Here's a picture of the throttle gear connected to the chain:
http://w6rec.com/duane/bmw/control/control4.jpg
And here's the procedure for timing the gears on the throttle properly. It is a POS to do it right the first time, but once you get the hang of it, it's not a big deal.
http://w6rec.com/duane/bmw/control/index.htm
quote:
"BMW uses a unique system for pulling on the throttle cable. It provides a straight pull at the control end. No other company, of that era, had a system nearly as good. It uses a bevel gear drive to pull on a small piece of chain. Other companies allowed the cable to wrap around the bar and each throttle opening it flexed some. Eventually it would break. This bevel gear system is more expensive, but with proper lubrication will last almost forever. It does require some adjustment that is a bit strange the first time that an owner does it.
The brass gear is turned over to show what it looks like. Look carefully at the lower teeth and see some wear. That one is still useable, but not great. The tube shows no such wear. This brass gear is for the /2. The /5 had a cast pot metal gear. This wasn't a set, just some old parts that I could find for the photo.
The bevel gear isn't a linear pull system. It is made in such a way that the first few degrees of twist, it only pulls a small amount. During the mid to upper range it pulls much faster. This gives you precise control of rpm at the lower end. I mention this because this (sort of) requires the bike to be adjusted in a certain way. It is all easy, but not obvious at first.
a. Basic throttle cable adjustment for the /2
The /2 and /5 have almost identical throttle assemblies. The /2 has no special markings on the gear teeth like the /5. In that aspect, the /5 is slightly easier. If your /5 has good cables and you want to apply this procedure, go ahead.
These directions assume that the cables are not stretched or made with too much wire sticking out. You can't know that, so proceed and see what happens. You are not going to get into trouble. If one, or both, of the cables are "stretched" then it will be impossible to synchronize them correctly.
Rubber boot
Slide the rubber boot up and off of both carbs at the cable adjuster. Only the slide carbs have this rubber boot. The CV carbs don't have it as they don't need it. More on this later.
Both of the adjustments probably looks like this.
They should have the same amount of threads showing. If not, then one cable "inner wire" is too long. You can tolerate some difference, but if you find one adjuster all of the way down and the other one almost out of threads, buy new cables.
They should look like this, so adjust them till they do. I like to have one thread up from the bottom, it gives me some "play" room. More on this later. I would consider them ideal of they both look like this.
Note from Mark Huggett. The carb tops displayed above are original and require a right and left. Now BMW provides a universal top with a screw in brass fitting that can be rotated to adapt to each side. The problem is that this brass arm is 10 mm longer than the original and this takes away from the free play.
Magura throttle gear adjustment on the BMW motorcycle
Now the throttle cables have some play. The play has been increased by the amount that you turned the adjusters down. We will now "fix" it at the top end. This involves a rather clumsy operation the first time, but would seem easy if you were to do it several times in one day. You may have that chance, especially since this is your first time, but we hope not.
The /2 and /5 have marks to show the proper teeth matching. The reason that this is important is because the brass gear is a cam. It pulls on the cables slowly at first and at a higher rate as the throttle approaches 1/2 or more throttle. This is designed to allow the amount of opening to be carefully controlled by the rider at lower rpm. At slow speeds the amount of power can be easily controlled. At higher settings it will open faster.
While this is nice in theory, one does have some amount of adjustment in the teeth to use for proper control of the slack in the cables. It is more important to have the cable slack taken up fully by the bevel gears than to allow it to remain.
Feel the throttle "play" or looseness that is now evident. Remember this amount of play. Turn the throttle grip just up to the point where it would start to pull on the cables.
1. Remove the screw holding the throttle cap. The /5 and later have a rubber boot to keep dirt out. It must be slide back quite a ways to allow this next operation.
At this time you could use a short-cut and leave the screw in only one thread. Lift the cover in the right side and it will look like this.
