Hi.. I have a 1978 R100/7 with 32K miles on it. I'm having a problem with it sputtering/cutting out/stumbling at high speeds, and more recently at lower speeds. I'm going to try to get all the necessary information out of the way in the OP, so sorry for the length. Here's a picture for fun..
So I just took it for a 1600 mile loop through Michigan/UP/Wisconsin with camping gear, luggage, and my girl on the back. From day one it would stumble very briefly while on the highway (55-70mph, 4th or 5th gear, 3800-4200RPM) maybe only once every 30 minutes or so. The sensation is very brief loss of power, a sharp deceleration almost like you're hitting the reserve on your gas tank, but it recovers immediately.
(Backstory: This happened on the first trip I took it on after I bought the bike at the beginning of this summer, on a hot day from Ann Arbor to Cincinnati. First occurred after about 45 minutes of riding, the frequency of the sputtering increased to a couple times a minute so I pulled over and did some damage control, thinking the engine was overheating but hoping to make it all the way down. I raised the float level thinking that the bowls were running dry, then re-gapped the points and retarded the ignition a little bit, and swapped the plugs back to some Bosch plugs that came with the bike, having previously put in new NGK BP6ES plugs. Kept riding, and the problem resolved itself. Checked the timing with a light down in Cincinnati before heading back, and it was dead on (which is strange because I had checked it before leaving and then retarded it at the side of the road). The ride back was flawless. I then went through some more maintenance, discovered I was getting a frustrating dual image, bought a used mechanical advance in decent shape because the one I had looked a little worn, though the new advance had the same dual image problem, took it to a local airhead expert who installed new points which (strangely) solved the dual image problem, he reset the float level by setting the line on the floats parallel to the carb bottom when the valve closes, re-timed and synched my carbs. Oh, also at the beginning of the summer I measured the compression at 130psi in each cylinder.
Everything was great for about 1000 miles, though I noticed oil consumption was high at 1500miles/liter.)
So back to the trip I just got back from.. the first few days up into the UP the sputter was regularly occurring every 30min to an hour of highway riding. The first 45min to an hour of the day usually had no issues though. Then after holing up at a cabin for 3 nights without riding, the sputter increased in frequency to once every 5 minutes or so. I thought maybe my airfilter had become clogged from riding on some dirt roads, and I realized I had never checked it. This continued until I got to Chicago, though strangely the last hour into Chicago in the rain on I-94 at 70mph was sputter free. Had a chance to do some maintenance in Chicago, and upon pulling the plugs saw that they were extremely dark with sizable deposits. Here's a picture:
My thought was that this explains my high oil consumption (in addition to pushrod seal leaks), and may be compounded by a rich running condition from the clogged airfilter, causing intermittent fouling. So I went down to Motowerks Chicago, which is a great place by the way, and got a new airfilter and showed them that plug. The fellow recommended some higher temp plugs, NGK BP6ES. So I installed fresh plugs and the airfilter, and when I took the old airfilter out I noticed... about half a cup of dog food under my airfilter inside the airbox. Which blew my mind. I've seen mice nests in airboxes before, but never with dog food and nothing else. So I thought some asshole had at some point deliberately put dog food down the air snorkel, but who knows, and I don't think it caused any problems. Just an odd digression for you.
Anyway Over the next 150 miles, the problem came back, and much worse, stumbling sometimes several times in a row. Often the stumble didn't make any noise that I could hear other, but occasionally now if it did it several times in a row I would hear a sharp sound, though not apparently a backfire. It would happen at any throttle position, always feeling the same when it stumbled, and sometimes going faster or slower would momentarily relieve the problem. Re-timed with a friend's timing light in Grand Rapids after swapping out the condenser for an old one, thinking that my condenser was going, and this time it was a good 10 degrees retarded at full advance, and the split timing was back. Checked the plugs, which looked like below, except half of the insulator was rather white. Blew out the fuel lines, checked the flow from the petcocks, cleaned out my carb jets, and nothing seemed out of the ordinary. The last 150 miles were smooth for the first hour, then it got way worse, and we just barely made it home, limping. Now it was sputtering as low as 30-45mph, when it hadn't before. Since then I have ridden it down the street and it has the same issue even at low speeds in 1st gear. Idle seems totally fine.
The new plugs, with 300 miles on them look like this:
so it seems they're going the same way as the last plugs and oil fouling, though I should mention this picture was after idling for half a minute with the choke on and going up and down the street.
Okay so that's the story..
here are my thoughts, and what I've done since then.
I thought it was pretty clear that this is an ignition problem, even though it's tough to get a good read on the carb mixture with all the oil burning. So having re-set the points and the timing, swapped a condenser, I thought that the coils were failing. My bike has the two stock 6V Bosch coils, like the one on the left on the first picture on this page: http://www.largiader.com/articles/ignition/.
So I disconnected all the coil wires and used a digital multimeter to check the resistance of the primary and secondary windings, finding that the primary coils for both were 1.2-1.3 ohms, within the specs in my Clymer manual, but the secondary windings were 6.68k and 6.73k, both just below the spec of 7.5-9.15k in the same book. Could I have found the problem? I did the test sequence explained here: http://w6rec.com/duane/bmw/points/index.htm
and everything checked out. The spark looked good, I think
. I checked the resistance of the plug wires, which both read close to 5kohms, and cleaned the spade connectors to the coils and the battery's terminals and engine ground (none of it resolved the issue).
That's my hypothesis for now, does anyone else have any ideas of what to check before I order new coils? Should I just order an electronic ignition at this point? Maybe the oil I'm burning is causing the misfires and my coils are okay?
I just noticed this while photographing the plugs, the cap end of the old plugs with about 3k on them has this distinct black mark:
I checked the inside of the plug caps, and the brass part this presses into does look a bit corroded. Strange thing is, the new plugs don't show evidence of this, and the problem persists. Maybe there's so much grime in the caps that some of the spark is being wasted through this connection? I'm going to get some contact cleaner and check..