750 Crankcase Ventilation

This may start a food fight, so here goes.

I'm fixin' to put together a 750 Combat engine. The first thing I want to do is get the crankcase breathing set up properly. It seems that Old britts, Andover-Norton and Colorado Norton Works each have their own way of achieving proper ventilation.

Old Britts says to drill some holes in the timing side and relocated the breather tower from the back of the engine to the blanked out mag cover.

CNW wants to block Old britts holes and drill one small hole to equalize oil level in the timing chest. Then install one of their check valves.

Andover-Norton whats to drill holes as well as doing some grinding to prevent oil cavitation.

Dare I ask, What's the best modifications to do to the Combat cases?

while those are all improvements, the stock setup seems to work just fine for most all of us, no?

Where's my popcorn?

the 72-73 750 case's have the breather in the best possible location IE at the source so it does NOT have to find it's way out via extra holes. DON'T do the old britts sump mod if you are not racing it than just use the CNW - comstock bolt on reed breather and your done. if you want to relocate the oil pickup send me a PM and I will share with you a better way to do that.

On my 2 Combat the factory system worked fine until enjoying the 2S cam power band range then elastic engine states let over pressured oil out seams. I had great success modifying Peel Combat via big vents into TS case with idea to lower pump loss there and increase crankcase volume to lessen the pressure pulse spikes and used a Krank PCV up by the oil tank so even over rev event stayed dry. The Comnoz CNW method is tested best way to lower crank pressure even with good bit of ring blow by. Also think about moving oil sump hole to rear which is simple if cases split, but not big deal as Combat breather carry slung back oil straight to the tank pass any one way valve in line.

grandpaul said:

Where's my popcorn?

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Come on now, it's been a few months.
Pass the Milk Duds.

At the end of a brisk ride on my other Combat there is a small pool of oil on the crankcase just underneath the carbs. I added the valve from Mike's XS late last season. It seemed to improve things.

Is there a reason one shouldn't do the Old britts sump mod for non-racing engines. Is it because it's not neccesary or it other problems?

Andover Norton's article was very adement that all the combat problems were caused by poor scavenging, not inadequate main bearings.

http://www.andover-norton.co.uk/SI%20Combat.htm

Fred explains the the situation pretty well here, although it is sort of a commercial for the oil pressure guage, I love mine.
http://www.oldbritts.com/n_c_case.html

You gotta admit, it a pretty funky design with the stupid little hole to suck from the front of the bottom of the crankcase. And the breather low back to draw oil and choke the breather gauze. You can't really fix it, but you can make it exceptable.

I did the mod including plugging the front sump hole and drill the new one toward the back more. Although many love to disagree, I have always contended that installing a reed type breather valve on the lower left case of the 72 is moronic. Even if you machine away the baffle and improve the sump draw, oil is going to be slung and drawn into the breather area the effect the function of the reed, like trying to play a sax with a mouth full of 20w50.

Then there is the new school of closing off the timing side altogether. I guess that may be all well and good but isolating the breather to the rear left of the case again does not seem all the great. I mean the 73 on 850 bikes all breathed out the old mag area and really didn't seem to suffer as far as I know.

I even incorporated Grandpauls reed into the old mag boss. It is quite audible when kicking the motor over. I also have run the xs reed out the lower left however I mount it high to create a 90 degree right to the oil tank. Nice, neat, and functional.

Some oppose this endevour and it really is for isolated to the bull crapped designed 72's, but it sure works for me and i would surely do it again.

I run the original elbow breather on my '72 Combat plus an 850-style breather with a 5/8" hose that loops over the air-filter backplate and terminates in a mini K&N-style pod located just at the frame rail behind the gearbox. This permanently eliminated all pressurization problems when I installed it ten years ago. It's amazing how much air moves through both breathers at idle. The exaggerated loop must keep the oil out of the external breather--the bike rarely needs oil between changes.


Tim Kraakevik
kraakevik@voyager.net

grandpaul said:

Where's my popcorn?

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:mrgreen:

Back to the straw poll: I relocated the scavenge point to the rear of the case - once you see the rationale, it becomes something of an imperative in my book.

I relocated the breather to the back of the timing case per 850 practice, with a Mike's XS breather next to the tank. I also machined out the baffle. Again, given all the options it made most sense.

Available breather kits clearly work well and have the great advantage of being an in-situ mod - and look rather Gucci as well :wink:

My engine was already stripped and I'm geared up for this stuff. The bike is oil tight and I'd do it the same next time.

Lots of ways to skin this cat. I'm so sold on crank case evacuation beyond just good enough I'll be trying to get an exhaust extractor plumbed in and not suck the oil out with it. If anyone has practical pointers on a 2-1 header on 360 twin please pipe up. Such a simply complex machine to devote inventiveness to.

grandpaul said:

Where's my popcorn?

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Are you thinking this is going to be a long movie.

pete.v said:

grandpaul said:

Where's my popcorn?

