Does your BMW motorcycle idle too fast when warm? Maybe it is the mechanical advance unit. |
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Causes of a high idleYou may have one or more of these factors going on at the same time. Test for them and then correct as needed. Don't go off buying things without a proper diagnosis. 1. The mechanical advance not working properly. Read about the test, springs and lubrication below. 2. Throttle cables holding the slide up a bit. Try rerouting them, or replace with real BMW cables. 3. An intake leak can contribute to a high idle. This page about the /2 carbs. 4. The normal procedure is to set the idle while hot, but then it may be a bit low while warming up. That is common. What is a timing advance mechanism unit?As an engine goes from idle to higher rpm, the fuel in the combustion chamber wants the "fire" to start sooner. That fire is started by an electric spark. When the spark happens that is called the "ignition timing" and we are quite concerned about having this happen at exactly the right time in the engine cycle. To change when the spark happens, we need some way of sensing the speed of the engine and then telling the spark to happen. This is done by the timing advance mechanism. How does the BMW motorcycle timing advance unit do this?The spark happens when the points open up. The points are opened up by the camshaft. The front of the camshaft has the advance mechanism mounted on it. It has a "bump" on it that comes around and forces the points to open up. We call that bump the cam lobe. The advance mechanism unit has two of these lobes.
The arrows point to the cam lobesAlways examine the cam surface for a smooth surface to rub against the points. A worn surface will quickly wear out the points rubbing block.
This shows a "smooth" lobeHow do we get the "advance" aspect?The advance unit has two flyweights mounted on the ends. As the rpm increases, so does the force to allow the weights to pivot outwards. As the weights "fly out" they cause the cam lobe to rotate a bit. You can demonstrate this easily. With the advance unit off of the camshaft, pull the weights outwards. Notice that the part with the lobes will rotate. That means that as the rpm increases, the weights fly out and make the lobes open the points at an earlier time. The ignition timing is advanced and the (fire) combustion starts earlier. More power is generated because the fire is started enough in advance to complete the burning of the fuel. A simple test for the high idleA high idle that creeps up during and after the warm up time may be due to weak centrifugal advance unit springs. (These are the springs that are always trying to return the fly weights to the resting position.) In this case, no amount of carburetor adjustment will return it to normal. Here is a very easy test for this. While the engine is running at the "too high" RPM, gently pull up on the ignition key. Don't allow it to jump out, lift it only enough to kill the engine, maybe 1/8". On the /6 and later, just turn the key to kill the engine. Allow a few seconds for the RPM to drop down to below normal, almost stopped, and then push the key back in. The ignition system will work again, the engine will start firing again and the RPM should go back to normal. The rpm should stay down at normal. If it stays at a normal rpm, then your springs need to be replaced. It may creep up again in a few seconds, but this proves that when the weights got pulled back in where they belong, the engine runs at a normal RPM. If the rpm goes up quickly then I would suspect the carb adjustments. You could easily have both issues, weak springs and poorly adjusted carbs. To be sure, remove the front cover and put your strobe light directly on the advance mechanism. As the key is pulled to drop the rpm and then reinserted, watch the weights carefully. They should return to the relaxed "pulled in" position. If they return and then go out again with increasing rpm, that shows that the advance is working and the rpm is held up by fuel, not ignition. The advance unit carries the lobes that opens the points. When the points open up, the spark occurs. The lobes change position on the shaft according to RPM. The faster that the advance unit rotates, the more the weights swing outwards. As that happens the cam lobe rotates in the direction that makes the points open earlier in comparison to the piston. This is what it meant by timing advance. When the springs get weak they allow the weights to fly out at a lower RPM. This "advanced" timing causes the RPM to increase quite a bit. The solution is simple, replace the springs. It is easy, but they are a bit expensive. There has also been a heavier set of springs available for the /5 to change its advance curve Care and feeding of the BMW Timing Advance UnitThe advance unit has two types of springs. One spring for each flyweight to return it to the normal resting place, and the flat spring to limit the maximum rotation of the lobes. That is two coil springs and one limit spring. Both type of springs are important to the proper operation of the engine.
