These HF copper washers are .90mm thick. However, last night I had the lower banjo nut and spring off overnight, with only the ball in the hole, and it didn't leak out. The oil tank is full as well. I'll bet if I removed the oil tank cap, then it might have leaked out.

You must not realize that the oil tank is vented.
If oil will only flow when you remove the cap, how would the pump get any oil?

Deet, I know that the oil tank is vented. I'm just saying that when I had the banjo fitting off the pump, with only the ball bearing in the hole and no spring holding it down, it did not leak overnight (I had a pan below it just in case). However, when I removed the prime screw, oil did flow out from gravity, but when I put the prime screw back in, it still did not leak where the banjo fitting goes. Apparently just the weight of the ball bearing, and the viscosity of the oil, was enough to stop it from leaking. What I was saying was that if I removed the tank cap that would have greatly increased the ventilation, then perhaps it would have overcome the ball bearing and started leaking. Or, maybe it still wouldn't have leaked?

As far as the thickness of these HF copper washers, they are 0.9 mm, versus what you say is 0.5 mm for the Yamaha copper washers. With two washers that's a combined increase of 0.8 mm. Do you think that's really enough to cause spring not to be effective? When the spring is sitting on the ball bearing without the banjo fitting, it protrudes from the hole about a quarter inch. The shoulder inside the banjo screw that stops the spring is actually below the pump hole opening when the screw is installed, so I think there should still be some spring pressure on that ball bearing, even with these thicker washers.

“ What I was saying was that if I removed the tank cap that would have greatly increased the ventilation”
The atmospheric pressure differential is the same for the vented tank whether the cap is on or off.

I won’t speculate on the suitability of your washers. My experience says use the thin stock ones.
If it didn’t matter, why would Yamaha go to the trouble of matching the original thickness?

"The atmospheric pressure differential is the same for the vented tank whether the cap is on or off."

Then assuming the vent tube is not clogged, which is something I'm going to check today, it probably would not have leaked with the tank cap off either, and just the ball bearing alone in the outlet hole.

Deet,  I completely respect the fact that you only use OEM-spec parts, but I'll live dangerously and use these washers. If something negative happens, I'll post it here so people can learn from my mistake  :o)

Edit: I checked the vent tube and it's clear. Also ran a wire through the loop coming out of the top of the tank and it's clear as well.

I'm half tempted to calculate the area of the oil that the check ball is exposed to and using the height and weight of the oil and the weight of the check ball (factoring in the angle from vertical) to see if gravity and a full tank of oil applies enough force to lift the check ball from its seat without a spring. 

I've never used "aftermarket" sealing washers...  I've only used Yamaha OEM washers on the banjo.  (Most of my bikes have the later "press in" pump outlet fitting) I'd like to say that this is because I was smart enough to think of I thought of the check valve spring preload reduction caused if the washers were thicker...  but I never even thought about it!

Thanks DEET!  You made a good point and definitely something to consider!

I just thought of an alternate and/or additional reason for the spring. Perhaps it's not as much to hold the bearing tight to the bottom, as it is to keep the bearing from flowing up and shutting off oil to the cylinder? I have no idea how to calculate the pressure exerted on the bearing by the tiny opening it rests on, the amount of oil in the tank, and the tank height above the pump, but that would be interesting to know.

"I'm half tempted to calculate the area of the oil that the check ball is exposed to and using the height and weight of the oil and the weight of the check ball (factoring in the angle from vertical) to see if gravity and a full tank of oil applies enough force to lift the check ball from its seat without a spring"

Remember.... the way that the pump works is that the shaft intake hole aligns with the supply port... it sucks in oil,  then rotates to the discharge port and sends the oil towards the pump outlet.

When the pump is at rest, the oil finds it way from the tank AROUND the outside of the shaft and then towards the outlet.
THEREFORE.... the greater the clearance between the shaft and the body, then the easier it is for gravity to flow oil through the pump.
This is also why some pumps do not pump their design capacity.
The shaft and body wear.... then the discharge takes the path of least resistance: back around the shaft.


I see folks all the time reducing the complexity of these pumps down to a nut and bolt.
There is so much more to them and even with all the hundreds of pumps I have restored, I feel like I have just scratched the surface in understanding them.
 

".......I have no idea how to calculate the pressure exerted on the bearing by the tiny opening it rests on, the amount of oil in the tank, and the tank height above the pump, but that would be interesting to know."

It's not that hard.  The static pressure is only dependent on the height of the oil in the tank above the outlet fitting.  If you know the specific gravity of the oil, which might be on the oil spec sheet, it's pretty easy to calculate the pressure. 

And yes, the only factor is the height difference.  A one quart tank or a one hundred gallon tank with the level 12 inches higher than the pump outlet will exert the same pressure at the outlet.
Here's a calculator:
www.sensorsone.com/liquid-level-to-pressure-calculator/

Then you need to know the area of the ball the oil is pressing against. Then you just multiply the area (in square inches if using psi) by the pressure created by the oil level to get the pressure (in pounds, or actually a very small fraction of a pound) and compare it to the weight of the ball. 

Of course if the hole the ball is in is not straight up and down to allow gravity to push straight down on the ball, you'd have to factor in the angle to see how much of the weight of the ball is sealing the outlet hole.  Obviously if the hole was parallel to the ground the ball would just roll out of the way with the slightest pressure. 

And of course the above can all be done in metric units of measure or whatever system you're comfortable with. 

If it's not one leak, it's another 

I've put over 40 miles in it since putting the HF copper washers on the banjo fitting, and now no 2 stroke oil is leaking. HOWEVER now I've noticed a black streak of oil, and a very small pooling below. I've wiped it out several times and rode, then it came back. This is from the latest ride. I replaced the inside seal a year ago, and have a good, intact gasket between the pump and case, but have not used any sealant on it. I also bought 2 new mounting screws to have standard #2 Phillips heads, and can tighten it better now.

The trans oil is very clean, so I'm a bit confused why this streak is black. I'm also wondering if this could be mixing with the 2 stoke oil inside the pump. When I undo the cylinder banjo fitting and pump oil through, it looks like clean 2 stroke oil. Any ideas? Did that seal fail and/or should I use sealant on the gasket?