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Come on now, it's been a few months.
Pass the Milk Duds.

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Nah, I'd rather have whisky

Hobot, the problem with parallel twins like ours, and flat twins like BMW's is that when the pistons head towards the crank case, the pressure builds up. Cant stop that, but what you can do is fit a breather with a valve in it that stops the air going back into the cranck case when the pistons head away from the crank case. How do BMW solve it.
Believew it or not, when was a kid I drove a jowet flat twin. It had a large breather valve made of a round plate with 5-6 holes in it. On the out side of this plate was a shim steel star like device that was blown away from the holes when the pressure built up and sucked back against the hole when the vacuum built up. This worked. Hence when I had problems blowing oil out of my commando before I discovered the rings were f---d I fitted a jowet breather valve modification. My old shitter no does not leak oil now. Don't know anything about BMW flat twins, but I wouldn't be surprised if they have a breather valve like the jowet.
\
The other little problem I see with the commando is they have a 3/8 hole in the hose coming out of the engine then they step it down to a smaller size before it goes onto the oil tank.??????????? I stil run with this and its working, but not my idea of good design.

kerinorton said:

Don't know anything about BMW flat twins, but I wouldn't be surprised if they have a breather valve like the jowet.

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BMW one-way breather valve as used on the /5, /6, & /7 models up to the mid-seventies when it was replaced with a simple reed valve.

"The other little problem I see with the commando is they have a 3/8 hole in the hose coming out of the engine then they step it down to a smaller size before it goes onto the oil tank.??????????? I stil run with this and its working, but not my idea of good design."

I THINK YOU MEAN 5/8TH HOSE COMING OUT OF ENGINE BREATHER (cos mine is)............ I dont think its such a bad idea, whereas with 5/8th you have "volume" to accomadate initial crankcase pressure on statup.. IMO, once/if a reed valve type PCV is fitted high up by oil tank the crank case pressures are minimalized as much as possibly with the "volume" of the crankcase and 5/8th tubing included..
If a reed valve is fitted the C/C pressure will "float" with negative pressure and minimal/little/ and no flow (1) of pressure will be experienced in the PCV tubing. So its restriction down to 7-8mm will not matter..

Maybe pressure also "floats" in a standard system but is on the the postive side, hence the notiourious Norton oil leaks from C/C pressure...

(1) Pressure will fluctuate that quickly precevable flow wiill not be felt.......

Maybe im wrong but this theory has work for me...

stuck amals said:

At the end of a brisk ride on my other Combat there is a small pool of oil on the crankcase just underneath the carbs. I added the valve from Mike's XS late last season. It seemed to improve things.
Is there a reason one shouldn't do the Old britts sump mod for non-racing engines. Is it because it's not neccesary or it other problems?
Andover Norton's article was very adement that all the combat problems were caused by poor scavenging, not inadequate main bearings.
http://www.andover-norton.co.uk/SI%20Combat.htm

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RE; “Andover Norton's article was very adamant that all the combat problems were caused by poor scavenging, not inadequate main bearings.”
If I can add my opinion, and it’s just my opinion mind, when the Commando first came out they inclined the engine forwards, taking the oil pump further back which left more oil in the crankcases, so the oil pump was unable to scavenge as efficiently as it did before this mod, hence oil left in crankcases and more bubbles on in oil tank.

The following is from Andover advice sheet;
“This new crankcase, and its breather arrangement, had been so cleverly redesigned for 1972 the sump was not drained properly at higher revs. Normally the problem occurs in the region of 4.000 to 4.500rpm. As a result oil foam formed in the crankcase that was then thrown out of the breather, mostly unnoticed by the rider sitting in front of the breather outlet, until there was no oil left.
I have heard from owners of this engine type they never had a problem. However, having ridden a race engine on the track many years ago that- due to our ignorance of the problem at the time- sported an unmodified crankcase, I can tell you the oil literally pumped out of the breather and I put a good layer of oil onto the track, making myself highly unpopular with my fellow competitors!”

Hi, go Comstock way.........I have no interest ,but he seems to know!

Bernhard said:

If I can add my opinion, and it’s just my opinion mind, when the Commando first came out they inclined the engine forwards, taking the oil pump further back which left more oil in the crankcases, so the oil pump was unable to scavenge as efficiently as it did before this mod, hence oil left in crankcases and more bubbles on in oil tank.

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However, when they did so, I believe they moved the scavenge hole to about the 'lowest' point, as I don't think the 'upright' engines have this, and, as far as I'm aware, it was only after the introduction of the 1972 crankcase modifications that the scavenging problem occurred. It doesn't seem to affect the 850 models which reverted back to the original strainer design so the problem would appear to lie with the '72-'73 (750) crankcase design and not due to the inclined angle of the engine.

I have Bolt on CNW, no mods to cases needed.
Works a dream, and ZERO oil leaks on the engine. (72 combat, 5000 miles since rebuild 2.5 years ago)

Cheers............ 8)