Breathers

Schwany

Schwany

Joined
Apr 22, 2020
Messages
3,157
Country flag
First gotta say my P11 motor is not stock, so my observations below will not apply to basic proper P11 restorations.

I have a cNw reed breather on the back of the timing case of my P11. (See **note at the end of this post if wondering why.) I believe the standard installation recommendation with the cNw breather on the back of the crank case was to block off the cam timed breather, so I did even though that is not where I installed the cNw breather. Never had a problem with it on the back of the timing case until I went out and started riding like a teenager and passing cars on mountain roads. Told myself I wouldn't do that again and see if I could extend the tire life, but old habits die hard. Anywho, the cylinder base gasket started to weep a little due to the thrashing, not pour out oil, just a little weep. So because I am not a fan of leaks, I got to thinking maybe I should hook that timed breather up and see if I get better crank case pressure evacuation. I have not had a chance to run the motor up to excessive RPM for any extended period of time, but there was another positive side effect I felt immediately. The motor is about 50% easier to kick over stone cold with the timed breather hooked up. I'm leaving it hooked up because of that feature alone.

Breather plumbing is currently T'd together about 8 inches before entering the oil tank. I plan to find a tight angle Y connector and connect the plumbing together just before the oil tank so there is no chance of any weird backward pressure pulses against the reed valve with the two pressure sources coming together too soon in the plumbing circuit. May not even be a real issue, but still want to do it that way.

Point is having both the breathers hooked up lets the motor breath better if using a breather off the timing case. I will update this thread if I notice any negative performance issues at higher RPM. My motor will wind up higher than is recommended for a stock motor. It still pulls even if not making any additional HP. 85mph in 3rd is 7000 RPM with my gearing and my motor was still pulling before shifting into 4th the day I did that. I don't do that regularly, because I doubt it would last that long doing it, but had to try it once to see how it sounded and felt. Kind of intoxicating, but best avoided on the street.

One thing I have not tried is having only the timed breather hooked up. It might be sufficient for the engine. It was for the motor when it was bone stock.

**Only reason I had installed the cNw breather on the back of the timing case in the first place is because I had a 2S cam in the motor previously that was not designed to support the timed breather.
 
Schwany said:
First gotta say my P11 motor is not stock, so my observations below will not apply to basic proper P11 restorations.

I have a cNw reed breather on the back of the timing case of my P11. (See **note at the end of this post if wondering why.) I believe the standard installation recommendation with the cNw breather on the back of the crank case was to block off the cam timed breather, so I did even though that is not where I installed the cNw breather. Never had a problem with it on the back of the timing case until I went out and started riding like a teenager and passing cars on mountain roads. Told myself I wouldn't do that again and see if I could extend the tire life, but old habits die hard. Anywho, the cylinder base gasket started to weep a little due to the thrashing, not pour out oil, just a little weep. So because I am not a fan of leaks, I got to thinking maybe I should hook that timed breather up and see if I get better crank case pressure evacuation. I have not had a chance to run the motor up to excessive RPM for any extended period of time, but there was another positive side effect I felt immediately. The motor is about 50% easier to kick over stone cold with the timed breather hooked up. I'm leaving it hooked up because of that feature alone.

Breather plumbing is currently T'd together about 8 inches before entering the oil tank. I plan to find a tight angle Y connector and connect the plumbing together just before the oil tank so there is no chance of any weird backward pressure pulses against the reed valve with the two pressure sources coming together too soon in the plumbing circuit. May not even be a real issue, but still want to do it that way.

Point is having both the breathers hooked up lets the motor breath better if using a breather off the timing case. I will update this thread if I notice any negative performance issues at higher RPM. My motor will wind up higher than is recommended for a stock motor. It still pulls even if not making any additional HP. 85mph in 3rd is 7000 RPM with my gearing and my motor was still pulling before shifting into 4th the day I did that. I don't do that regularly, because I doubt it would last that long doing it, but had to try it once to see how it sounded and felt. Kind of intoxicating, but best avoided on the street.

One thing I have not tried is having only the timed breather hooked up. It might be sufficient for the engine. It was for the motor when it was bone stock.

**Only reason I had installed the cNw breather on the back of the timing case in the first place is because I had a 2S cam in the motor previously that was not designed to support the timed breather.
Click to expand...
For many many years I used a reed valve from an oxy acetylene gun on the back of my timing cover teed into the timed breather hose and then the oil tank on my commando
When I removed the reed valve to rely solely on the timed breather the cylinder base gasket started leaking at the back
 
baz said:
For many many years I used a reed valve from an oxy acetylene gun on the back of my timing cover teed into the timed breather hose and then the oil tank on my commando
When I removed the reed valve to rely solely on the timed breather the cylinder base gasket started leaking at the back
Click to expand...

