Bob Gorman and Jon Diaz from Chicago. It is a fast and simple procedure that can yield dramatic riding improvement.

The 1994-96 models have a crossover synch cable; the 1997 and 1998 models have left and right throttle cables attached in an upside down "Y" pattern. Use these instructions for the 1994-96 models. If you have a 1997 or 1998 model, look for the ['97-'98 ] and follow the instruction inside the brackets and disregard the instruction given immediately prior to the ['97-'98 ] brackets.

Overview:

The throttle bodies will be synched at two rpm levels -- low (idle) and high (operating rpm).

The low rpm synch will be done first. With the bike at idle (1,000 to 1,200 rpm), the large brass bypass screws will be adjusted. (LBBS in the Glossary at the end of this article will help you find this part.)

The high rpm synch will be done last. With the bike at 2,500 to 4,000 rpm, the right side crossover synch cable will be adjusted. (RSCSC in the Glossary at the end of this article will help you find this part.) ['97-'98 right side throttle cable will be adjusted.]

Tools needed:

Setting Up:

Remove the right three-sided black plastic fuel injection plug cover for access to the right side crossover synch cable ['97-'98 remove cover for access to right throttle cable]. If you have a lower fairing, remove it to allow air to reach the cylinders and to prevent cooking the fairing. With the bike on the center stand, retract the side stand, put the transmission in neutral, turn on the bike and warm up the engine. Jon Diaz suggests a more enjoyable way to warm up the engine -- take a ride.

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. The headers can become red hot and, if they do, turn off the bike and resume work after they cool considerably.

Secure the mercury manometers (carb stix) on the right side of 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 a ceiling hook. I removed the dampeners from the carb stix and got better results. Be careful and move the throttle slowly if you remove the dampeners; if you move it quickly you could suck mercury into your engine.

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. If they are too long, I put a loop in the plastic tubing to shorten it and keep it away from hot head or exhaust parts. There is no need to plug the black vapor recovery tubes.

Starting the Adjustment:

Loosen the right side crossover synch cable by loosening the 10mm lock nut and turning the knurled knob counter clockwise to ensure that there is slack ['97-'98 loosen right throttle cable] .

Count the turns you make to lightly seat both the right and left large brass bypass screws by turning them clockwise with a flat screwdriver. Record this information for possible trouble shooting later. Record this information for possible trouble shooting later.

As a starting point, back out both large brass bypass screws 1 1/2 turns for an RS or RT; 2 turns for an R or GS.

Start the engine. The bike may idle rough because it is not in synch. Do not be concerned about a rough idle at this time. If the bike will not idle at all due to air starvation, back both large brass bypass screws out by 1/4 turn (ccw) until the bike maintains a rough idle.

If the bike is idling too fast, reduce the air flow by screwing both large brass bypass screws clockwise (cw) in increments of 1/4 turn. If the bike is idling too slowly, increase the air flow by screwing both large brass bypass screws counter clockwise (ccw) in increments of 1/4 turn. At some point you will not be able to decrease or increase the idle speed by moving the large brass bypass screws. Return to the setting immediately prior to the point where the idle was not changed by moving the large brass bypass screws.

You now have the large brass bypass screws at the starting point for the low rpm synch.

LOW RPM SYNCH:

With the throttle at idle (1,000 to 1,200 rpm on the tachometer) and the bike at normal operating temperature, turn the right large brass bypass screw clockwise until the mercury columns in the carb stix are at an equal height. If moving the right large brass bypass screw does not accomplish equal height mercury columns, then reset the right LBBS to its starting point and turn the left LBBS clockwise until the mercury columns in the carb stix are at an equal height. The amount the mercury column rises or falls, its actual height, is not important. What is important is that both left and right mercury columns are the same (equal) height.

The goal is to balance the throttle bodies with the large brass bypass screws open as little as possible (minimum counter clockwise opening). Bob Gorman notes that, when adjusted correctly, the screws should be within 1/2 turn of each other.

If the bike does not adjust by turning the LBBS clockwise (in), or if the idle speed is too low (below 1,000 rpm), change the starting point of the low rpm synch by backing out both LBBS (counter clockwise) by an additional 1/4 turn.

HIGH RPM SYNCH:

When performing the high rpm synch, you will be moving the throttle up to 4,000 rpm. Move the throttle slowly to avoid sucking mercury into the engine.

The right side crossover synch cable has already been loosened ['97-'98 right throttle cable has already been loosened]. The knurled knob should turn without turning or binding the crossover synch cable.

