OK....I see the two sides now.

Vacuum is in inches of mercury - pressure side is in pounds.

So.....if it is 0.5 psi he read on the the gauge.........that equates to something around 1" of mercury.  The end result is he reduced the crankcase pressure by 1.5"....but more importantly he went from a pressure to a slight vacuum.

I guess DBM needs to set us straight - he might have already done the correction.

I wanted to get a better feel for how much vacuum this thing might pull.  The gasket failure didn't seem consistent with such a minute pressure differential.  So I cleaned up the area with mineral spirits and smashed a big mound of modeling clay over the leak zone.  I really piled on the clay to make sure its was sealed good.  I only tested it with the KrankVent installed because I figured vacuum will hold the clay dam in place, but pressure will blow it off.  With the leak plugged, it pulled 1.0" Hg vacuum.

These tests were done in my garage.  I just started it up and ran the engine unloaded up to about 5000 rpm.  I never rode it with the vacuum gage in place.  My intent was to ride it, but I found the leak.  That shut the test down.  For all I know, the blown gasket could simply be a coincidence.  The missing chunk of gasket could be inside the engine, or it could have blown out of the engine.  I wont know until I take it apart.

Regarding crankcase vacuum and oil pump suction.  Yes, I agree, the crankcase ambient pressure on both sides (suction & discharge) should be the same, so the DP across the pump should not be affected.  But this little pump doesn't appear to have any suction head at rest, and when the engine is operating, it looks to me like it will go into suction lift because the oil level in the sump will go down.  The temperature of the pumpage is elevated, possibly as high as 300 degrees.  My concern over pump suction was related to flashing.  If the contraption pulled too much vacuum it could reach the saturation pressure of the liquid.  I mentioned that it seemed like an unlikely problem, and now that I have a better feel for what sort of vacuum we are dealing with, I'm very confident that pump suction wont be a problem.  The oil pressure gage backs that assumption up.

The stuff I've been reading indicates that there are oiling problems associated with crankcase vacuum.  Wrist pins and exhaust valve guides can run dry.  But those problems don't come into play until crankcase vacuum approaches 10" Hg.  This thing isn't anywhere close to that.

Thanks for the suggestion Dave.  Actually, that pretty much is my style of workmanship when it comes to troubleshooting.  As long as the blob doesn't screw up the surface cosmetically.  I did consider covering it with silicone, but the leak was going to get repaired and I didn't want to wait a day for the silicone to cure.  The engine is currently on the deck and ready for disassembly.  Copper base gaskets are on order.  I might regret not trying to test further, but its water under the bridge now.  

So let's say I get it apart and fix the base gasket.  Where's the next weak link.  I'm gonna say "alternator cover gasket".  Where can I get a copper gasket for that?  After that, I predict that the next weak link would be the output shaft seal (might start to run dry).  Is the minor improvement (perceived) worth all the trouble?  Of course, if you learn a bunch of good stuff in the process.  

Hey Armen, is this what you are looking for?  It doesn't even require welding.  It's aviation grade man.  

https://antisplataero.com/products/crankcase-vacuum-kit-complete

DBM

The reason I think you can't pull a lot of vacuum with a check valve, is the breather port is pretty small compared to the size of the piston.  As the piston comes down and the crankcase pressurizes - only a small portion of that squeezed air can get past the wire screen scrubby thingy and out the hose to the other side of the check valve.....and there is only a tiny fraction of a second before the piston starts back up and removes the pressure and begins to create the vacuum......and whatever blow by comes down the cylinder/piston/ring assembly is adding more volume to the air.

Without the check valve - I believe there is actually very little exchange of air going in and out the breather, as the small hole doesn't allow much air to rush in or out.....and there is just a constant +/- pressure pulse going on inside the engine - the vent only is bleeding off the increase that is the piston blow by caused by the combustion process.

I think it is pretty amazing that you are able to create any crankcase vacuum with a check valve - and I wonder if you will have any vacuum at all with the engine at full throttle and high rpm.....under a heavy load there may be so much blow by that the crankcase ends up being pressurized - and I wonder what the crankcase pressure is when the engine is at "full chat"?

As an example.....I was having some problem with an oil leak that was really difficult to find.  My bike would stay clean and shiny....but every now and then I would find oil up on the front of the fuel tank on the right side.  I thought maybe it was a fork seal - but they were fine and I never had any oil residue on the sliders.  I thought maybe the brake master cylinder was leaking.....nope.  Eventually I found that is was the O-ring down on the left side of the engine where the speedometer drive inserts into the engine case.  It only leaked when I would accelerate under full throttle - and I can ride for a whole month without running through the gears at full throttle.....I find riding around curves more fun than full throttle drag races.  When the oil came out around the speedometer drive, the swirling wind would carry the oil upward and forward and deposit it on the front of the tank....it was really a surprise to find the oil drops were carried so far forward without leaving a mess down by the source.  I surmised that the only reason it leaked was the crankcase pressure increased when I was at full throttle and pushed oil past the seal.  When I replaced the O-ring on the speedometer drive - the leak went away.  The piston is a 95mm Wiseco that was bored and received a silicone carbide treatment by Boretech, and it doesn't burn any oil and has good compression - so I don't believe my engine has any unusual amount of blow by.  This engine is notorious for sending oil out the breather when operated at sustained high rpm on the interstate - so I think it is somehow a "special feature" of this engine.

Here's a shot of the top side of the gasket.

I cleaned up the old gasket material and installed the cylinder.  It mates up nicely with the crankcase.  I can't insert a .001" feeler, and if I lift the cylinder slightly and insert the feeler, when I rest the cylinder back down I can't pull the feeler out.  

I don't think the blow out is related to a bad sealing surface or gasket.  But I also find it hard to accept that the whimpy vacuum this thing pulls was enough to suck the gasket into the drain.

It's a nuclear phenomenon.

When I get it all back together with a nice copper base gasket, I will test it some more and see what happens.

I ordered the cylinder base gasket from Copper Gaskets Unlimited, Phoenix, AZ.  I sent him a new OEM gasket to use as a template.  He makes a CAD drawing and then uses the drawing in his equipment (I assume some sort of laser cutter).  So he will have the LS650 gasket CAD file and it will be easy to order from him in the future.  A variety of thicknesses are available so it will be easy to adjust deck height.  The stock gasket is .018".

coppergasketsunlimited@yahoo.com