The engine is a V-twin with a dry sump oil system.  The engine crankcase is basically sealed
and runs at between 5 and 10 inches of vacuum generated by the scavange pumps. 
The breathing to atmosphere is done at the remoted oil tank.  The tank is vented
to atmosphere through the filler cap. We are restricted by the fact that the oil
tank on these motorcycles are only approximately 3 qts capacity and we are only
running 2 qts of oil in order not to overflow the tank during operation.

This basic setup works well in all but sudden high rpm throttle bursts.  At
those instances, the sudden flow of oil from the scavange back to the tank
causes oil to come out of the vent in the tank.

What solutions are out there for proper venting of oil tanks? Is an air/oil separator the solution?  We are considering regulating the flow from the scavange to the tank.  ]

Back in my days at Harley, the biggest oil leak problems were simply due to overfilling the oil reservoir tank. This led to oil migrating out the breather tube to the air filter when expansion (heat and aeration) raised the level in the tank. "It's gonna leak anyway so I might as well add more oil" was usually the root culprit. The FXR got a relocated vent tube and a change to the other end so vapor was sucked directly into the carb, along with an oil capacity label on the tank. Dyna series solved most of the issues by putting the oil beneath the powertrain where it belongs.

Consider a balance line between the tank and the engine. This can be jetted to maintain partial crankcase vacuum.

Otherwise - as Pat said - the return line shoud inject the oil into the tank against a curved wall. This helps remove some of the air from the oil (deaeration) which keeps the volume lower.

TESTING: Make a polycarbonate window in the tank so you can watch what happens. If you can't duplicate it on the dyno, rig up a helmet camera for the test track. We had windows in the primary cover to watch the wet clutch oil patterns and the aeration change was spectacular.

Timing has an awful lot to do with the outcome of a rain dance.

Check your check valve. 1 psi may not seem like a lot but you need to find actual cracking pressure under operating conditions (heat, vibration). You may also find that after time the chk valve could freeze open or closed due to contamination, sludge, etc. It sounds like you are using this line to vent pressure spikes from c'case and still maintain c'case vacuum.

Somewhere in any oil system is a vent to atmosphere - typically going into the intake (carb/inj) manifold to burn blowby hydrocarbons. This usually creates a slightly lower pressure for the vent.  Without this, all the pressure must go into the tank, then to atmosphere. There may be a much greater flow of air than you may have allowed for - thus the velocity is pulling oil with it and exiting the oil cap. Try running a 1/2" hose from the very top of the oil tank up as high as you can get it. This way oil will have a chance to drop out of suspension and flow back into the tank. Use a small in-line gas filter at the terminal end to keep dirt out.  I'm guessing this is a race application since you are free venting.

Remember that scavenge pumps are positive displacement so if you have a 3 gpm pressure feed and (2) 3 gpm scavenge sections, you have to contend with 6 gpm total flow, 3 of which will be oil mist. Unless this is redirected to the c'case, that's what you have to vent off. Using an air balance line with a jet inside will let you pull some of this air volume back into the system while still maintaining negative c'case pressure.

Timing has an awful lot to do with the outcome of a rain dance.

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