Crankcase vent question.

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10drum

10drum

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Greetings,

I have not be into my 72 interstate engine. The engine is a combat, has the crankcase vent on the back of the engine.
As we all know these bikes wet sump if allowed to sit for any length of time. After a few days of setting the oil level drops about a 1/4" on the dip stick. I start the engine, then shortly the oil is being forced back into the tank.

I assume it is just the crankcase pressure that is forcing the oil back to the tank? With that being said, is there a point to where there is too much oil in the crankcase and not enough pressure to force the oil back to the tank?

I've owned the bike a couple of years, and have drained the oil when riding season is over both years.

I'm not going to install anti-wetsump check valve, I'm just curious how long it could sit without issue??

Thanks for all replies.
 
You will receive many more educated answers than mine as for oil level in the crank case. My bike sumps pretty good and still has great oil pressure and return. The oil will just sit in the case and keep your bottom end and cam properly lubricated. If you have to much down there and start it, you have a good chance of oil coming out the crank seal and filling your primary case up. If it’s sits for a long time and your level is low you can drain the case some and put back into your tank.

Others will chime in soon.
 
Any oil coming up the breather is from crankcase pressure but at the same time the pump will also be returning excess oil as its return gears have approx double the capacity of the feed gears. Once the sump is cleared the return side of the pump will be a mixture of oil and air.
 
A reed valve breather to replace your existing breather is a much better investment than the Anti sump valve.
A couple of primer kicks will clear the sump, then run it.
Reed valve also creates a negative pressure and lessens oil leaks. Win, Win, WIN...

coloradonortonworks.net

1972 Engine Breather Modification

Experience an increase in engine performance Eliminate persistent leaks at gaskets, seals, tach drives, head, crank case joints and more Lower the operating temperature of the engine oil The valve can operate consistently during a far higher rpm range than the Commando engine can ever reach The...
coloradonortonworks.net coloradonortonworks.net
 
I put a read valve breather on mine and it works great! about 95% of my leaks cleared up.
 
nopdog said:
Sorry to hijack but which breather fits a 71.
Click to expand...

If you mean the Jim Comstock type, they will fit the 1971 frame as the crossmember is not in the way for the sump mounted version.
That version will not fit a pre 1971 frame. with the lower frame crossmember more rearward and in the way.

Edit.
The 1971 20M3S engine has nothing to get in the way including internally but some 850 cases have more material on the divider that the tower can hit (The later version probably has a shorter tube)
I elected to machine a relief channel with a 12 mm ball nose cutter to retain the tube height after a PM with JC.

Crankcase vent question.
Crankcase vent question.
 
Last edited:
This is a 72 so no large sump plug for a breather sadly as it is the best solution, so instead the easiest way is to fit an XS650 type reed valve in the current breather pipe as close to the cases as you can get. You can get them off eBay by searching for EGR valve in motorcycle parts. Go for a one with the inlet and outlet at 180 degrees for a 72 so it can fit in a straight run of pipe.
 
These are Maney rather than '72 cases, but the original Comstock designed CNW breather fits right there. Maney provided for the two optional popular breather locations.

.....and in my opinion this is far better than the XS650 style of reed simply because it is located at the oil pick up and does the sump clearance job as well as breathing.


Crankcase vent question.
 
"I assume it is just the crankcase pressure that is forcing the oil back to the tank?"
No, that would be your scavenge pump, working as designed.
 
Mart UK said:
That's a non-standard set up? On my mk3, no oil cooler and the oil filter is in the return route.

I added the xs650 valve and it's the best mod I've done and cheap. The engine sounds much happier and no oil leaks (primary still leaks though!).
Click to expand...
An even cheaper method is/was to use a 1/2" or 3/8" pneumatic or steam check valve. The best type to use are fitted with a steel ball, just remove the spring if fitted and mount it vertically and let gravity seat the ball. I fitted these to my Triumphs etc in the mid 60s. The first one I fitted to a Commando was in the early 80s to a Mk1 850 and fitted it in the tank breather hose. The theory being to equalise the pressure between the crank cases and oil tank ie not to restrict scavenging. However, I currently have installed an SX650 Valve fitted and in the crank cases to tank hose as everyone else.
 
I bought two of the JC sump located units but had considered machining one for the 20M3S using Boyesen reeds located at the timing side and replacing the removable plate.

I used a scooter sourced reed unit on this one but like a Commando anything was better than the stock arrangement. (A small disc and spring)

Crankcase vent question.

Motorcycle engines were light years behind the (racing) automotive world when it came to crankcase evacuation (and windage control on wet sump engines)

You might wonder what a vacuum in a Commando does to the scavenge side of things, maybe there is an improvement with the oil coming off the rotating parts settling to the case floor (or not)
 
Mart UK said:
That's a non-standard set up? On my mk3, no oil cooler and the oil filter is in the return route.

I added the xs650 valve and it's the best mod I've done and cheap. The engine sounds much happier and no oil leaks (primary still leaks though!).
Click to expand...
That was a generic dry sump schematic I found, to show basic architecture, regarding your quoted assumption. It shows the scavenge pump inherent to any dry sump system.
Crankcase vent question.
 
Where does one get an XS650 reed valve? From Yamaha? Aftermarket? Do you have a part number? I've been using a VW Rabbit power brake booster check valve for 20 yrs or so on several bikes. Recently I sent a friend to the NAPA store for one for his Ducati single and he came back empty handed. Perhaps he didn't know how to coax the counterman to look under vintage volkswagen, I don't know. The XS650 valve looks like a better solution anyhow.
 
Thomasa said:
Where does one get an XS650 reed valve?
Click to expand...

www.mikesxs.net

PCV Reed Valve - Crankcase Breather

Color/Finish: Silver Material: Metal Dimensions: Overall Length: 86.50mm Width (at widest point): 28.85mm Engine Inlet OD: 14.15mm Engine Inlet 11.85mm Filter (Air) Outlet OD: 12.00mm Filter (Air) Outlet ID: 9.45mm Fits: Universal Note: Easy Installation (Linked Below)
www.mikesxs.net www.mikesxs.net
 
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