TOPIC:

Hi guys.
I performed a leak-down test this evening (thanks Patton) to find where oil is getting into the exhaust, and gas into the crankcase oil.
I removed the cylinder-head cover to get a good listen at the individual valves.
At 30 psi into the plug holes (both), there was a gentle whoosh sound coming out of the crankcase opening by the timing chain (red arrow), and nothing at the valves, so at least my re-seating did the trick.
So: should there be no ingress of air past the piston rings, or is a little bit expected due to the ring splits?
Thanks in advance.

Head Gasket blown or rings stuck/worn. Depends if it is truly coming between cyl and head or blowing down liners and up-sweeping thru crankcase tunnel. How much air/% of loss?

. . . leak-down test . . . gas into the crankcase oil . . . .

I suspect that a leak down test won't help much with regard to diagnosing gasoline intrusion into the crankcase. Which gasoline intrusion can be happening while the bike is parked without the engine running so long as gasoline is able to pass through the petcock.

Gasoline may get into the crankcase when a defective carburetor allows the fuel level to keep rising up past the normal fuel level and on into the carburetor's main bore. This raw gasoline in the carburetor's main bore can flow backward toward the airbox or pods. But may also flow forward into and through the intake port past the valve head and into the cylinder where it then seeps past the piston and rings and on down into the crankcase.

The excessively rising fuel level inside the carburetor may be caused by failure for whatever reason of the fuel valve (operation of the needle and seat and float mechanism) whereby fuel keeps passing through a supposedly closed fuel valve. The excessively rising fuel level inside the carburetor can happen even when the float itself is functioning properly but the float needle is failing to properly fit inside its seat to stop the flow (often due to an unclean needle and/or seat, or a damaged needle and/or seat). The excessively rising fuel level inside the carburetor can also happen when the float isn't functioning properly for whatever reason (such as a leaking float, a sticking float, a damaged or improperly adjusted float tab, a float installed upside own, etc.), whereby the float mechanism fails to close the float valve (and allows the float valve to just stay open), thereby allowing fuel to keep rising inside the carburetor.

Some carburetors are designed with an overflow circuit that -- when functioning correctly -- serves to allow excessively rising fuel to escape by running out underneath the bike onto the ground. When the overflow circuit is obstructed or otherwise failing to properly function, excessively rising raw gasoline keeps rising up past the normal fuel level, and past the overflow intake, and on into the carburetor's main bore.

Some carburetors are designed without an overflow circuit, including manual slide carbs (such as Mukini Smoothbores), as well as some CV (constant velocity) carburetors.

A badly leaking float valve on a carburetor without a funtional overflow circuit can allow raw gasoline to be entering the crankcase even while riding.

Filling the tank with dirty gasoline -- even from a service station -- can result in small particles of crud getting past the petcock and into the fuel line(s), which can obstruct and prevent proper operation of a float valve. Hence a reason to fit good after-market inline fuel filters. I don't allow any gasoline whatever to reach my carbs without first passing through a good after-market inline fuel filter.

A clear tube test should reveal an excessively rising fuel level.

Good Fortune!

Thanks guys.
Asphalt, no actual tester here; I just used Patton's "po-man" testing method.
Patton, thanks for the lead on looking at the carb overflow. The floats must be shot, as gas seeps out the overflows except when running. I was hoping the test would reveal how oil was getting into the cylinders (lots of white smoke), or perhaps indicate why the left cylinder reliably overheats.
Back to the drawing board.

OldguyJoe wrote: . . . The floats must be shot, as gas seeps out the overflows except when running. I was hoping the test would reveal how oil was getting into the cylinders (lots of white smoke), or perhaps indicate why the left cylinder reliably overheats. . . .

While it might possibly be a float issue, in my experience, gas seeping from an overflow is more likely due to a float valve issue rather than due to a float issue.

Sometimes it helps to polish the float valve seat orifice with a q-tip used as a drill bit.
When replacement is needed, it's better when possible to replace the entire float valve (both needle and seat).

Also remember when disassembling and reassembling the carburetor to keep the same float needle with its same seat, and replace the same float in its same carburetor.

White smoke might be due to running with gasoline contaminated crankcase oil (more easily gets past the oil ring and into the cylinder).
White smoke might be due to a worn valve seal (especially noticeable when starting a cold engine, that lessens as the engine attains normal temperature), and exacerbated by the gasoline contaminated crankcase oil.
White smoke might also result from a worn or damaged oil ring.

Is overheating of the left cylinder an issue at idle, or normal cruising, or both?
Overheating can sometimes result from a too lean pilot circuit (for whatever reason).
Does overheating occur only on the left cylinder while idling with a fan blowing directly on the engine?

If not already done, would assure that the valve clearances are within specs,
Results of a compression test and a leak-down test may not hold true when the engine is running at normal operating temperature.

Good Fortune!

To add to Patton's extensive replies.. The vacuum petcock should shut off the fuel when the engine is not running. I polished my seats, the o-ring bores and the o-ring bores in the petcock to make sure they all seal as they should. If I remember right, I had to change the diaphragm o-ring in the rebuild kit, since it is a K&L aftermarket kit that is always close, but no cigar type stuff.

Thanks guys for the ideas.
Carb rebuild is in my near future. Will polish the float valve seats first.
The gas tank fuel cock shuts off fine, it's just that when I switch on fuel before starting, gas immediately waterfalls out the overflow nipples.
Overheat in left cylinder occurs within 4-5 minutes at all throttle settings. It was a crisp and breezy -10° outside the last time it ran and overheated, so there's plenty of cooling going on. Low-speed idle needles are opened out 3 turns to get lots of gas in there.
I already honed the cylinder walls during the top-end rebuild, and lapped the left-side valves while I was at it. I'll tear it back down and lap all 4, replacing the valve-shaft oil seals while I'm at it. Sound reasonable? I'll get shopping on Z1.

Edit:
Upon carb teardown, left carb high-speed port was thoroughly clogged. That'll learn me for not being more thorough in the first place. Hopefully this fixes the overheat problem.
Have a few carb and valve seals on order. Further updates as events warrant.