Now drop down to step #7 and proceed. I recommend going through the whole thing at least once. You will learn more and also assure proper lubrication.
2. Pull the upper throttle cable, with your left hand, back (to the left) until you see the wire, like this. The lower cable is "captured" and can't jump out.
3. Very carefully lift and wiggle the cap up and off, being sure to not let anything move. The throttle cable ends can come out easily, so be careful. It isn't a disaster, only a nuisance.
You may now relax the throttle cable in your left hand. The cap has a hole for the outside of the cable to hold it in place. The slot for the cable wire is so small that one can only get the wire through. That is why the cable must be pulled back. Understand this part, it is important and you will use it later.
4. Make sure that the bevel gear area is well greased. Clean out old dry grease. Make sure that you grease the channel that the pull block runs in too. Use any thin grease. The grease will need to be checked again in a few years.
5. During normal operation the throttle grip tube won't come off of the handle bars.
The tube has a slot in it like this. The cap tang.
6. The cap that you just removed has a "tang" that sticks down into that slot.
The photo above is what you would see if the casting were invisible. The cap holds the throttle grip tube from coming off.
7. With the top removed, the grip can be pulled out, to the right.
Only pull it out enough to unmesh the teeth of the bevel gear drive mechanism of the throttle. It is only about 1/8" or so. On the /2 there is a spiral spring inside the tube at the outer end.
Photo of the /2 spiral spring, by Bernd Kupper, thanks
The purpose of the spring is to provide resistance from the grip easily returning to idle. It is a crude "cruise control." It's tension is only adjustable by the amount of grease, or lack of grease between it and the bar. You don't want to allow it to come off of the bar. If it does, it isn't the end of the world, but now it must be started back on. That takes a turn in the wrong way to allow it to "walk" onto the bar. The /5 and later don't have this feature. They have an adjustable throttle screw as a type of "cruise control."
The screw with the spring around it and underneath the control is the /5 and later "cruise control."
This is what the part looks like. The spring is "captured" on the screw. The white plastic is what rubs against the throttle tube to provide the drag. This part can fall off and we sold lots of them. Just loosen it enough that it no longer provides friction and it won't fall off.
8. Turn the grip in the idle direction. The amount is the amount of play that you were told to remember at the start of this. Gently shove the grip back into the casting so that the teeth mesh again. Again, as in step 2, pull the top throttle cable back. Replace the cover and gently wiggle it so that it sets down into the slot in the throttle tube. Relax the top cable so that it fits into the hole in the cap. You don't need the screw yet. Hold the cover down and test the throttle grip for play, it should be very little. If it is still too much, repeat steps, starting at #2 and try to catch the next tooth. The amount of play that one tooth will take up is about 1/8" at the grip. You can't usually get all of the play out, only most of it. The lower adjustment can easily take up any residual play. Replace the screw. Leave the throttle fully in the idle position.
9. The cables are now out of synchronization. Basic sync is very easy. Grab one cable at the carb and gently pull on it. You should be able to feel the play. Rotate the adjuster out to where there is about one thread of play. You must leave some play. Adjust the other cable.
10. Start the engine and let it warm up a bit. This next step takes two people. Have your helper very gently turn the grip just barely off of idle. You go to the rear of the bike and lean over and put your head up to the rear fender. Make sure that you are centered. Make sure that you can hear with both ears. As the throttle comes off of idle, one side may pick up first. The stereo affect will make it obvious. Go to the other carb (the one that didn't respond or did so later) and turn the adjuster out a bit. Test again and adjust as necessary.
This is an alternate method and doesn't require the extra person. With the bike warm and idling, bring the adjuster up until it just starts to pick up the slide (increase the rpm) stop and adjust it back down one turn to give some slack. Do the other side exactly the same way and amount of slack. This will get you extremely close and good enough for most of us.
Now that you are happy with the sync, here is a very important test. Turn the handle bars from side to side and watch for any change in the idle speed. You might need more "slack" or "free play" in the cables to prevent the bars from affecting the speed. You might need to reroute the cables to fix it.