Thanks for that comment. I feel using the timed breather is an improvement. Hoping it works on stopping the base gasket weep. I think it will, but still need to do a couple of hot day 90 mph runs to be sure.

I'm too lazy to make the modifications to remove the cNw breather. Besides the cNw breather looks trick and works. What does not look trick is a big hose coming off the timed breather and going up over the intake manifolds. Looks like a prototype thought on a flat track bike.

I'll get around to re-routing that hose and gearing the bike up again before the cold weather comes back. Low stock gearing is great in town, not so great on a long ride. Bike always sounds like it is being raced around by a madman when it definitely is not going fast at all. The madman part might still be true though.
 
Is keeping the stock breather configuration (66 N15CS) a 'really bad thing'? I just didn't have the knowledge about this subject and have been researching. I hope so, as it's all together with the little disc in there. Maybe put a reed valve on the vent hose?
 
gpzkat said:
Is keeping the stock breather configuration (66 N15CS) a 'really bad thing'? I just didn't have the knowledge about this subject and have been researching. I hope so, as it's all together with the little disc in there. Maybe put a reed valve on the vent hose?
Click to expand...

I tried that ..... putting a XS650 REED VALVE in series with the timed breather. Measurement of sump pressure with and without the reed valve was exactly the same.

I plugged the timed port and now run a Comstock (same as cNw) sump plug breather. I have not measured the sump pressure in this configuration, but the bike seems happy.

Slick
 
Last edited:
How about connecting the (XS650 or a similar unit) reed valve to the timed breather port? The timed breather vents at both 360 degree cycles, so once crankcase has been evacuated (which may take a bit longer due to small orifices), the combo breather should work as well as a crankcase mounted breather, no? Has anyone tried this?
This arrangement has the benefit of avoiding obstructions by froathing, which affects the (low down) crankcase mounted breather.

- Knut
 
mdt-son said:
How about connecting the (XS650 or a similar unit) reed valve to the timed breather port? The timed breather vents at both 360 degree cycles, so once crankcase has been evacuated (which may take a bit longer due to small orifices), the combo breather should work as well as a crankcase mounted breather, no? Has anyone tried this?
This arrangement has the benefit of avoiding obstructions by froathing, which affects the (low down) crankcase mounted breather.

- Knut
Click to expand...

Mine is a reed valve that screws in where the original breather elbow screwed in. I forget where I got it, I think it's an older style Jim Schmidt breather, I don't think it was on his site anymore last time I looked. I removed the old timing plate and now have only this reed valve set-up. How well does it work? Dunno yet! Unfortunately I didn't get it running before I left the US. Oh, I just realised that I'm replying to mdt-son, who actually has my bike in his garage right now.
P6180132.JPG
 
Junglebiker said:
Mine is a reed valve that screws in where the original breather elbow screwed in. I forget where I got it, I think it's an older style Jim Schmidt breather, I don't think it was on his site anymore last time I looked. I removed the old timing plate and now have only this reed valve set-up. How well does it work? Dunno yet! Unfortunately I didn't get it running before I left the US. Oh, I just realised that I'm replying to mdt-son, who actually has my bike in his garage right now.
Click to expand...

Jim still sells that reed breather. $199US.

I thought motorson had your bike. mdt-son is in Norway according to his profile.
 
Schwany said:
Jim still sells that reed breather. $199US.

I thought motorson had your bike. mdt-son is in Norway according to his profile.
Click to expand...
Dang it. You're right. I never think of him as "motorson" unless I am on the site. I've known him for 30 years by his real name. Sorry for the confusion.
 
I could be repeating myself... The cNw breather I have is intended for installation on Nortons that had a breather on the back of the crank case. It is not designed to go where the sump plug goes on Nortons with sump plugs. It is supposedly designed by James Comstock, but it is not that one that NYC Norton sells, which is also designed by James Comstock. Jim Schmidt sells a sump plug breather as well as the reed breather Junglebiker is using.

Stopping frothing with a reed breather is overly optimistic. The timed breather off the cam is a frothing son of a gun as is a reed breather on the timing case of my P11.

A reed breather valve on the timed breather would prevent the sucking cycle at low RPM, if there is one. It would not prevent froth though. On a P11 the exit port from the froth tower pukes up froth (oil mist and condensation mixed). Condensation is normal and as far as I know happens on every vehicle combustion engine. On my P11 that froth eventually goes into a catch can that is mounted on the side of the rear fender and located just in front of the drive side shock. Crank case pressure in the oil tank from the breather(s) is vented out of the oil tank from the froth tower and blows through the catch can and out an air filter close to the back of the rear fender. I'm not concerned about original appearance. Performance is all I care about. Not adequate stock performance, but something that keeps me awake.

