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Setting timing on '76 R90/6

geisterfahrer

Bill the Cat
I was working on my new old bike this evening, trying to sort out a hard start issue, when I discovered that at even at maximum retard on the ignition plate, the timing was still at full advance at idle. :scratch

I used a Dremel to elongate the holes in the ignition plate, and was able to get the idle timing set to the "S" mark.

Now the bike starts easily, and pulls like a tractor. :thumb

My question is, is this a common problem? I have no idea what brand of points/condenser was put on this bike by the PO, so are there aftermarket points that will cause this condition? If I put in a set of genuine BMW points, will I be able to set the plate back to a more "normal" position? :dunno

TIA,

Kevin
 
The advance mech. is stuck in full advance. Remove the bean can and lubricate the wieghts.
 
Not a bean can ignition on this machine. This one has the old-school points and condenser system. And, I have checked to make sure the advance does work correctly (it does).
 
Oooops, didn't know they didn't have a bean can. If the advance is working are the springs strong enough to resist it going full advance before it should?
 
I have tested the advance using a Tiny Tach (the cluster tach is busted) and a timing light, and it seems to be working well within spec.

I haven't worked with a points/condenser + mechanical advance ignition system since my '73 Beetle, so I'm definitely a little rusty when it comes to diagnosing problems. I'm just wondering whether a set of aftermarket points can cause the timing to be so far advanced, and why?
 
Have you checked your points gap to be positive it is set correctly? Go ahead and get some "real" points. Is the points cam clean? Did you try setting the points using a static method?

Just thoughts.......Dennis
 
Dennis,

I have checked the point gap several times. I had to reset them each time I took the timing plate off to elongate the slots. I have the original BMW toolkit feeler gauge, and I am a toolmaker by trade, so I am pretty comfortable with the gap setting. The cam is clean, and has no visible wear on the bearing surfaces. I set the initial timing using the static method, and then fine-tuned it with a strobe timing light.

I will go ahead and get a set of "real" BMW points, and see if they make any difference.

In the meantime, I'm going to put some miles on the bike as is, and see how she feels. The difference between the way she runs now and how she ran when I picked her up is astounding. If I had the traction, I could probably pull a triple-gang plow with her now :p
 
You initial problem was that the points gap was not correct. It may have been correct based upon your measurements, but the gap and the rotation of the plate need to be played against each other. There should be no reason to elongate those holes in the points plate. A small change in gap results in big changes in the firing point.

In the big picture, there just needs to be a gap. The time period of no points gap during the rotation of the camshaft is the saturation period for the coil. When the points open, the coils create the spark. If no gapping, no spark; if there's gap all the time, no saturation. The exact amount of gap is not so critical on a 2-cylinder engine...more important though on those old V-8 motors.

So, I think the right thing to do would have been to widen the gap a skoosh. A wider gap means the points begin to open sooner during the rotation. That would have resulted in the ignition firing just a bit sooner, eliminating the retarded condition you first found.
 
point gap

Kevin: Notwithstanding all the advice already given, I had the same problem with a 77 /7. No matter what I did, I couldn't get the points properly set without lengthening the adjustment hole on the plate. A few passes with a round file and I was able to get the right timing and gap. When I finally did get another plate and compared the holes, the newer plate did have a longer one by a few mm. For what it's worth.:dunno
 
Check the slack in your timing chain. Rotate engine one direction till the cam moves, rotate it back till cam moves and measure the degrees of play. May have jumped time a little or have too much slack. Better to check now than later. :lurk
 
Purchasing Points

Note that if you're purchasing new points, many are now made in China even though they are BMW parts. The rubbing block is too high on the one's available from most suppliers. I understand that Ted Porter's Beemer Shop is buying from a german supplier and the block is at the correct height.

Check here: http://www.beemershop.com/
 
I just ordered a point/condenser set from Max BMW. I hope they are good parts. Does anyone have the specs on the height of the rubbing block?
 
There is quite a bit of slop in the index for the advance mechanism on many bikes. Retarding the advance mechanism's location frequently will allow the timing to be corrected without filing the points backing plate.

It's also worth rechecking the points gap after moving the plate. Apparently (I've never checked this) the pates are not truly round.

However, it's also acceptable to open the slots a bit, as long as the points don't interfere with the wire bracket on the bottom screw.
 
setting gap

I heartily endorse the tool referenced above (although I purchased it from a different supplier, mine looks the same). Without this tool it is very difficult to set the gap properly because the advance mechanism is in your way. Without having the gap set properly, it may be difficult to set the timing. I had the same problem as Kevin, and it was resolved when I set the gap carefully (with the aid of said tool).
Theo Marks
74 R75/6
 
Without this tool it is very difficult to set the gap properly because the advance mechanism is in your way. Without having the gap set properly, it may be difficult to set the timing.

I would agree that the tool makes setting the gap easier. My contention is that the gap that is set with this tool may or may not be "proper". I mentioned previously that the gap is not that critical for our bikes. IIRC, the gap is ideally 0.016" but could be anywhere from say 0.014" to over 0.02". I had this experience recently setting the points on my /2. I set them (without the tool!) to best of my ability but eventually had to change the gap to get the static timing to be more or less correct. In the end, I believe my gap was over 0.02" but the important thing was static timing was correct.

On top of that, static timing is only just a starting point...no pun intended. It puts things in a good postion to get the bike started. What really matters is where is the full advance mark when the bike is at 3000-3200 RPM. You want the mark centered at that RPM to get the most out of the advance curve and timing. If adjustments have to be made to get the full advance ignition point correct, you might find that the static timing mark is no longer where you started. No biggee...of course if it's way, way off, better investigate...might be something wrong with the points and/or the points plate.

My 0.02...
 
Thanks for all the responses, guys.

Lostboy, if I understand you correctly, you are saying to make sure the advance mechanism is twisted as far in the appropriate direction that the "D" shape will allow.

Kurt, I think you are saying that the more important consideration in timing is that you obtain full advance at 3000 to 3200 rpm, and that the idle timing is not as critical. I think that makes sense, but my initial problem was that I was already at full advance at idle, and it got more advanced as rpm increased.

I think I'll get this tuning thing down eventually. It seems to be almost as much art as it is science :p
 
Kurt, I think you are saying that the more important consideration in timing is that you obtain full advance at 3000 to 3200 rpm, and that the idle timing is not as critical. I think that makes sense, but my initial problem was that I was already at full advance at idle, and it got more advanced as rpm increased.

Yes, full advanced timing is more critical than static timing, but you can't overlook static timing totally. In this case, your readings at idle were telling you that something was not right.

Remember that the advance will eventually stop, or at least should. You said that it increased with RPM. Likely you didn't go to an RPM where you saw it actually stop. But once you get the static settings nearer the proper location, you should then be able to witness that the timing will continually advance and that at the suggested RPM, the advance mark should appear in the center of the window. Any more RPM will not change the full advance mark at that point.

I also agree about putting on the advance unit in a consistent manner. Be sure and find that detent...it will do damage to it if you're close and then you tighten things down. I tend to twist mine CW (from the front of the engine) because that's the direction that I'll be tightening the bolt. And by tightening, it really just needs to be snug or just past snug if that hasn't been said already.
 
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