Why it is hard to tune

 

 

BMW motorcycle timing, tuning and carbs (twin cylinders)

by Duane Ausherman

Here are some BMW tuning considerations.  It is only natural to want to keep your BMW tuned.  Questions keep coming up about the two carbs and how to synchronize them.  Do you have a “hard to start” BMW motorcycle?  These are only some of the factors in tuning.  This is a mental exercise only and is not intended to be important to anybody at all.  The net effect may be to confuse some riders.  It’s amazing that they run so well with all of these factors affecting the tuning.

1.  The BMW twin is basically two engines with a common crankshaft.  In all tuning aspects, they must be treated as being two separate engines.

2.  The BMW exhaust systems are of a different length, due to the staggered cylinders.  So they tune differently.  A cross wind can change the back pressure between the two sides.

3.  The BMW twin intake is not exactly symmetrical for each side.

4.  The right side, of 1970 until about 1980, has some crap (crankcase fumes) dumped into it from the crankcase breather.  This changes the tuning slightly, it’s especially noticeable at lower rpm.

5.  The Bing carbs are pretty consistent, but I have found differences between them, don’t assume.

6.  The BMW valves may seal unevenly.  Same for the rings.  Remember the time that you rode for 2 hours in a cross wind? That is when you forever changed the two sides because the cooling was totally different.

7.  If the compression is different between the two sides, then one can’t expect to balance them.  Compression is measured at a very low rpm.  At higher rpm they can act differently, and we can’t even measure it.

8.  Do you think that the camshaft was made with equal lobes?  Maybe so, but do you think that they wore equally?  Not likely.

9.  Often the ignition timing is slightly different between the two cylinders.  If fact, why even think that these two cylinders even need the same timing?  Proof that they don’t is that when one encounters a ping (pink in the UK) it is usually worse, or only on one cylinder.

10.  The BMW timing advance curve is mechanically derived, not “need” derived.  The ideal timing is to be retarded enough to be just under the ping timing at all rpm.  To determine that, one must have the ability to independently change the timing of each cylinder under load.  Then find the optimum timing increments of 50 or 100 rpm.  Then you would see the ideal timing curve.  There is no way to get that mechanically or electrically.  The ideal curve for each cylinder would be very different from each other and different from the curve provided by the advance mechanism.  That’s why one can find that the pinging will be slightly different in each cylinder as it goes through the “ping” rpm range.  One may even find that it only pings on one cylinder.

11.  Have you ever used vacuum gauges to tune the carbs and found them to be way off?  It is possible, even easy, to get the vacuum equal with the settings at idle and have them run quite differently, even badly, especially at a higher rpm.  A person with experience and sensitivity can tune them faster and better, by ear, than anyone with only gauges.  It is natural to want a “magic” fix, but all of the gauges and level tubes filled with this and that, are a very poor way to do it.  Sorry, but you need to “know something” to do the job well.

12.  To adjust the cables to pick up at the same time is one way.  The error in that thinking is that the two engines (cylinders) won’t rev up at the same horse power vs. rpm rate.  Then, at road speed they may be putting out different amounts of hp.  We call that “vibration.”  Some vibration is normal and is due to the offset cylinders.

13.  Another way is to take the engine up to 3500, or so, rpm and short out one spark plug at a time and adjust the cables for equal rpm.  This won’t produce equal HP (low vibration) at that rpm, but it will get you into the ballpark.  The reason is that the two cylinders usually produce a different amount of horse power at any one rpm.  The two sides aren’t able to contribute equally to the work.    Note, I really don’t approve of that method of balancing the carbs because it doesn’t work well.

14.  The choice is easy for the touring rider.  One will find that a narrow range of rpm will be normally used on the road.  At this rpm, adjust the throttle cables for minimum vibration.  Just know and accept that it won’t be optimized at any other rpm.  More info on this procedure at the end of my controls page.

14.  The sport rider must accept that it will never be “perfect” at any rpm.

14.  Now let’s throw in the fact that your engine will want different timing and mixture at different ambient temps, pressure and humidity.

The bottom line on carb tuning for the BMW twin

A good BMW twin tuner with a lot of experience will almost never find a perfectly tuned engine.  Each of my mechanics went through a “crazy period” when they were learning to tune carbs.  While riding their own bikes, every few minutes they would reach down and do a slight tweak on a carb.  Eventually, they learned to accept that it would never be perfect.  The very best that you will ever achieve is a workable compromise.  Be happy.

Trivia

The Bing carbs are or were, made by a company named Fritz Hintermayer Gmbh in Germany.  There was no company called Bing when I was in the business.  They were very good to me.  I purchased dozens of new carbs and the delivered price was about $5 a carb.  That included the carb, shipping, and customs.

To adjust the Bing carbs on the /2