Oil Level?

I've done a little searching of this forum on this topic, but so far haven't found a whole lot. My MK III sometimes smokes a fair bit from the left exhaust for a short time after start-up; guess I need to get a compression tester to make a first assessment (Harbor Freight, here I come). I'm also wondering if it could have something to do with the fact that the oil drains out of the oil tank into the crankcase when the bike sits overnight or longer? A local Brit-bike shop has told me that Commandos tend to be prone to that condition, and that it means the oil pump is leaking and needs replacement. I also see that in the INOA Tech Digest there is a discussion about "Wet Sumping" (p 1-25), where it suggests a fix involving drilling out a couple holes in the timing cover and inserting spring loaded ball bearings in those holes to function as check valves. Any other thoughts?

oediehl said:

I also see that in the INOA Tech Digest there is a discussion about "Wet Sumping" (p 1-25), where it suggests a fix involving drilling out a couple holes in the timing cover and inserting spring loaded ball bearings in those holes to function as check valves.

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'Hole' not "holes" for a check (ball) valve, but a Mk3 should already have an anti-drain check valve (a spring loaded plunger, not a ball) in the timing cover.

at this point early in diagnosis I would ignore the wet sumping fix the oil pump outlook and pursue the faster and easier ways to find the smoking problem

get a decent compression gauge and read up on how to do it with carb slides open and a couple good kicks
then read up on how to do a "leak down" test to further find out if it could be a ring, stuck valve,guide, etc problem

something is letting in oil into the left side combustion chamber that is burned as smoke on startup..

If you leave your bike parked on he side stand maybe oil is following gravity and pooling a bit in the left cylinder. That said a good compression and leakdown test will prove invaluable.

If ALL the oil actually drained into the crankcase overnight, I suppose that could cause some oil burning on an initial start due to all the excess oil in the crankcase being tossed around by the crankshaft and over-lubing the cylinder walls, some of which could migrate into the combustion chamber. One cylinder could be more prone to it than the other depending on how, exactly the oil gets thrown onto the walls and/or on the condition of the piston ring/wall seal in that cylinder.Once the sump was drained to the "normal" level by the normal operation, there would be no excess oil to be tossed around by the spinning crankshaft.

OTOH, I cannot imagine the oil pump clearances being so bad as to allow that to happen overnight. On my Commando, it takes a nearly three weeks of sitting to drain the tank into the sump. Admittedly, when I first received the bike it would drain the tank in 4-5 days. But attending to the pump as per the Norton service manual made a big difference, reducing the rate to its current amount.

I personally would never consider adding a check valve to eliminate wet sumping, feeling that the cure is far worse than the disease but, as you can tell from searching/reading posts on the subject, there are strong views both ways.

But I agree that the first thing to look at relates to the usual suspects for oil burning - valve guides/seals and piston rings.

Simple test:

Run the bike then let the bike sit overnight. The next morning drain the crankcase oil and pour it back into the oil tank. You now know how much the bike sumps overnight. Now start the bike once you've finished draining the sump. If it blows smoke on the left cylinder you know it isn't because of sumping, because you drained the sump.

o0norton0o said:

Simple test:

Run the bike then let the bike sit overnight. The next morning drain the crankcase oil and pour it back into the oil tank. You now know how much the bike sumps overnight. Now start the bike once you've finished draining the sump. If it blows smoke on the left cylinder you know it isn't because of sumping, because you drained the sump.

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The above is the best advice.

The valve in my MK III would occasionally stick and allow it to wet sump which created smoke on start-up. The spring is very light and a very tiny piece of carbon can cause it to stick.

When I bought my bike the valve and spring were missing. Perhaps some one thought it shouldn't have one or it simply fell out and, since previous models didn't have one, it just didn't get put back.

Searching this topic you'll read plenty of good advice NOT to put a valve, automatic or manual in the supply line; heed it! Fix the stock valve as it works.

My MK111 did the same thing. Pulled the head to find an intake valve seal hard as rock and riding up and down with the valve stem. Replaced both seals with Kibblewhite teflon type.