Surging or Hunting

Edited by the US Surging General, Michael Boomgarden

Mid-range surging is a common affliction among owners of R1100-series motorcycles. Typically occurring between 2500 and 4500 rpm. (it varies from bike to bike) and most noticeable in low gears when the engine is not pulling hard, surging manifests itself as continuously changing engine speed under constant throttle. It is exceedingly annoying, but, thankfully, many R1100RT owners report that their motorcycles' surging has gone away as the engine began to break in. If you have a low-mileage RT that is surging, it may be worthwhile to be patient and work closely with your mechanic rather than attempting the remedies described below.

Not all R1100-series BMWs exhibit surging; it is apparently more common among U.S.-specification motorcycles than among R1100-series motorcycles sold in other markets. Surging is commonly attributed to the extremely lean fuel mixture utilized in the U.S.-spec R1100 engines and can be more noticeable in oilheads that have not been carefully adjusted by a competent mechanic.

BMW's first attempt at a "fix" for the surging problem was a lower-friction throttle cable. Throttle body synchronization is key to a smooth-running oilhead motor and early R1100-series models had "sticky" throttle cable that made throttle body synchronization difficult. BMW warranty service bulletin #2748 dated 7/3/96 outlines the procedures for replacement of the cable for R1100 RTs with a VIN under 0440499. Most, if not all, affected motorcycles are likely no longer under warranty, but, if you own an early RT, it is worth checking to see if the cable has been replaced. Furthermore, since the cables do stretch and wear with use, BMW recommends that your throttle cable assembly be replaced every 24,000 miles.

A variety of other "fixes" have been reported, including (1) replacement of spark plugs, (2) throttle body synchronization, (3) "zero = zero" adjustment of the throttle position sensor, and (4) replacement or removal of the cat code plug and other components.

WARNING: While owners have reported many instances in which these modifications or adjustments have cured their motorcycle's surging, you should be aware that these modifications - particularly if attempted by a someone other than a competent BMW mechanic-may decrease performance or cause damage to your motorcycle. If BMW determines that there is a causal relationship between any non-BMW-approved modification and subsequent damage, any warranty claim will almost certainly be denied.

Replacement Of Spark Plugs

A number of R1100 RT owners have reported that simply changing to different spark plugs has eliminated or drastically reduced their machines' surging. Among those plugs which have been reported to reduce surging are the following:

Bosch is the OEM Supplier and makes several types of plugs:
Bosch FR5DTC (3-electrode)
Bosch FR6DDC (2-electrode)
Bosch 4418 (4-electrode)

Autolite 3923 (1-electrode)
Champion RC9YC or RC9YC4 (1-electrode)
NGK 4563 (3-electrode) (old #BCP6ET)

The NGK 4563 plug, which is reported by Dan Abbott to have significantly improved his RT's performance, is a non-resistor type plug, which will likely cause interference with other electronic gizmos-such as stereos-that are powered off of your RT's electrical system. Some RT owners have expressed concern that the use of non-resistor plugs could damage the Motronic system, though, at this point, no instances of actual damage have been attributed to non-resistor plugs by contributors to this BBS. Dan indicates that the resistor-type equivalent is the NGK 2164. As of this writing, no RT owners have reported using the NGK 2164.

A very important caution: You should be very cautious in substituting plugs of a longer length than the OEM Bosch plugs. While all of the plugs described here as possible substitutes have reportedly been used by oilhead owners with good results, the installation of an overly long plug could result in it contacting a piston, with disastrous mechanical consequences.

Throttle Body Synchronization

Improper throttle body synchronization is reportedly the cause of surging in many oilheads. A competent BMW mechanic should be able to accurately synch your motorcycle's throttle bodies, but, unfortunately, a number of owners report poor results when they have turned to their dealer's mechanic for this adjustment. For those with the equipment and the willpower to give it a shot on their own, Stephen Karlan has posted an article on throttle body synchronization procedures on the IBMWR web site. His article can be found at: (incidentally, the IBMWR web site has a number of excellent articles on mechanical issues).


