Jump to content

TB sync musings and some thoughts on surging.


Recommended Posts


I did my 12k service today and everything looked pretty good. A tweek here and a snerk there was about all that was needed, but for one glaring exception. The cable to the left side throttle body was about 1/4 turn out of adjustment.


Prior to starting the service I had noticed that the RT would "hunt" a bit in the 3500 to 4000 rpm range. I wouldn't quite call it a surge, but the rpm's did seem to want to wander a bit. I also had a slight issue when rolling off the throttle at low rpm's such as when riding low speed in a parking lot. It seemed as though the throttle would slam shut in that last fraction of a turn as I was rolling off to idle.


The left body cable adjustment cured both those issues and that got me to thinking (you could tell I was thinking because there was smoke coming out my ears grin.gif) about surging and whatnot. So I set the cable adjustment back to where it had been and sure enough, the hunting and slamming shut symptoms were back. Re-adjusted, and they're gone again. This cable adjustment was the only major change I had to make in order to get the bike in tune ( a 1/8 ccw turn of the left air screw and some very, very slight valve tweaking were the other changes). I looked up some info on the throttle bodies to try and get a better understanding of how things work and that gave me some thoughts. Please correct me if I am off base or misinformed smile.gif


Basically the throttle plates have to open at precisely the same time, any variation and there will be an imbalance in the velocity of air delivered to the cylinders. The injectors are mixing in fuel to a specified ratio, but there is less volume of the mixture going to one cylinder or the other if the throttle plates are off. Symptom, rough running or vibration. Am I correct here?


Now, the surging. I noticed my "hunting" problem putting around town, light throttle, mostly downhill or holding low speed on flat road. I'm thinking that maybe the surge is caused because the difference in the mis-adjusted throttle plates is exaggerated during those periods where the throttle is just barely cracked open and the rider is just trying to provide "maintenance throttle". In this case you're walking the razor edge between on TB shutting down and one still remaining open. Thus as soon as the first throttle plate closes (the one with the longest cable, I suppose) that cylinder basically goes off line while the other is still providing the throttle requested from the rider. That's how I think the "slamming shut" symptom I had was originating. Wouldn't the vacuum coming from the cylinder add to this problem by having a tendency to suck the plate with the loose cable, shut? (or is there something that resists that tendency?) In a similar fashion I would think there would be a throttle position where that errant throttle plate is going to be on/off/on/off again, giving the surging symptom while the plate controlled by the tighter cable is still suppying the requested amount of fuel confused.gif Add to the mechanical problem, a rider who gets jolted and in turn, ever so slightly, jolts the throttle. Now you have throttle plates banging open and shut providing the surging.


I don't know how that would explain the sucess people have had with th aftermarket solutions like the Teclusion. Perhaps they're mor flexible or forgiving than the BMW fuel mapping program confused.gif


Anyway, just some thoughts on the subject. I thought I'd throw them out there for the experts and see where my logic is wrong or right smile.gif


BTW: Has anyone tried just a TB sync on a surging bike to see what happens, or is it always done with the valve adjust?





PS: BMW synthetic gear lube rocks! thumbsup.gif I put it in the tranny and for a second I thought I was shifting a Japanese bike grin.gif

Link to comment

You are more or less correct. If the throttle blades are opening at different angles, the air flow will be different (assuming equal cyclinder pumping capacity). The Motronic doesn't know this and trys to put an equal amount of fuel into each cylinder. In your case, the surging thing and the "snap shut" feeling you were getting due to the loose left side throttle cable made the situation worse as the throttle position sensor is attached to the left throttle blade. That would allow the right throttle blade to open without any knowledge of the Motronic, causing the right cylinder to go lean.


You really need to understand that this is a system. Any of the components can influence the overall operation. Yes, proper TB sync is part of getting the system to function properly, as is valve adjust, ignition timing, TPS setting, proper throttle free play and the rider's grip on the throttle. No one part is more important than any other despite the proclamations of the now departed DEF.

Link to comment

Drive train lash also is a contributing partner in the abrupt feel of the throttle on-throttle off surge dance I suspect.

Link to comment

Your understanding of the balancing process is quite sound and you should have no trouble getting near-perfect TB sync with your concept of the process.

As you've already identified, the major part of establishing smooth running is to have as close to identical air-volume to each cylinder. The fine tuning mechanism for this is controlled by the cable and brass airscrews. Prior to the TB sync, the valve adjustments should be set as accurately as possible to try to equalise the potential air delivery to each cylinder.

So it's always in this order:

1. Valve Adjustments

2. TB sync.


There's another element in the behaviour of the Motronic. With a fully closed throttle, there is no fuel delivery above 2000rpm. As the engine slows through this speed, you can feel the engine give a slight kick as the revs drop below this figure and fuel is delivered normally. Make sure that you don't confuse this behaviour with surging.

Link to comment

Uneven throttle butterfly openings will result in different air volumes entering the respective cylinders. This partly explains for the surging that you experence ie different air volumes in each of the cylinders will unbalanced power output in the 2 cylinders.


From my experience the surging can also be due to the oxygen sensor located just before the catalytic converter. Too lean a combustion will result in the Oxygen sensor telling the Motronic unit to up the mixture to rich and vice versa. This can result in the surging as the Motronic is constantly trying to "hunt" or adjust the mixture between rich and lean and I suspect it can be also be "triggered" by the uneven throttle butterflies opening giving the Oxygen sensor different exhaust gas pulses.


This is the downside of BMW boxers ie surging issues due to uneven throttle butterflies. I would recommend Twinmax synchronisers which can be order via the internet. Its easy to use and balancing throttle vacuums with this unit is very effective indeed and best all it is not difficult to master ( no mercury spills etc ). With experience, it only takes less than ten minutes to balance my bike. Great stuff. I hope this helps.

Link to comment

Chris, how did you determine that it was the left side cable that was out of adjustment? Was there no slack in the right cable and just a slight amount of slack in the left? Or did you dial in the exact same amount of slack into both cables? I have seen examples of both tuning methods, slack and no slack in the cables.


I need to do a careful TB sync on my R1150R during the next home service, because it's hunting quite a bit now, and seemingly over a far greater RPM range than before. Some days it's better, some days it's worse... although it vibrates less now at high RPM than it ever did, go figure. Seems like you can't have it all, there's always a compromise in terms of driveability and vibration somewhere in the RPM range, at least on my bike.

Link to comment



I really don't know why I chose to tighten up the left side confused.gif The right was drawing more vacuum so I gues it seemed logical the left should be tightened up. The adjustment was only 1/4 turn so it wasn't much and I definitely felt as though I was drawing some slack out of that cable. Had the adjustment required much more, I probably would have begun to loosen the right side just to keep things on an even keel and not have one side grossly over or under adjusted.


I need to do a careful TB sync on my R1150R during the next home service


Get to it grin.gif

I half-assed watched one done before, but this was the first I've done myself. I'm suprised how easy it was. That being said, I didn't have any outside issues complicating the procedure either.

Link to comment

With a manometer, the cylinder with the throttle plate that opens first will show a drop in mercury or whatever you are using. That is because the manifold pressure rises, resulting in an increase in power. The other cylinder that is lagging will show an increase in mercury or a drop of manifold pressure because it is actually being accelerated by the other cylinder. You must look closely just as the throttles are cracked open to see what is happening. When they are synched correctly, the manometer should show an equal drop from both as the throttle is cracked from idle. Then you can use the idle air screws to even the manometer between the two cylinders.



Link to comment


This topic is now archived and is closed to further replies.

  • Create New...