Older style /2 advance unit with the "question mark (?)" shaped advance limit spring. This type of spring was used up through 1967. It was replaced by the one below. This spring failed often, as the tip of the spring would break off at the point where the arrow points to. This failure of the spring would allow the ignition timing to advance too far. That would allow some overheating of the pistons. That was a common factor in piston failures.
Newer style /2 advance unit with the "U" shaped advance limit spring. Be very careful if you try to adjust the spring to change the limit. They break easily, don't ask how I know that. A few more great advance pictures from Bernd can be seen at http://home.tiscalinet.de/strichzwei/index.htm What is a worn out /2 advance unit?
Photo by Rico Carballo, thanksThis is the old style advance unit with the ? mark advance limit spring. On the left is one where the weights are out too far. The one on the right are far better. Hold the weights out and see how you advance unit looks. You can also see the wear marks from the attachment bolt on the left spring. The one on the right shows little wear. What I can't show here is how the weights on the left unit are also floppy. The one on the left will work, but one would need to make sure that the full advance doesn't go beyond the "F" mark at high rpm. The timing light flashing directly on a worn advance mechanism may also show a less than smooth movement of the weights while changing rpm. That isn't very important. More important is that it must not stick open while the rpm is coming back to idle. The /5 and later BMW motorcycle mechanical advance units
This shows the /5 engine with the advance unit at the position where it opens the points, or where it fires the plugs. The nut is the one that so many over energetic owners over tighten and strip the threads off of the camshaft. That amount of oil leak is common and not important for proper operation. It is only a cosmetic problem. WarningThe 10 mm nut that holds the advance unit onto the camshaft can be over tightened very easily. The usual result it to strip the threads from the camshaft. You don't want to even think about replacing the camshaft. It only takes one finger on that short wrench to very gently tighten that nut. It is only slightly more than finger tight. I have never seen one fall off from under tightness, or any other reason, except stripped threads. If your threads are stripped, you still have an easy option. The cam can be drilled out and a screw used to hold the advance. To get the tool to drill it in alignment, contact Paul at repairhead@sbcglobal.net
This is the /5 advance unit. The red paint is just to mark the advance unit for easy recognition in aligning the tip of the camshaft.
Here is the same unit with the weights held out in the full advanced position. Ignore the (temp stops) two round light colored things that are used to hold the weights out just for this photo. It can't be seen in the photo, but the lobes are now rotated (advanced) with respect to the face of the unit. The locating flat can get worn, but usually the camshaft has a flat that wears more. Between the combined wear on each part, it makes a lot of "slop" to contend with. Be very gentle when working with this unit to protect the flats. I like to hold the advance mechanism in the "clockwise part of the slop" when tightening it down. The weights can eventually become sloppy on the posts and flop around quite a bit. In that case the timing is harder to get it to repeat itself. Probably most of us must replace the whole thing. Altering the advance curveThe advance curve can be altered. One would only want to do this for some special reason. Pinging (pinking for the Brits) isn't a reason. If you remove weight from the two flyweights, it will advance even later in the rpm range. If you make them heavier, it will advance at a lower rpm. LubricationIt is very important to lubricate the unit. The most common fault is for the cam lobes of the advance unit to get stuck on the camshaft. Clean off the old cruddy grease on the camshaft. Clean out the inside of the unit very well. Use a high temperature grease on that part of the cam. Put some grease on the cam itself and on the felt of the points plate that rubs against the cam. Try to get some light grease, or oil, into the place where the weights pivot. Testing the BMW timing advance mechanismThe first test is just after tightening the unit back onto the camshaft. With your fingers, open the fly weights and let them go. They should open easily and "snap back" smoothly and quickly, without any sticking. When one uses the timing light to check the timing marks, also put the light on the advance unit and rev it up and allow it to come back to an idle. The strobe light will reveal any sticking or uneven operation. More BMW motorcycle tuning info |
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This page was last edited:
12/07/2006 - copyright
Duane Ausherman |
