Stumble, hard start, and backfire...

[Duane Ausherman]

One can do a lot of testing with no spare parts.

I wouldn't replace the condenser just yet for two reasons.
1. They rarely go bad. That doesn't mean that they NEVER go bad. I have seen quite a few new condensers fail in short order. People want to assume that something that is "new" is also good. Not true.
2. This bike is running and that means that it is very easy to test. Remove the engine cover and with little ambient light, run it through the RPM range. At low rpm you may see some normal minor arcing at the points that will reduce at mid range and may increase again at higher rpm. In no case will the arcing exceed normal.

Also, before you start it up, check the points wire at the top of the engine where it is help by a metal strap. It is about 2 inches long and sticks straight out and wraps around the wire with a rubber grommet to protect it. That metal strap is close to something electrical. I can't now remember exactly what, but the metal strap vibrates, usually at mid range and causes an intermittent short that causes the engine to miss and stumble. I now forget the exact details now, but we found several that had this defect. Just bend the strap away a bit and all is well. I wish that I had one to look at and could describe it better. Probably early dementia.

Now you ask "How do I know what normal arcing is?" and that is a valid question. Go to Google (as usual) and enter something like "bmw motorcycle points" and you will find my page on this issue. A couple of paragraphs down you will see a section on this very issue.

My purpose for building that website was to make it easy for owners to solve technical problems such as this one. Would you use it more if you had to pay for it? Several people have suggested that humans value things more highly if they have to pay for it. Several have suggested that I charge for the information. Maybe I should try that for a few years to see if it works.

[dwire]

If everyone is all fixated with the condenser being bad, there are A) Cheap DMM's that include a capacitance tester in them. I have one that only does that and one built into another one; a standard $10 H.F. type Cen-Tech junker I'm willing to let another human touch... Both DMM's and the function are horrible for a number of reasons as far as getting an exact result, but I'm not sure you need one [exact result] do you? The placement of one's hands with an apparatus like that could change the reading from in spec to junker - really it's true; capacitance is something very hard to measure as one would other things... An analog meter is best for this but you can go to the Ohms range and you can charge the cap one way and watch it when you reverse the leads; did it charge up - did you see how the needle deflection was different charging and discharging???

Finally why is the condenser there? So that your points are not utterly destroyed from arcing. Are the points all burned up? If they are not, I would tend think it less likely there is anything wrong with the capacitor. On small engines, just as Duane has often lamented about their longevity, we only replaced them if they were part of a kit; otherwise they stayed put. Some for 20 to 30 years with no issues and while I'd have to put my thinking cap on, I'd say a magneto type ignition system might be a little bit harder on EVERYTHING than our 12 volt Tesla type tank circuits that are in our bikes... Just my opinion - so surely that now means I'll be 110% wrong and the condenser will have failed!

[dwire]

Also, before you start it up, check the points wire at the top of the engine where it is help by a metal strap. It is about 2 inches long and sticks straight out and wraps around the wire with a rubber grommet to protect it. That metal strap is close to something electrical. I can't now remember exactly what, but the metal strap vibrates, usually at mid range and causes an intermittent short that causes the engine to miss and stumble. I now forget the exact details now, but we found several that had this defect. Just bend the strap away a bit and all is well. I wish that I had one to look at and could describe it better.

Duane, this is what I described myself that I was likely the culprit - it is on the early /5 I know; production '71. There is a simple little clip to help hold the wire down in place as well as help keep the rubber grommet from falling out as you put the cover on. In my case, I had buggered it up somehow and there was a tiny bit of the copper wire rubbed through - spark with the cover off, yet not enough to run the engine with it on. I can totally see how what I ran into could hit a sweet spot of oscillation and cause the sort of mid-range stumbling, cutting out issue you and the OP describe...

That clamp is the on a bottom screw below the shaft if I recall...

[vespajg]

Jean...ran like a top last weds, put it up at night, and very difficult time starting the next morning. It was on the side stand over night so starting produced a rather typical puff of smoke. I was able to get it running, and seemed to resolve after some initial rough running and a seemingly normal 30- 45 min run to make sure the battery was charged up. Topped off the tank and it ran pretty well the next day, but difficult to start at the end of the day after work. Parked in my garage ever since. I bet I could get it running again, but waiting for the weekend to dig in ... even when running smoothly, it seems to be missing a bit of horsepower, but that could be me overly paying attention to how it's running.

[Jean]

vespajg, I wrote a nice note a few minutes ago and NOW it's lost.
Item: check for air leaks around the "hoses" connecting the carbs to the heads. Also check for leaks around your header pipes and crossover(backfiring).
For sure, look at the wire coming off the points as mentioned by Duane as that is a sure fire way to have problems if it is pinched and the insulation cracked.
Running OK one day and NOT the next is always a PITA. What you describe CAN be attributed to an advance unit that is not working properly! Maybe you lost/broke a spring? I ned to read an earlier post...I'll be back.

[Jean]

Read your first post again.
Then I read my notes on the "problems" I had with the R100RS that I had to push UP the hill...recall that comment?
Change the capacitor. (Have they gotten so expensive/rare that it's a problem to get one? )
You do not have to have a BMW one...*they are quite "forgiving" as far as electrical size goes. The long lead is the major difference between most old auto caps and the one on your motorcycle, but that can be "fixed". Be sure the case is adequately grounded when you install a new one.
*'67 Dodge capacitor works fine in '68 230 MB for instance...

[dwire]

Duane, as far as the wire clamp, that has been at minimum three years ago when my bike was a bike and not mostly pieces-parts in boxes and such, so I must have been standing on my head when recalling I believed the clamp to be located at the bottom; the parts catalog shows it located just as your memory serves; at the top...



I can say I do not recommend a lot of the NOS points available as they had a fabric insulation coating with what appeared to me as a latex layer over top as insulation. The rubbery latex type insulation self destructed on the two OEM kits I had which leave one even more vulnerable to shorts and such if you are either unable to get heatshrink tubing that REALLY shrinks down (so it can fit over the end) or some sort of dip or something to apply to what likely will be all that is left on those 40 year old wires; a piece of fabric... Just food for thought anyhow...

[jtoml]

I've had this problem, and in my case, it was caused by one of the crimped connections on the capacitor (condensor) getting slightly loose. There was nothing wrong with the capacitor itself, but current to the points was intermittent, and that certainly was a problem.

[vespajg]

Installed new condenser on Saturday morning and all is running well. Looked for shorts against the case and rubber/insulation all appeared in good order. I ran Duane's points diagnostic and all seemed good - bigger sparks at extremes, even mannered in the sweet spot. Old condenser was not original, or, from what I have read, at least the bracket was not. Stable running for two days now...no long runs, but it got me around town on Saturday and to work this morning. Full power appears to be restored and it runs very smoothly. Once I got it up and running, I also replaced the spark plugs for good measure.

Thank you all for your assistance...

[Jean]

vespajg, SO glad you got running OK.
I have been hammered 3 times in my life because of a faulty capacitor, so I always suspect it when weird things start happening that cannot be explained (such as you describe). I used to assume it would only show at certain times...not so. Maybe will cause hard starting, another time missing at speed.
Recent replacements seen to fail more often.