When I got home, I pulled valve cover and lots of oil spilled out - more than I’m accustomed to seeing from these engines - even when parked on the side stand (but I always use my center stand). The oil was surprisingly dark with only a few hundred miles on it. It also smelled like fuel. Not good.
Looked down push-rod tubes and they were un-occluded - head gasket oriented properly.

Removed the head and another few oz of oil spilled out of the combustion chamber.

Your engine has "fuel wet sumped" (as opposed to "oil wet sumped"). 🙁

That is, fuel has made it's way from the carb(s), past the rings, and into the sump. This raises the fluid level in the engine's sump, so that when you run the engine the engine breather tries to throw the gasoline/oil mixture out of the engine. BUT, the breather on a BMW is connected to the carb intake, and so the engine simply ingested the liquid being thrown out of the bottom end... into the top end.

CAUSES
The problem was not anything to do with your engine.... the entire issue is in your petcocks and float needles.
► Somehow fuel was allowed fuel to seep into the engine over night. This is the most probable case, and speaks to 1) not turning OFF the fuel during storage, or 2) simple wear or hardening of one or both petcock packings.

► Trash in the fuel system could have also lodged in the float needles and forced them to stay open. Usually this situation is accompanied by a soggy shoe. It's also highly unlikely fuel could seep into a running engine. But we need to account for this possibility.

THE REAL KILLER
The worst news is that you've been running your engine with gasoline diluted engine oil which is not good for any moving part inside the engine. All you can do at this point is to drain all your engine oil and change the filter. And start checking your oil level before each ride. This could have been prevented with a simple check of the dip stick, which would have indicated a higher than expected oil level.

SOLUTIONS
• I'd also replace both petcock packings, add fuel filters, and change out both floats and float needles as insurance steps.
• Start turning OFF your fuel every time you stop.
• When returning home, start turning OFF your fuel 1/2 block from the garage in an attempt to empty the float bowls. That way if fuel does leak in, it simply re-fills the float bowls.

Hope this helps.

I think Wobbly has it right. I wonder about the disparity in the compression.. I also wonder about the crank case ventilation system with the valving mechanism being stuck open. If your R100 has the system where the the crank case vents to the right side carb only, that could be a clue. That system works O.K. on my 750, but 1000 cc motors could overwhelm it.

Hey thanks for the replies - Yep: when I smelled fuel in the oil I got an instant mental picture of the bearings...not pretty - so I did change the oil before I attempted to start it again, but the compression is what's preventing it from running at this point.
I'm puzzled.
The petcocks were disassembled and cleaned thoroughly so will focus on that L side carb. - after I discover what's happening with the compression.
Fuel gets turned off every time, but draining float bowls before shutting down sounds like a good practice.

There's only: rings, valves and head gasket

2 things come to mind....
• Liquids, being incompressible, they may have blown the head gasket
• A great deal of gasoline around the valve guides may have made the valves hard to close

There's only: rings, valves and head gasket

2 things come to mind....
• Liquids, being incompressible, they may have blown the head gasket
• A great deal of gasoline around the valve guides may have made the valves hard to close

Right.
Except I eliminated valves and head gasket with the leak-down test - which allowed pressurization of the crank case, not air escaping from valves or head gasket....This points pretty definitively at the piston/rings.
Since I was riding down the road, I think an instantaneous combustion chamber full of fuel (a really unlikely hydro-lock situation) would have resulted in a visibly bent rod, or worse, don't you?
If that were the case, I would expect lower compression, but the rod looks straight as an arrow.
I do agree that that a faulty fuel system component has allowed excess fuel to get into my crankcase... but I think this is a separate issue - a condition that occurred over several miles since I was riding the bike when the compression loss occurred. Regrettable just the same...I'll get a look at the crank bearings this week.
I mean, unless there's something else I'm not looking at...I guess all I can do is install some brand new rings and hope that does it. If not, I'm looking at Nikasil.:whistle:

Right. Except I eliminated valves and head gasket with the leak-down test - which allowed pressurization of the crank case, not air escaping from valves or head gasket....This points pretty definitively at the piston/rings.