Adjustment of the Throttle Position Sensor (TPS)

Sean Franklin writes "Some consider Rob Lentini's "Zero = Zero" advice gospel on the subject of correcting surging." Rob Lentini, an officer of the BMW MOA and a well-regarded mechanic, has devised a non-factory-approved, method of adjusting the TPS which can help in smoothing a surging oilhead. It can be found at:

Lentini cautions that this procedure should be attempted only by "a reasonably skilled mechanic with knowledge of Motronic fuel injection theory of operation." If you meet those criteria and possess a digital voltmeter and a mercury or electronic manometer, this procedure can result in a reduction or elimination of surging. However, it is worth noting that even Lentini cautions that it is worth simply trying to swap spark plugs (he recommends Autolite 3923s) before attempting this more drastic procedure.

Tomb9 utilizes a different procedure: "I've spent some time diagnosing my surging, and I have a slightly different take than the zero-zero adjustment. As mentioned in other posts, the surging is a result of the overly lean mixture on US bikes. The zero-zero procedure attempts to adjust the TPS to richen the mixture, but misses the point. The fuel injection uses the TPS to indicate throttle opening, so it can be adjusted to increase the amount of fuel without increasing the amount of air. The results are a richer mixture. The procedure is simple. Pull your left fairing cover. Get out your trusty DVM and attach the minus side to the battery negative terminal. Attach the plus side to a pin and stuff the pin into the end wire of the TPS connector (red/white). Turn on the ignition, but don't start the bike. The DVM will now read between .300 and .400 volts. The optimum setting is .395. Loosen the screws that hold the TPS in place, and rotate the TPS until you get 0.395 volts. Tighten the screws and crank on the throttle a few times. Reset the screws if required to get the correct voltage."

Removal or Replacement of the Cat Code Plug

Engine control and emissions components used by BMW vary significantly from country to country. U.S.-market R1100 RTs employ components to conform with (and, in fact, exceed) the stringent emissions-control requirements of the U.S.A. Some owners report that removal of the cat code plug, combined with the installation and proper adjustment of a potentiometer, reduces or eliminates surging.

TomK reported: "I had my dealer change the fuel and exhaust system to Canadian specs. For a cost of about $200 the change was dramatic." TomK indicates that the BMW part #s for the required modifications are:

  • 13621461425 CO Potentiometer
  • 13621341499 CO Potentiometer carrier
  • 13541460594 Vacuum nipple cap

According to FrankT, the procedure was outlined in "the July issue of Rider in the Tech Q&A column on page 89. It indicates using a foreign market cat code plug and the potentiometer. The article indicates that the potentiometer gets adjusted with the use of an exhaust gas analyzer to 2.5 percent CO at idle, or if no gas analyzer is available, pretend you are adjusting a carburetor for the best lean idle and smoothness."

Again, Rob Lentini has experimented with several variations on this theme, including substituting the Swiss-market cat code plug. The results of his experiments can be seen at:

Of course, the usual caveats regarding the possible impact on your BMW's warranty apply. One additional caution is in order: some of these modifications will damage your catalytic converter. If you ever decide to restore your motorcycle to its stock configuration, replacing the cat is a big-dollar item. Moreover, Shotgun1897 reports: "If you use an exhaust with a [catalytic converter], you can not adjust the mixture using an exhaust gas analyzer with the CO potent & the cat will clog and will cause back pressure and heat build up. If this happens it will take out the exhaust valves."

Surging is a tremendous annoyance, but if you are riding a low-mileage RT there is an excellent chance that the surging will disappear as the miles add up. If your motorcycle is still under warranty and the surging is unacceptable, your first step should be to take your motorcycle to your BMW dealer; surging can often be rectified by a competent mechanic using factory-approved procedures. Be persistent with your service advisor and dont take any excuses for a bike that runs poorly. 

If all else fails, or if you simply want to try to further improve your RT's smoothness and power, the procedures outlined above may be the way to go. When attempting these modifications, stay within the limits of your mechanical ability; some of these modifications, if done improperly, will damage your motorcycle. Bear in mind that any damage caused by your modifications may negate your warranty.