I have considered using Jim Schmidt's reed breather on the timed breather output, but I'm also a cheap skate so I keep putting it off. I would have two reed breathers in that scenario. I would be really special then. lol

My Norton engine works fine with both breathers I'm using plumbed into the P11 froth tower with appropriate venting. A breather setup that does not work will kill the power output on a Norton motor because of too much crank case pressure. Plus, oil leaks would be a problem. The more pressure one can relieve the crank case of the better.

People should do whatever they want with breathers, especially if it makes them feel like they are doing something that improves the performance of their engines. You do you. I'm not suggesting anything other than the timed breather can be hooked up on a P11 if a reed breather is used on the back of the timing chest. The timed breather can also be blocked off, if another adequate breather solution is in place. It certainly looks better blocked off.

Off for a ride to see if I stopped the weep at the base gasket at sustained high RPM.
 
I've never had a timed breather apart but In would assume it is designed to open on the downstroke and close on the upstroke to limit intake.
Much like the reed does, but under pressure.
That being said, not sure how much a reed with the timed breather helps, unless one removes the interior disc.
 
MichaelB said:
I've never had a timed breather apart but In would assume it is designed to open on the downstroke and close on the upstroke to limit intake.
Much like the reed does, but under pressure.
That being said, not sure how much a reed with the timed breather helps, unless one removes the interior disc.
Click to expand...
I'll buy that. The timed breather would be a far less useful if it sucked on the upstroke which is what it would do if all the parts were removed.

I've had the breather apart more than once but wasn't real concerned about the science as long as all the pieces were there, and it worked. I always figured it does what it supposed to do.
 
During my test ride today, I got in a fun but childish straight ahead HWY run with a very nice looking and healthy sounding custom Harley with more than 2x the cubic inches of my P11. I could hear him coming and just had to do it. Stayed right with him the whole time. We never went over 90MPH due to coming up on traffic. Fairly good test for the base gasket weep. Although I was not WOT much. More WOT would have been a better test.

Anywho, the base gasket did not weep, and I personally feel it pulled a touch stronger earlier in the RPM range than it did before hooking the timed breather up. Placebo effect most likely, but a pleasant one. Definitely have no plans to block the timed breather off again.
 
I always wondered why Norton eliminated the timed breather in favor or the hose off the timing chest. It seems to me that only reduces the compression ratio within the crankcase by adding the oil tank head space. On my 850 I installed a VW power brake check valve in that hose after mucking about unseccessfully with various PCV valves. That stopped the oil leaks from under the head and elsewhere, so apparently it works. Been that way for 20 odd years now. I did a similar thing to my G80 Matchless though it resulted in an ugly hose running from the timing chest to the end of the rear fender. It was a race expediant and looks the part; it did stop the leaks though. The factory breather is a little disc inside a fitting with no spring and a 3/8 hard line going who knows where from the factory but to the rear chain when I got mine. It did an admirable job of oiling the chain and everything the chain might fling it to. It did not stop the oil leaks in the engine. Only the check valve worked for that.
 
I would imagine when it started out as 5k rpm 500, it worked fine.
As size and RPM's increased it got less efficient.
They actually moved it to the rear sump first, then the timing chest.
Turns out the sump with a proper reed valve appears to be the best.

Thomasa said:
I always wondered why Norton eliminated the timed breather in favor or the hose off the timing chest. It seems to me that only reduces the compression ratio within the crankcase by adding the oil tank head space. On my 850 I installed a VW power brake check valve in that hose after mucking about unseccessfully with various PCV valves. That stopped the oil leaks from under the head and elsewhere, so apparently it works. Been that way for 20 odd years now. I did a similar thing to my G80 Matchless though it resulted in an ugly hose running from the timing chest to the end of the rear fender. It was a race expediant and looks the part; it did stop the leaks though. The factory breather is a little disc inside a fitting with no spring and a 3/8 hard line going who knows where from the factory but to the rear chain when I got mine. It did an admirable job of oiling the chain and everything the chain might fling it to. It did not stop the oil leaks in the engine. Only the check valve worked for that.
Click to expand...
 
I measured sump pressure with the timed breather and found it to be negative 18 inches of water from approx 2K rpm up. Then I put a XS650 reed valve in series at the timed breather port and measured again. I was amazed to find no difference in pressure. Surely, I thought, the rotary disc allowed leak back which I expected the reed valve to stop. Thus with no improvement in negative sump pressure, I conclude the timed breather is actually quite efficient, although I have to admit I do not understand why a reed valve offered no improvement.