Adjust the knurled knob so that the mercury columns rise together as the rpm rises. You may need several turns to take up the slack you created earlier. Some riders adjust the mercury column heights with the rpm steady at 2,500, some with the rpm steady at 4,000, and some make this adjustment so that the columns are fairly equal while the throttle is being opened from 2,000 to about 4,000 rpm. The mercury columns should be fairly equal if the throttle is held steady or while the throttle is increasing rpm from 2,000 to 4,000.

After adjusting the knurled knob so that the mercury columns are equal, tighten the 10mm lock nut and recheck the mercury levels. If tightening the lock nut moves the knurled knob and upsets the adjustment, hold the knurled knob with small pliers or needle nosed pliers to maintain the adjustment. If holding the knob does not work, measure the amount of mercury column error after tightening the lock nut. Recording this measured error, loosen the lock nut and add the amount of measured error to the mercury column before tightening the 10mm lock nut. This objective is to have equal mercury column heights after the lock nut is tightened.

Test your work by checking the mercury level at idle and at 2,500 rpm. If not equal, you should begin again with the adjustment process at "Low RPM Synch".

Trouble shooting:


Where to buy carb stix, mercury, TwinMax:

Precision Manufacturing Services at 800-237-5947 was selling the Motion Pro brand for $36.95 + shipping in 1996.

J.C. Whitney has a Motorcycle Accessories and Parts Catalog for $1. They show several types of vacuum gages and mercury manometers. Their address is P.O. Box 3000, LaSalle, Illinois 61301, phone 312-431-6102; fax 312-431-5625.

Donelson Cycles Inc. at 800-325-4144 was selling the Motion Pro brand for $34.45 including shipping in 1996.

Ask you favorite cycle shop or mail order company for their price.

If the mercury comes out of the reservoir (because it tipped over or the cat hit it), you can try companies that calibrate instruments for replacement mercury. You don't need the highly refined mercury they sell, but it may be the only material available.

TwinMax

The electronic equivalent to the mercury manometer is called the TwinMax; technically it is an electronic differential depression gauge. It is contained in a small rectangular box measuring 5" x 3" x 1.5" and has an illuminated null meter, a knob to zero the meter, a sensitivity knob, a power cord that plugs into bike's electrical outlet, and two clear vacuum lines that use BMW adapters that plug into the throttle body nipples in the same way that the carb stix's vacuum lines plug into the throttle body nipples. This information is from John Baxter <jbaxter@macs.net> who claims it gave his bike " a totally new world of smoothness ". Order this from Adventure Motorcycles Gear, P.O.Box 366, Springfield, VA 22150, phone/fax 703-913-7261, at a cost of $99.95.


Glossary:

RSCSC - Right side crossover synch cable ['97-'98 right throttle cable].

On the right side of the bike, forward of the fuel injection plugs and between the engine and the throttle body, you will find this vertical cable. The cable is secured on a bracket with a lock nut on the bracket and a knurled knob above the lock nut. After loosening the lock nut, turning the knurled knob will either lengthen or shorten the cable. If the top of the cable is pulled gently up, it should come out of the knurled knob. This is the mechanism that must be adjusted to vary the mercury columns.

LBBS - Large Brass Bypass Screw:

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.


Note: The throttle cables on many 1994-96 bikes were replaced with Teflon coated units (warranty service bulletin #2748 dated 7/3/96) because the inner cable was sticking. The improved cable was installed at the factory after the following VIN number:

R1100RS        0312537    

R1100GS        0381479

R1100R         6379238

R1100RT        0440499

R850           from start of production.  

If the older type is installed on your bike, and if it has not been replaced, see your dealer because some older cables caused significant balancing problems.


Author's note: When the bike is in the garage and sitting over night, I use tape, rubber bands, string or bungi cord to twist new throttle cables to full open throttle position; that will speed-up stretching the cables and result in fewer adjustments.

The original throttle body synch work by Jon Diaz is posted on the Internet BMW Riders web site; Bob Gorman's original throttle body synch work appeared in a local newsletter and the original tune-up manual. 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.

Comments, corrections and questions may be directed to
Stephen Karlan (Dali Meeow) at (305) 255-1010
or via email at dalimeeow@adelphia.net.
Updated November 23, 2004.


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R-Tech pages maintained by: John Petty for the original author(s) and the Internet BMW Riders
Last Update: 09 September 2010