Sorry, but it would be next to impossible to "pressurize the crank case". First, there are crank case breathers which are designed to stop the accumulation of any crank case pressure. Then there are multiple oil seals that won't hold air under pressure. (Oil yes, air no.) Then there are the piston rings themselves which are specifically designed to act on pressure coming only from the combustion chamber. The results of pressure from the opposite direction is completely undefined !

Besides all that, the firing cycle occurs in a fraction of a second. A "leak down test" that pressurizes parts of the engine for minutes proves very little. Therefore, I am high skeptical that even a new engine could pass your "test".

What you need to do is a real, honest-to-goodness combustion chamber compression test. Adjust the valves, remove both carbs, and start turning the engine over until a sparkplug hole compression gauge stops climbing.

Since I was riding down the road, I think an instantaneous combustion chamber full of fuel (a really unlikely hydro-lock situation) would have resulted in a visibly bent rod, or worse, don't you?

All that's great, except that it avoids one obvious point... it takes hours, sometimes even days for enough fuel to seep past the piston rings to raise the sump oil level. So the fuel didn't get there as you rode down the road. No sir, your bike left the garage that morning with an overload of oil/gasoline mixture already in the sump. That mixture in turn, may have taken a while to work its way into the combustion chamber.

No offense intended, but IMHO whomever is guiding you in your thought process, is not helping you. You'd do better to exclude that person or engine testing resource from your knowledge base.

Wobbly.
Anybody who has taken the time to read this exchange understands you’re making childish, semantical arguments.
From what I’ve seen, this is what you do - you’ve done it with me and I’ve seen you do it to others. (For a recent example: R100lt Exhaust Smoke with Low Compression https://www.airheads.org/forum/my-airhead-needs-help/831-r100-lt-exhaust-smoke-with-low-cylinder-compression ). In that recent thread, a much more helpful Airheads Beemer Club administrator actually tried to help the OP after you pissed on that thread with your insulting, interrogating “you-have-no-business-working-on-your-own-airhead” demeanor.…And now the thread is presumably dead - 10 weeks inactive with no solution from the OP. That's a shame, because these true-case histories are what I always thought this forum was about. But reading your comments (your very first one!) it’s no wonder that person is looking for a solution elsewhere. And I, for one, would like to know what they find out but unfortunately, I’d be surprised if they come back.

But let’s talk about how you've applied your legendary skills to my thread, Wobbly. This one.
It's all in the recent posts which anyone can read for themselves, but I’ll pull a few highlights.

…I eliminated valves and head gasket with the leak-down test - which allowed pressurization of the crank case, not air escaping from valves or head gasket....This points pretty definitively at the piston/rings.

Sorry, but it would be next to impossible to "pressurize the crank case”.

Anybody who has used a leak-down tester understands I’m talking about air escaping past the rings and into the crankcase, and that you’re arguing semantics. I have no idea what you're trying to prove, but this is not productive - it’s childish, it’s adversarial and it runs counter to what the Airheads Beemer Club stands for - which is sharing knowledge and camaraderie.

Club canon #3: “...friendship over friction”, right?

(I think) you understand what I’m saying, but you’re duty-bound to have the first and last word on every thread, showcasing your vast knowledge by pushing others down with your rude, blame-assigning comments. Really, what’s happening is you’re making yourself look like an angry little fool.  I’ve seen you do this to others, and you tried to do it to a thread of mine earlier this year Unable to draw fluid to rear caliper. That one ended with you apologizing and me finding (then sharing) my own solution (master cylinder disassembly).

I see you as sort of the Cowardly Lion of the Airheads Beemer Club.  Congratulations. You're a recognizable presence.

A "leak down test" that pressurizes parts of the engine for minutes proves very little. Therefore, I am high skeptical that even a new engine could pass your "test". 

I’m starting to see that maybe you’re *not* familiar with a leakdown tester. It’s really a common and very useful tool, Wobbly. A leakdown tester pressurizes the combustion chamber. It allows one to discover the source of a combustion chamber leak. I’ll let you do your own research and then get back to the group.
If you don’t appreciate me condescending to you this way you might examine the way in which you interact with others.
Here's some more high-brow instruction from Wobbly:

What you need to do is a real, honest-to-goodness combustion chamber compression test.

