help narrowing down slight oil leak into left cylinder

tracing what I think is an oil leak into the left pot, she smokes slightly at idle only, and I've eliminated fuel/ignition issues. I've got the head off right now, and the copper head gasket appeared dry, but I know that can be deceiving, so I'm going to try a flame ring type this time around.

- top end checks: valve seals in place, petrol poured into combustion chamber, intake and exhaust ports, as well as into the intake manifolds show no leaks from the valves... this leads me to suspect the head gasket

- bottom end checks: compression tests show even on both cylinders, hot and cold (100 cold, 180 hot). While the head's been off, I've poured a couple ounces of 30w into each jug and let that sit for 2 days now, no discernable drop in level. She wasn't consuming oil when she was running, and nothing accumulated in the catch bottle I have the oil tank breather running into.

I'm wondering though if one of the rings on the problem side may be installed upside down, but would think this would produce a lot of smoke at all throttle openings, as well as excessive blow by... am I wrong here?

Smoking under power is generally piston rings, smoking at idle is usually the intake valve seal or guide. It's most noticeable when closing the throttle and coasting - that's when intake vacuum is highest.

Head gasket leaks between the pushrod tunnel and cylinder can cause smoke too, but I think you'd have other symptoms too.

Compression check is not that good for your situation and I would not normally bother. The leak down check removes the cam dynamics from the testing, it is to test your sealing surfaces and when skillfully used is more appropriate to find or eliminate possible path for your oil leak. It will not necessarily be the only test required.
Do you have a 850? Do you have a cracked RH4 head?

dynodave said:

Compression check is not that good for your situation and I would not normally bother. The leak down check removes the cam dynamics from the testing, it is to test your sealing surfaces and when skillfully used is more appropriate to find or eliminate possible path for your oil leak. It will not necessarily be the only test required.
Do you have a 850? Do you have a cracked RH4 head?

Click to expand...

71 750, have not pressure tested head, so otherwise unsure whether I have a crack in it. Comp test is all I could muster at the moment as I have no access to a leakdown tester.

The cylinder bores look good, can still see some cross hatch and no signs of scoring, so I was wondering if I did indeed install a ring upside down (don't think I did, as I was pretty careful.. but anything is possible), how prominent would the smoking be at idle as she doesn't smoke or foul plugs at any throttle opening above 1/4

My head builder is convinced I have a head gasket leak, due to the copper one I used. But I'm not even convinced entirely it's oil fouling the plugs... but new premier carbs, and no amount float or pilot jet fiddling cures the problem on that pot... (as noted, electrical has been ruled out)

A leaky head gasket doesn't make smoke on an air cooled engine. Piston rings will cause smoke under load. Intake valve seals and/or guides cause smoke at idle. That's where you should be looking.

Barrels off, middle ring gap measures .014, so out of spec... top ring still at .011

luckily I have a fresh set of rings on standby and will replace the middle ring

acadian said:

Barrels off, middle ring gap measures .014, so out of spec... top ring still at .011

luckily I have a fresh set of rings on standby and will replace the middle ring

Click to expand...

You might want to read this, theory is the 2nd ring gap is about twice as wide as the top ring gap.

https://www.motorstate.com/TheGreatPistonRingControversy.htm

acadian said:

Barrels off, middle ring gap measures .014, so out of spec... top ring still at .011

Click to expand...

Slightly worn but not out of spec. The dimensions given in the manual should be regarded as the minimum gap for new rings in new bores. Rings can wear to 0.020" - 0.030"+ without problems.

Something else to read:
http://www.diagnosticengineers.org/jour ... 20Gaps.php

I had the same problem in the early 80s after a complete engine rebuild oil was leaking out of the exhaust port, what had happened was the guy that rebuilt the head ( experanced with JAP bike heads only) I am not sure but think when he pushed the new guides in maybe didn't heat the head up or something like that but after a few weeks the leaking got worst, found 2 crackes beside the guides, he ended up getting the head welded but I didn't let him rebuild the head again, at this time I started working at a Tec College (TAFE) where one of the motorcycle teachers was a ex Norton race team mechanic in NZ and knew his stuff he rebuilt the head ported the repairs done to the head and never had the problem again and after 40 years I am still running the stock valves that came with my bike, the head has been rebuilt over the years mostly for more performance.

Ashley