Location of breather

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PJL

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Mar 26, 2008
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Just made an interesting observation which backs up the CNW claim. I have a 72 Combat Commando on which, last year, I made the oil pickup modifications and, whilst apart, moved the breather to the timing case. It breathed through a Ducati reed valve to the oil tank. However, since then I've always had a slight weep from the cylinder head to block joint at the right side pushrod tunnel, but only if running at over 4000 rpm for any length of time.

Based upon discussions here and viewing the CNW site, I moved the breather back to the old location at the bottom of the crankcase, keeping the reed valve in the line. The oil weep has now gone, even when cruising at 5000 rpm.

I wish I'd never drilled the holes into and out of the timing case! Now, how best to put the timing case back to how it was before, preferably without taking everything apart again? There's probably not much point in worrying about the internal holes, but the one at the back of the timing case now bugs me. The motor is really running extremely well, pulling straight up to 7000 rpm through the gears if not careful about watching the rev counter.

Many thanks for such as informative forum - just wish the discussion on breather location had occurred a few months earlier!

Phil.

PS, now just need to deal with the weep from the primary chaincase so called "seal" behind the clutch and the occasional drip from the swinging arm. Still, they give me an excuse to fuss over the bike when returning from a ride. It's like being in love all over again.
 
PJL said:
Phil.

PS, now just need to deal with the weep from the primary chaincase so called "seal" behind the clutch .
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this is an easy fix, install a belt drive and run the primary case dry and problem solved :mrgreen:
 
I heartily second installing a belt primary, everything stays dry, mine has lasted nine years and look just fine, dry clutch plates,
bike feels smoother and quieter without the chain
 
It appears mounting the PCV valve directly to the crankcase, a' la CNW, is the way to go.
 
I "m not surprised. I tried every combination of breather I could come up watching the crankcase pressure waves with a transducer mounted in the tach drive hole and an oscilloscope before I gave up on a breather on the timing chest . It wasn't until I went directly into the crankcase with a valve that I began to see a negative average pressure in the cases. It sure would have been easier to put a breather into the timing chest. Jim Comstock
 
I also created the other day a mathematical model of a two compartment system with a restricted flow between them. Under no combination of intercompartmental flow parameters (except unrealistically large) could I get a timing case breather to match one on the crankcase. Proof positive, both experimental and theoretical. QED.

So, Norton were on the right track in 1972; pity about the poor oil pickup position and lack of one way valve. Even nature solved that millions of years ago - veins have reed type valves in them to ensure blood returns upwards to the heart.

Phil.

Location of breather
 
PJL said:
Just made an interesting observation which backs up the CNW claim. I have a 72 Combat Commando on which, last year, I made the oil pickup modifications and, whilst apart, moved the breather to the timing case. It breathed through a Ducati reed valve to the oil tank. However, since then I've always had a slight weep from the cylinder head to block joint at the right side pushrod tunnel, but only if running at over 4000 rpm for any length of time.

Based upon discussions here and viewing the CNW site, I moved the breather back to the old location at the bottom of the crankcase, keeping the reed valve in the line. The oil weep has now gone, even when cruising at 5000 rpm.

I wish I'd never drilled the holes into and out of the timing case! Now, how best to put the timing case back to how it was before, preferably without taking everything apart again? There's probably not much point in worrying about the internal holes, but the one at the back of the timing case now bugs me. The motor is really running extremely well, pulling straight up to 7000 rpm through the gears if not careful about watching the rev counter.

Many thanks for such as informative forum - just wish the discussion on breather location had occurred a few months earlier!

Phil.

PS, now just need to deal with the weep from the primary chaincase so called "seal" behind the clutch and the occasional drip from the swinging arm. Still, they give me an excuse to fuss over the bike when returning from a ride. It's like being in love all over again.
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Phil,

Part of the modification to this type crankcase is to remove the wall at the back of the crankcase that normally restricts oil flowing up the original breather location. Did you do this and when you put the breather back in the original site are you getting more oil going up the breather? I did the mod on my '72 750 in 1998 and would also like to put it back to stock. Wondering how to reverse the mod.
 