Slick
 
With a timed breather valve, at low rpm, the pistons descend, raising pressure in the crankcase, expelling air through the open breather. The breather closes, so the crankcase depression caused by the rising pistons does not draw air in.

All very sensible, but what happens at 6000 rpm, when the pistons rise and fall 100 times a second? I’m sure I don’t know.
 
I needed the extra pressure out of the crank case that the timed breather provides. I already have 3 3/8" holes drilled into the crank case on the timing chest side that the cNw reed valve breather takes care of, but those holes are not enough, or they are not drilled in the ideal locations. That said, if I rode like a sophisticated gentleman, I'm sure the cNw breather on the back of the timing chest would be more than an adequate breather alone. It actually was adequate until I started pushing the motor a little harder and got the minor base gasket weep. I built the motor with the parts I used so I could push it hard and needed to address the issue. A man has to have his enthusiasms.

Parts is parts: The timed breather in a P11, Ranger, N15, or Atlas has 4 three primary parts. A camshaft machined to support a timed breather, a stationary breather plate, a rotary breather plate (the disc), and a spring. Look at parts 81, 82, 83, and 84 in this exploded view. The above-mentioned breather parts are all used on my engine, except my cam is a JS2SS, not a stock cam. Only brought this up because Thomasa said no spring. Does that make me a bad person? lol
 
Triton Thrasher said:
Timed breathers only work at some engine speeds.
Click to expand...
Says who?

Triton Thrasher said:
With a timed breather valve, at low rpm, the pistons descend, raising pressure in the crankcase, expelling air through the open breather. The breather closes, so the crankcase depression caused by the rising pistons does not draw air in.

All very sensible, but what happens at 6000 rpm, when the pistons rise and fall 100 times a second? I’m sure I don’t know.
Click to expand...
I don't see why it shouldn't work at 6000 rpm also. We are not talking of mass transport here. The breather must account for the time delay of approx. 60 microseconds for the pressure vawe to hit it. The pressure wave has the length of a quarter of a crankshaft rotation cycle (90 degrees). At 6000 rpm it is exactly 2.5 microseconds long, which compares well to the cycle time of the camshaft and breather (20 microseconds). Given correct relationships, the overpressure will escape, reducing the crankcase pressure towards the minimum theroretical value (see below).

texasSlick said:
I measured sump pressure with the timed breather and found it to be negative 18 inches of water from approx 2K rpm up. Then I put a XS650 reed valve in series at the timed breather port and measured again. I was amazed to find no difference in pressure. Surely, I thought, the rotary disc allowed leak back which I expected the reed valve to stop. Thus with no improvement in negative sump pressure, I conclude the timed breather is actually quite efficient, although I have to admit I do not understand why a reed valve offered no improvement.
Click to expand...
The minimum pressure obtainable assuming no piston ring leak and isotrophy is easy to compute. Assume ambient pressure at piston BDC (overpressure is completely evacuated to free air). Then p1*V1 = p2*V2. ==> p2 -p1= p1*(-CC/(V1+CC))
where CC is the common engine displacement (750 ccm or 0.75 litre). All you need to know is the maximum crankcase volume. p1 may be taken as 101325 Pascal (N/m2) or 14.696 psi.
V1 is the total crancase volume at BDC including the timing side (unless blocked off). The top end may be disregarded. Matching your reading, I have assumed 7.7 litres at BDC (initially I had assumed 10 litres).
Using this figure, the pressure drop p2-p1 computes to -8993 Pascal. Qusistatic read-off is the average negative pressure (50% of the figure), i.e., -4497 Pascal or about -18.05 inch on a water column gage.

The effect of the engine's temperature change should have negligible effect on the readings. The volume V1 has yet to be confirmed, but sounds plausible.

It does look like the timed breather works quite effectively, and this may be the reason why your reed valve doesn't improve the readings. The rotary valve will close off the crancase due to the oil film seal when apertures don't align.

- Knut
 
Last edited:

Similar threads

S
New Breather on 1972 Combat
Replies
11
Views
418
ilikevelos
I
Bonzo
A quick NYC/Comstock Reed Breather Valve Question...
Replies
19
Views
1,163
Mart UK
Mart UK
cliffa
Crankcase breather / separator
2
Replies
21
Views
1,257
Stu Bodycote
Stu Bodycote
Lineslinger
NYC Norton Reed Breather Plumbing
2 3
Replies
47
Views
4,313
acadian
acadian
Bonzo
Comstock / NYC Norton reed breather banjo washer size?
Replies
3
Views
473
Bonzo
Bonzo
Back
Top