Mk III Anti Wet Sump Valve

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Dec 15, 2006
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Another question related to the "Modern Commando". It apears that the Anti Wet Sump valve on my Mk III is not functioning properly. Sitting overnight (24 hours) the oil level on the dip stick went from 2/3 up between the low & hi marks, to below the low mark. I recently had the timing cover off for a cam chain adjustment & found the plunger & spring present in their cavity. I cleaned these parts and carefully inspected the plunger for scoring and free movement in the bore. One thing I noticed was that the spring seemed pretty weak, and that it's free length apeared a bit too short. I reassembled the timing cover to the engine using the existing parts, with the exception of a new rubber seal (Mk III specific) on the oil pump outlet.

I've never been one to remember to park the machine with the pistons at TDC. And if the valve were functioning correctly this task should make no difference.

Any insight would be appreciated - thanks for your time!
The anti-wet sump valve on a MkIII was a feeble attempt to solve the problem. They often hang up in the bore and never seem to quite seal properly.

A well set up oil pump with a gasket and a fresh pump to cover seal will greatly lessen the wet sumping to the point that it won't appreciably wet sump for up to two weeks standing. One of the inline valves (manual or automatic) will do better. Manual valves scare me because it only takes once to forget to open it. The automatic valves are better as long as they don't create too much of an obstruction to returning the oil to the tank, but yet seal well enough to prevent drainback.

I just have gotten used to draining the crankcase before starting my Nortons if they have sat for couple weeks without starting. As long as there is some oil in the tank, it will not run dry and starve the engine if you start it and let the scavenge side of the pump clear the crankcase.
Ron L,
Thanks for your reply. I agree with you; having been afflicted with "Sometimer's Disease" myself, it would just be a matter of time until I'd forget to turn on the valve. I believe the source of oil that feeds the wet sump condition is through the supply line, from the tank to the pump vice the return, and all automatic valves I've seen are installed in the feed line. But installing a check valve upstream from a positive displacement pump is just asking for a problem. I understand that a new anti wet sump valve is available that has a clear portion incorporated in the body so you could theoretically verify flow visually. I'm still not going for it.

It seems to me that perhaps both problems you mentioned with the Norton design could be overcome with a bit stronger and/or longer spring, although it may pose a problem during assembly. I doubt that I would experiment with that concept until I verified that the pump clearances were tight, especially on the pressure side. I've watched the oil pressure on my 750 drop to near zero while at Interstate speeds for long periods. A loose pump working with hot oil may not develop enough pressure to un-seat too strong of a spring.

Appreciate your time,

Think your desision might be right...if you forget things...they will indeed bite you. I'm afficted with the same disease you mention. Mine includes forgetting to turn off turn signals...deadly at an intersection, so I have a nasty, annoying buzzer under the headlite. Now I just use hand signals if no cars are in sight in town...buzzing is gastly stuff. I won't use an inline valve either, same reasons as yourself. But.....

last week, the "sometime" thing really did it's thing...scary. I drive a Vespa T4, 1961, too...good for short trips and fun to drive, agile as heck. I rarely lock it...too lazy...and it's is usually within sight anyway. But I went into a factory to speak to someone...not about to leave it unlocked out there in the parking I remove the key and put it in the pocket. Out I come ten minutes later...helmet on...kick her over and take off. About 5 miles later, other end of town...a birdy starts to chirp real loud in my ear...Key...Key...Key... Oh Poop! Pull over and get the key out of the pocket and unlock the forks. That little baby will lock up the steering when you go to a full lock with the bars, left or right. I had made it across town in heavy traffic, somehow, without doing that...and that's the only reason I didn't have it lock up and flip me across the street and into some building. Gave me a good case of the cold sweats. That is the third time that has happened over the last few years...vehicle inspector did it to me one time too...test rode it and gave it back to me with it locked...thought it was the ignition key... :D indeed listen to your will have the best advise.. :wink:
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