If we put this breather business into a historical perspective, Norton got it right before the Commando, with the timed breather on the camshaft ala Atlas (and first edition 750 Commados). Look at how it works. It opens when the pistons decend and shuts when they rise. The only thing they didn't really sort out was adequate volume. The holes in the camshaft flange are pretty small as is the exit holes. Going to direct crankcase ventilation may have seemed an improvement, but judging by the many recent successful applications of one way PCV gadgets fitted to Commandos, it seems to me that the original idea was the best, just not adequately developed ....like a few other things I can think of!

Mick
 
Your right Mic, the timed breather is the best and worked well on the early Norton motors until the engine got a bit of wear on it and then the blowby was more than the tiny holes could handle. A timed breather with a 1/2 inch hole would likely work great.
 
When we do the CNW valve we normally still remove the baffle. That lets the oil collect at the rear of the crankcase so it can be picked up by the new scavenge pump hole drilled back there. You can just remove the cover plate and bolt up the CNW kit that is designed to fit there and you will be good. There is no question some oil does go back to the tank through the reed valve but it will not cause any problem. Jim
 
When modifying the oil pickup I removed the weir as per pictures seen on Old Britts and blocked the original oil pick up hole. It's essential to have the oil pick up towards the rear of the crankcase as this is where the oil is swept by the spinning crank. I've simply replaced the original breather and capped the timing case exit hole I'd drilled. There is still an overhang in front of the breather hole (see photo below). I've kept the Ducati reed valve in the line to the oil tank. I've not checked to see whether oil is again going up the breather (but will do when next out on it).

Before I did the modification last year, when watching the oil flow to the tank and revving it to about 4000 rpm, the flow to the tank would stop, then a few seconds later oil would come spewing up the breather line, frothing! The engine also leaked quite a bit at speeds corresponding to these rpm (cylinder head to barrel and exhaust rocker covers). This it doesn't do any more.

I can't be sure about the power output, but it don't half pull well when those 32 mm Concentrics are opened right up.

Phil.

Location of breather
 
Just a quick update - been out today and checked whether oil goes up the breather when get up to 4-5000 rpm. Answer: not that I can detect. Oil returns normally via the pump.

Best regards, Phil.
 
Hi there, nice mods, but you are speaking about the post 200.000 cases, and what's about the 850 cases,? how to fit the XS PCV ? that mean , remove the oil baffle?? drill the left case just like the post 200.000 ? and fit the XS PCV ? :roll: , must modify that XS valve :(
 
marinatlas said:
Hi there, nice mods, but you are speaking about the post 200.000 cases, and what's about the 850 cases,? how to fit the XS PCV ? that mean , remove the oil baffle?? drill the left case just like the post 200.000 ? and fit the XS PCV ? :roll: , must modify that XS valve :(
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The XS 650 PCV valve is an inline valve. Since the 850's breath through the timing case no case mods are needed. Cut the breather hose and insert PCV valve. If you want a PCV valve mounted on the 850 crankcase, a' la Jim Comstock/CNW, you'll need to machine the left crankcase to provide a breather port and plug the stock breather port on the backside of the right crankcase. CNW say they modify the timing case, also. Don't know what mods they do to the timing case.
 
We modify the timing cover by plugging all the holes between the crankcase and timing case. We then drill a new small oil return hole down low in the timing case. This reduces the volumn of the crankcase and makes the reed valve work better so it makes more vacuum. It also lowers the oil level in the timing case. I had found that the timing case was running nearly half full of oil on a stock motor . Lowering the oil level reduces the amount of oil heating from being whipped by the gears. A substantial reduction in oil temp was observed. Jim
 
Jim,

Would you mind divulging the size and location of the oil return hole you spoke of?
 
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