unusual oil leak... HELP!

don't think it is a combat, how would you know,being a Dunstall the head is stamped "M" modified where a combat is stamped with a "C". With engine number being 210105 it may be in a series where ther wet sumping may be an issue.

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I was using 'Combat style' as a general term for '72 model year bikes with the rear breather and front oil pickup/return. Having a high comp head and hot cam is not likely to be contributing to your problem, not this way.
Assuming the cases are being pressurised (the leaks) something is not working properly. Have you tried running the engine and watching the oil tank and breather return outlets? When you start up and with oil cap off there should be a good deal of 'chuffing' for a few seconds until the breather has done it's initial job and evacuated the cases. If you then block the breather inlet at the filler tower with a finger you should only feel pressure and no suction. That should tell you if the breather is at least working. I'm also assuming that your case joints are relatively good and that it's only extra pressure that's causing the problem.
However there might be conflict between both outlets into the Yam valve (not sure of the fluid and pressure dynamics here as I'm not in the Jim Comstock league!). But, if the rear breather is passing a lot of oil during a ride then it's clear it's not being scavenged by the pump. It may also be that this oil flow does not allow the valve to shut properly (floating on a stream of oil ?- just a guess really).
Whatever is going on or not you have to take note of this engine year oil pickup problems. It is documented though not as clearly as it should be even if some owners don't have a problem with the standard setup. But, why would Norton revert to the rear pickup for 1973 and beyond? The fact that you mention 5000rpm rides and have lots of oil going up the rear pipe kind of confirms that.
I know this doesn't really help but until you solve the root cause of all that oil sitting in the cases no one way breather will help that. I use the Norvil valve which is just a spring loaded nylon poppet in a brass housing. I'm told that works with the rear breather OK, well, Norvil sell them with fittings for those engines....doesn't mean to say it works!!
If I were you and don't want to strip the engine during riding weather I'd replace the old breather, block off the timing breather and go back to standard. At least you know there's oil coming up that big pipe. That's unless it was even worse than before you added the Yam valve, I have to guess it wasn't?
Finally, how is the oil tank vented? To a bottle, open pipe or to one of the air filters?

First I would like to thank all of you for your input to my problem with this odd oil leak, upon closer inspection I find that the oil is in fact comeing from the base so close to the case bolt it was hard to see with my ageing eyes to see. :x So now the engine comes out and I will look for those wet sumpping issues as well. Thanks again

Keith1069 said:

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I know this doesn't really help but until you solve the root cause of all that oil sitting in the cases no one way breather will help that. I use the Norvil valve which is just a spring loaded nylon poppet in a brass housing. I'm told that works with the rear breather OK, well, Norvil sell them with fittings for those engines....doesn't mean to say it works!!
If I were you and don't want to strip the engine during riding weather I'd replace the old breather, block off the timing breather and go back to standard. At least you know there's oil coming up that big pipe. That's unless it was even worse than before you added the Yam valve, I have to guess it wasn't?
Finally, how is the oil tank vented? To a bottle, open pipe or to one of the air filters?

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My guess although Nortasaki has found the leak, is that with the standard rear breather teed onto the new timing case breather as per the 850 a lot of oil is being pumped up the rear breather (As it is known to do) then because of the flow restriction caused by the XS650 valve it is draining down into the timing cover hence the 'hot' timing cover mentioned in one of the first posts.I am sure he mentioned a cool filter and hot timing cover so best guess is most of the oil is going from crankcase to rear breather up the pipe then back into the timing case so empty oil tank..does this make sense to anyone :?

I am sure he mentioned a cool filter and hot timing cover so best guess is most of the oil is going from crankcase to rear breather up the pipe then back into the timing case so empty oil tank..does this make sense to anyone

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I thought that after I posted but then with the kind of pressures that he must be developing in the cases why doesn't the Yam valve open? It does make sense though. A flow tee might help ('Y' piece) to direct the oil etc.

The crankcase joint behind the cylinder is a notorious leaky area on the '72s. I used to get this on mine (202093) but it stopped completely after putting the $3.98 HELP! brake booster valve in the breather line where the hose diameter changes near the battery. The XS650 valve should do the same thing maybe more elegantly. I still have the stock '72 setup with the forward oil pickup and rear crankcase breather. But I don't normally run over 5000 rpm for long periods either.

I think it is a great diagnosis that the breather oil is diverting to the timing case with that tee arrangement. The pressure pulse from the pistons going down is taking the path of least resistance and blowing into the timing case space rather than causing the valve to open.

Also it might be good sometime to examine the oil pump gears and check that the passageway from the sump to the oil pump is clear if it hasn't been done in a long time. There is no protection of the return side gear teeth if it sucks up something from the sump.

Keith1069 said:

I am sure he mentioned a cool filter and hot timing cover so best guess is most of the oil is going from crankcase to rear breather up the pipe then back into the timing case so empty oil tank..does this make sense to anyone

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I thought that after I posted but then with the kind of pressures that he must be developing in the cases why doesn't the Yam valve open? It does make sense though. A flow tee might help ('Y' piece) to direct the oil etc.

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I thought that at first as well but I guess the unknown is what is getting pumped up the breather it has been said by many that above 4500 rpm a lot of oil, not just oil 'mist' goes up the breather so what happens to this oil at the tee and the various contractions and expansions through the Yam valve would be difficult to determine except by a practical test my guess is it goes back down the timing breather pipe.(Never had to investigate any of this as I have kept my 850 breather as is). After reading all the info on this forum I have ordered an XS650 valve to tinker with and take some pressure readings.(I obviously have too much free time )

Two important facts have to be kept in mind when using any PCV valve. It has to be the one and only vent out of the motor if you're looking for pressure control. The second is that it has to be as close to the motor as possible. If you have the valve on the end of a long tube it won't work. Air is elastic, the air in the tube is going to act like a spring. It aint stealthy but it needs to be really close, or even in the motor. Coming off the timing case is bad enough. Ideally it would be right off the crankcase. You can have more than one vent, but they each need a valve.

Just to show the lengths racers go to on this matter for performance reasons. Drag bikes use three pumps to evacuate the motor, that's air not oil. NASCAR has to limit the size of scavenage pumps on the motors. They're using'em to pump air, at this point, not oil. Formula One engines are a stack of V-twins, not connected. They all are sealed, not to keep oil in, it's there for air management. Windage, oil drag, is so yesterday. It consumes energy to pump air above and below the piston. Now it's _air_ drag inside the motor.

hi i finally got tired of that same leak and a few more on my '72 combat (204061)
after reading the posts here i bit the bullet and went with the cnc breather and anti sump valve $220 and $75
they said it would work 90% without splitting cases and doing machine work they were right works bitchin!
still leaks a tiny bit when i get on it hard but what an improvement
only thing i didnt like they supply allen wrench bolts no way theres enought room to tighten top two
with out moving trany a couple hex bolts worked great
love this forum you guys and girls are the best very helpful
thanks mike