Right. This is how I got the figures from the very first post: 30 on the left, 110 on the right. But you didn’t read that far because you stated:

The problem was not anything to do with your engine.... the entire issue is in your petcocks and float needles.

A diagnosis is worthless when you fail to collect all of the information. That means you’ve contributed worse-than-nothing to what could have been a productive conversation. Nice work.

In response to the following, you seem to forget your comments from only a couple posts back, and you contradict yourself. Again.

Since I was riding down the road, I think an instantaneous combustion chamber full of fuel (a really unlikely hydro-lock situation) would have resulted in a visibly bent rod, or worse, don't you?

To which you pedantically replied:

... it takes hours, sometimes even days for enough fuel to seep past the piston rings to raise the sump oil level. So the fuel didn't get there as you rode down the road. No sir, your bike left the garage that morning with an overload of oil/gasoline mixture already in the sump. That mixture in turn, may have taken a while to work its way into the combustion chamber.

But why would I suggest something I recognize to be a “really unlikely hydro-lock situation”?
Let’s go back and find out.

• Liquids, being incompressible, they may have blown the head gasket
• A great deal of gasoline around the valve guides may have made the valves hard to close

I seems you, yourself suggested this really unlikely incompressible liquids situation to which I skeptically referred. It makes more sense when we see the whole exchange doesn’t it? You’ve contradicted yourself, Wobbly. Which I recall you also did in our last exchange about "complex" but oh-so "simple" R100 brake systems.

No offense intended, but IMHO…

That’s funny. Both parts.

…whomever is guiding you in your thought process, is not helping you.

Now this is one true thing you’ve said, and we are in agreement here. There’s only one person who has offered advice, (you) and as you’ve seen from the thread, I recognized that advice was flawed, but I was polite about how I rejected the idea, hoping a wise and helpful airhead would chime in with useful information.
No such luck. Seems you've already contaminated the thread.

You'd do better to exclude that person or engine testing resource from your knowledge base.

Yes, I did try that in May when I invited you to excuse yourself from the discussion, but it looks like I’ll need some advice solving that matter too.
I actually hoped you’d just let it go and allow other, interested airheads respond.
Instead, you read half of my post and foisted your condescending and never-to-be-refuted definitive (and also dead wrong) diagnosis of a leaky petcock.

Airheads Beemer Club Canons:
6. Airheads maintain their own motorcycles.
7. Airheads don't take themselves, religion or life too seriously.
8. Airheads like to share time, knowledge, parts and camaraderie with other Airheads

Wobbly, your childish, discouraging behavior runs counter to all of these.
You're contaminating the knowledge pool.
Tainting the aquifer.
Pissing in the well.

Now I want to address the administrators of the Airheads Beemer Club:
Twice this year, I’ve come to drink from what used to be a clear, deep well of good-will here at Airheads Beemer Club, only to find the water in my cup rancid, and of no value.
There's no excuse for a moderator to antagonize the people he/she presumes to serve.
I’ll be in the BMW motorcycle community for many years to come.

If Wobbly’s condescending, adversarial demeanor hasn’t already wrecked the reputation of Airheads Beemer Club, he/she is making good progress.
And if this is what our annual membership buys, some of us will have no reason to renew our membership with Airheads Beemer Club.
Please show your members you expect more from people who represent our club.

A cylinder that can only achieve 30lbs compression screams "broken rings" to me.

Things I know for sure;
1) a bent or burn valve will achieve 0 compression.
2) a cylinder out of spec and out of round will achieve 90lbs compression.

The crux of the biscuit in that statement is for sure.

Has Wobbly ever rubbed me the wrong way?, sure.
Have I ever rubbed him the wrong way?, probably.
Am I going to let it spoil my fun?, never.

I think if we could know the number of unique forum users here compared to the total membership, we would be talking about a small percentage of the whole.

Have we ever seen T. Cutter or R. Fliescher or L. Provin or M. Parkhouse reply on here? I think not. That leaves us to the care of 2nd or 3rd tier operatives like myself and/or Wobbly.

Let's resolve to be kinder and gentler with each other, O.K.?