I've been doing a fair bit of searching on the forum and I'm fairly sure what's ailing my K, but I thought I'd run this past you guys before I start throwing money at the situation.

Got the bike with a dead battery, so in goes a fresh one and the bike runs great for the first few weeks. After fixing a number of other issues attributed to sitting improperly stored for a very long time, the bike started dropping cylinders after it had warmed up. Only at idle, it would run normally when put up to maybe 1500rpm, and ran famously up to redline. It makes a "Snapping" noise when the cylinder(s) kick back it. Sort of like a relay slapping over, or maybe just the coil arcing back? I have no idea, but it's prominent.

So, I have the bunk black tower coils installed, I've done the nighttime test to look for arcing, but didn't see any. I've testing the primary resistance, and got 2.9 ohms on both, but couldn't get a reading at all on the secondaries. I found a crack on one housing and patched it up with a bunch of JB Weld, and it may actually run a little better, but there are a lot of variables involved.

I'm guessing the coils are likely pooched, but I'm curious about the fact it runs fine until it's nice and warm. Do coils run more poorly when warmed up? Should I maybe try running with the coil cover off for more airflow and see if that changes anything before splashing out for new coils?

I do have a bad cranckcase hose that I've bodged with some form-a-gasket. I know that can apparently cause idle issues. But it seems unlikely to me it would cause cylinders to miss.

Thanks in advance for your advice, gang.

Josh

It is not uncommon to have a crack in the coil that opens when heated up.

Most likely problem is one of the Hall effect sensors that live behind the 'T' shaped cover on the front of the engine.
They can break down as the engine warms up.
Remove the cover and heat the sensors with a hair dryer with a cold engine. If the problem is there as soon as you start the engine, then that's your problem.
Easily fixed.

With coils that are as bad as you describe then that is where I would start. Replace the coils as you are going to have to do it very soon even if it isn't the problem at the moment.
The breather tube usually doesn't cause the sort of problem you are having but will give a very bad idle and a flat spot just of idle.
Check the hall sensors as John has suggested. I wouldn't expect them to be the cause but electrickry is strange stuff.

Thanks guys, I'll check the hall sensor, and start cruising ebay for some orange tower coils

j

Or, rather than buying used and possibly someone else's duff bits and bobs, buy new from Rock Auto for pennies on the dollar as compared to what your local BMW dealer charges.
However, you may have to paint the tops of the new coils orange yourself.

brickrider wrote:Or, rather than buying used and possibly someone else's duff bits and bobs, buy new from Rock Auto for pennies on the dollar as compared to what your local BMW dealer charges.
However, you may have to paint the tops of the new coils orange yourself.

 Are those the one Japuentes recommended? They really don't look the same in the picture... But I'm all for an aftermarket replacement.

Yes.  I bought those coils on his recommendation and I am convinced they are the equivalent of what BMW sells.  However, they are entirely black in color, which initially made me doubt.  I had a close look at what EME sells and concluded they are the same as what Rock Auto sells. Their primary resistance checks out with figures others have posted on this forum and I am satisfied with their performance on my 1985 K100RS.