11. Now you have the basic sync done and it may be enough for you. Don't use the wrench to tighten the locking nut. It is not only unnecessary, but I have seen the threads pulled out of the carb top. Use only your fingers to tighten it. First, do your best with fingers on the nut while holding the adjuster. Then, grab both adjuster and nut and tighten both. Yes, you will slightly alter the sync, but it will be the same amount on both sides. Do the other side too. If you need to adjust it again after a few days or miles, you don't need a wrench, just your fingers. It won't "creep" as you ride.
12. If your bike is one with the boot on the cable, as on all /2 and later slide carbs, it is time to move it down again. The purpose of the boot is to try to seal off air and water. Notice how very tight it is? As you push it down, also rotate it for ease in getting over the carb top. Once it is in place you are not finished. You must rotate it to let it extend again. As it got pushed down, it gets fatter and shorter. When you release it, the thing wants to grip the cable and lift it up. This will mess up your sync. I have never noticed a mixture change from the boot being "up" or "down." My fear of air leaking into the carb via the cable is long gone. I use another modification. I cut off the lower neck of the boot. This allows it to move up and down without grabbing the carb top. The rain is kept out, as before, but now the boot has no chance to mess up the carb sync. It is now easy to slide the boot up, adjust the cable and slide the boot down again, all with no tools.
13. You may have a vibration at road speed that you suspect may be the sync. It is possible for the slides to lift off in perfect sync and then be out of sync at some road speed. Here is one way to deal with it. Loosen the cable adjuster lock nut on the left side. Warm up the bike and take it up to the speed that you suspect may be out of sync. With your right hand holding a steady speed, reach down with your left and slightly turn the cable adjuster, one way and then the other way. Judge by vibration changes any difference. The vibration in the mirror may be a great indicator. You could leave it at the place where the vibration is the least. You will probably find that the idle won't lift off equally as you previously adjusted at step # 10. You must decide which is more important to you, slightly less vibration at road speed or smoother "pick up" off of idle. It has been mentioned that riding with one hand may put you at risk for a wobble. That is true, but the bike has a really big problem if it is that unstable.
14. If #10 and #13 give drastically different results, go through them again. If that is the best it gets, then go to my page on tuning to read more about possible factors.
15. Another method has been used by shops and some owners with some success. It was used in my shop with some success. My best mechanic, Bryan Hilton, didn't like it. The idea is that if each cylinder runs, by itself, at the same rpm, above full advance, the two will put out equal horsepower. While it may get you in the ballpark, it assumes to many things that may not be true. This method takes some practice, skill and a large fan for cooling the cylinders while doing it. An experienced mechanic can do this whole thing in under two minutes. It involves removing the spark plug caps and inserting an extension. The extension is home made. I am told that Ed Korn sells this tool as a set of two. It is nothing more than a 3" long threaded rod with the ends to conveniently mount it. The top end uses a spark plug round threaded thingy and the bottom end has an end like in the spark plug cap. The purpose of this extension is to expose the spark plug voltage. It is now easy to "short out" the high voltage ignition. It will kill that cylinder. The mechanic runs the rpm up to what is needed to achieve full advance with one plug shorted out. Watch the rpm. Quickly un-short that one and short the other one, note the rpm. The cables are then adjusted to achieve equal rpm. Caution: the throttle is so far open that you must not let it run on both cylinders at the same time, or it could go past red line. Personally, I don't recommend this procedure for the casual owner.
The accessory electronic ignition systems for the /5 and later are working out well for owners. It has been suggested that those systems don't like to have a spark plug shorted out.
16. The R75/5 throttle cables must come straight up for some distance from the carb. If it immediately bends over towards the underside of the tank, it can affect the sync. I have seen owners tie wrap the cable to the spark plug wire to keep it vertical."
