well, it rained so i could not flog it so the first run was at 130*f oil temp and the engine did drop 100 rpm after a few seconds.
on the return, the roads were drier, have to be careful as she'll break traction. i was able to get temps a tad over 140*f and the rpm drop was more pronounced but did not die.
the way she acted did not seem to be flywheel related as i played around with it.
just me but i seem to think it was more of a mixture issue since i could alter the way it ran playing with the oil tank.
a rich mix will be sluggish and since we tune cleaner on, the carb will react to anything that changes flow within the cleaner.
when the tank is opened, it vents the majority of the pressure out of the engine, mine was huffing pretty good. with this pressure not going into the cleaner, it has to pull harder through the media with the resultant effect of a richer mix.
besides, the oil will still transfer out the compartment since flow is to the cam chest.

I can relate first hand that introducing air has this effect and the higher temps are a result as well. I believe it's partly because of the crank doing the backstroke in oil, pistons working harder, and the oil being suspended in the heads longer and getting hotter. Higher rpms magnify this issue greatly. I think the crank swimming in oil comes last after extended high rpm. The pistons working harder and oil not draining from the heads as quickly are immediate consequences.
Increased air pressure is NOT good. You wouldn't think failed umbrellas would have such a big impact but I'm convinced this is the case.

That would suggest the intake of the carb gets a substantial amount of air from the CC vents.

Some oil will still transfer to the cam chest for sure.
Lighter density oil will transfer faster than heavier density oil.
Heavier density oil will get slung into the crankcase walls as well.
So you at least did notice a change with the oil temp higher.
I realize that only a couple folks have mentioned this but I simply couldn't find many other's comments on the subject.

I didn't think to play around with the cap to get varying degrees of change.

There may be several things happening. In looking at the angles;
The vents are on/off so if that were the case,
Velocity in the intake could also be on/off on a normal day on every downstroke depending on the condition of the breathers?
As in both in different states of hardness / flexibility.
Then compensated by an increase through the media during out of sinc moments.
The vents don't put out that much air pressure normally I'd think.
Although the vent air does tax the intake track.

On the other hand,
The breathers are opened upon downstroke before much cylinder fill.
With the oil cap off, this relieves the pressure under the pistons faster during downstroke (wider escape but still traveling up the 1/4" vent hose).
In theory, better ring seal on downstroke also (increasing cylinder fill velocity)?
In that case, wouldn't the CV carb react in kind?

I'm curious about the oil temperature.
I'm leaning toward more dense air/oil mix since I considered what came first, the chicken or egg.
Apparently colder oil temps are less taxing on the issue.
Oil suspension is higher (faster) with hotter oil, lower (slower) with colder oil.
We speak of hot idle as the engine barely turning.
But the truth is 1000 RPM is still humping it. It'd be more visual if the engine case was transparent.

I can see my SBC crank pulley moving at 1000 RPM and then spraying oil on it.
It would immediately sling oil several feet each direction.
But the Sportster wheels sling oil inches away to the case wall cylinders/piston unders where some falls back on the wheels.
Higher suspended oil increases the oil density of the air/oil mix.
I don't think wet sumping is happening as I see the oil returning without issue.
Oil mist is slung around by the wheels by default of the machine.
But more dense air/oil suspension is heavier (what is swirling around the wheels).

If it's simply the mixture or balance of crankcase pressure, then the MoCo definately accomplished a change with the smaller breather bolt holes.
To be able to manipulate the mix simply by moving the oil cap closer or farther away from the opening proves how sensitive the engine is to CC pressure changes with air moving into the engine also.

Sorry if this sounds blunt or curt, but you guys are really confusing the issue. I can assure you that the carb and a/c have nothing to do with it. The carb flows, say up to 200 CFM? The air filter itself, if stock size but better flowing(K&N) can easily flow enough air to make 100hp. There is NO measurable pressure difference inside/outside the filter element itself. The airflow out the vents is negligible compared to these factors.

The vents however, if ported into the carb throat, DO deliver a small amount of "bad" air to the intake. Not much, but enough to rob a couple hp in the right conditions. Also they deliver some oil to the intake when a venting problem occurs.

Edit: HD vents into the a/c to pass EMISSIONS, to the detriment of performance, and not for venting dynamics.

That's kinda what I alluded to but I never discount bustert.
He's a good thinker.

That brings me to the 2010 oil tank.
Supposedly this was the year the MoCo installed the 10psi pressure relief in the oil tank.
Anybody got one that exploded can split it and take pics of the innards?
I'm curious where and how it's installed and it's function.

while that is true, that the air cleaner MUST flow more than the cfm of the engine needs, the media does in fact effect air flow. of more importance is how or the direction the flow must take. the best is a direct inline flow which does not happen with an hd ham can. if you introduce angles, they must be wide radius and the ducting of correct size as every curve or turn adds resistance and more importantly turbulence, best to have laminar flow.
my vents are not stock and probably flow more.
when the the vent exists the orifice, as mr. hippy mentioned, the velocity must increase but after the orifice the pressure must drop and drops greater as distance increases. when this vent hits the volume of the can, it becomes lazy and more so with little throttle demands at idle. now what happens when you get lazy flow, well anything in suspension will coalesce and drop out. this leads to oily/water mix. now is the really a vent issue, hummmm, well maybe and maybe not, i am more inclined to believe it is an engine condition issue since it progresses with age.
as an example, ever fooled with pontiac engines with trip-duce?
really could be anything but this is a personal example.
if the carbs are syn to open in one felled swoop, on a take off, the air flow becomes near atmospheric for a brief moment, the fuel drops to the manifold floor, this results in the two inside cyls on both banks to run rich and the further cyls to run lean. an ole trick was to groove the manifold floor so that any drop out was directed to the leaner cyls.

I read that post, not the whole thread yet, but:

I agree as I mentioned earlier, that the crankcase is at atmosphere while not running.
And a slight vacuum is pulled while running.
Then a push - pull is established while running.

But 'the next time the pistons come down, crankcase pressure will return to atmospheric."
Inbetween that you get this:
The first thing that happens on downstroke is that positive pressure (greater than atmosphere) is generated due to the restrictions of:
Case volume
Path to the vents
Vent hole size
Vent line (if applicable) length
Any induced air from the breather valve(s) not closing properly

These things will bring the pressure inside to higher than atmosphere, else there would be nothing to expell.
As the air is pushed out of the vent, at BDC, the air returns to atmosheric. Check.
That is the problematic condition, the higher pressure before returning to atmosphere.

The negative (vacuum) on upstroke I'd think is more stable than the positive pressure on downstroke due to ring blowby and others.
But ring blowby can also happen during upstroke depending on ring condition, thus oil entering the chambers.
Blowby adds to positive pressure which throws out the balance.
Let's use X (+1) as positve and Y (-1) as negative.
In a pefect world, X goes down and Y comes up ------ X+Y=0
Add ring blowby and you get ------ X+1>Y------ or the real result. Balance is off by nature of the moving parts.
The pistons move up and down almost together.
That makes the push/pull environment more violent.
Add a vacuum pump with Z (-1) amount of pull:
Now you get X+1=Y-Z... seems the balance (to zero) is restored even though positive pressure is compiled of blowby.
But the lower the negative pressure is at the beginning of the downstroke, the lower the positive pressure will be.
If you are generating 2 psi of positive pressure on downstroke but reduce it's beginning surge to -1 psi (Z), the result will be only 1 psi of positive pressure during downstroke.
-1 (+) + 2 equals +1.
The more the vacuum, even lower the positive will begin.
The rings act as a buffer between these two conditons as excess pressure could run both directions.
But positive pressure aides in oil scavenging.
So lowering positive pressure by default also hinders scavenging.
That's why racers with vacuum pumps use multistage scavenge pumps to account for the imbalance to scavenge and improve it.

Same thing with lowering the oil level in the tank.
You are lowering the amount to be scavenged later which lowers the amount of oil that could get suspended to mist thus lowering the density of what does get suspended.
Lower density of suspended oil drags the wheels less being lighter as it moves.

I've read posts where there was a concern with the plastic tank neck warping which would change the seal condtion of the O-ring.
If you do test your oil tank pressure, could you take pics of the apparatus ?
That'd be helpful to others I'd think.
More data is needed in the form of testing data.
If you pull a vacuum on the tank with a mighty vac to test it, how much vacuum or how much is too much will it take to do a proper test?
That depends on what the engine would pull at max volume... which is ?? volume.

I'm broke right now but I want to do some flow testing on the breathing system when I can so I can examine real data instead of figures out my arse.

All tanks no matter of construction have puked oil from higher pressure with the cap on just like the breathers puke oil.
The oil level isn't getting higher but the pressure above it is.
This causes the pressure to break the seal of the cap or bust the tank and bring oil with it from below.
With the cap off, the pressure is relieved.

Also with the cap off, you are connecting the entire pressure system including the crankcase and rocker boxes to atmosphere.
As far as air pressure is concerned, they are all as one.
You can't have more pressure in the rocker boxes than what's in the cam chest or crankcase.
The air has to back up from the source first and then reciprocate throughout the system.
That is why pulling the cap matters, the air throughout the bottom end including the top will react to the cap being off.

As cause and affect, if the tank is leaking, why wouldn't you also look into the breathing system?
Same as weeping rocker box gaskets. Why wouldn't you add the breathing system into diagnosis with bad gaskets.
I think it's the big picture not being diagnosed is what I'm getting at.
Hence, I agree with carrying on to other tests.

Too hot oil is a symptom but it can also cause other problems as in less residual lubrication and damage thereof to pistons, lifters, bearings etc.
As I mentioned, it can also cause more oil to be suspended, drag the wheels (less power), overpower the breathers (oil puking) etc.
Sometimes it's not as easily detectable before it does damage to an extent either.
I do have these as separate articles in the Sportsterpedia but have brought wetsumping into the CC pressure article as it relates.