Velvetelves breather valve

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Ron L said:
Has anyone figured out just what engine this valve was originally designed for? I recall some one posting that they saw one on the new Symba scooter, but I haven't been able to confirm it from a parts diagram. They don't seem to have been used on the Honda Cub, of which the Symba is a clone.
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Not that I know of.
 
I'm still trying to understand the oem breather process in a commando. Looking at the parts book, I see that the 750 has an actual "breather body," whereas the 850 just has a "breather tube." Is there any mechanical "action" being performed by the breather itself on an 850 or is it just an opening which allows the vacuum/pressure to move back and forth? I have seen references to a "timed breather" on (I believe) non-850 models. Could someone explain - or point me to an explanation of what these various systems actually do? I understand the need for crankcase ventilation and the benefit of some vacuum in the crankcase but I don't understand the specifics of what Norton did with the various "breathers."
 
My 70 model 750 has a timed breather. It vents through some small holes drilled into the cam out through the blind end of the camshaft. There's a rotary valve driven by the cam that allows the crankcase to blow out but not suck in. Only problem I see is the holes through the cam are pretty small.
 
MexicoMike said:
I'm still trying to understand the oem breather process in a commando. Looking at the parts book, I see that the 750 has an actual "breather body," whereas the 850 just has a "breather tube." Is there any mechanical "action" being performed by the breather itself on an 850 or is it just an opening which allows the vacuum/pressure to move back and forth? I have seen references to a "timed breather" on (I believe) non-850 models. Could someone explain - or point me to an explanation of what these various systems actually do? I understand the need for crankcase ventilation and the benefit of some vacuum in the crankcase but I don't understand the specifics of what Norton did with the various "breathers."
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Aside from much discussion on this board, the topic has been around for a LONG time. Just search "breather".
 
Thanks Dave. I had done the search and read most of that; still not sure exactly what the 850 breather is - looks like just a "hole" that vents crankcase pressure/vac in and out based on piston movement whereas the 750 breather was some attempt at actually keeping the crankcase at vacuum or neutral pressure - though, from what I read, it wasn't really effective.
 
MexicoMike said:
still not sure exactly what the 850 breather is - looks like just a "hole" that vents crankcase pressure/vac in and out based on piston movement whereas the 750 breather was some attempt at actually keeping the crankcase at vacuum or neutral pressure - though, from what I read, it wasn't really effective.
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The camshaft timed breather was used up until what is considered to be the end of the 1971 production. It was eventually replaced with what is sometimes erroneously referred to as the 'Combat breather', as this breather was in fact fitted to all '72-'73 750 production models-regardless of whether they were originally built as Combat models or not.

The '72-'73 breather consists of items numbered [47]-[53]: http://www.nortonmotors.de/ANIL/Norton% ... &Plate=004

You are right in saying that the 850 breather is nothing more than an open vent at the rear of the timing chest which has a pipe connection to the oil tank, but there's nothing particularly wrong with that setup, as it appears to be an improvement over the two earlier breather arrangements:

http://www.oldbritts.com/n_c_case.html - 'Moving the breather'

Oil lines and breather: http://www.oldbritts.com/oillines.html
 
L.A.B. said:
You are right in saying that the 850 breather is nothing more than an open vent at the rear of the timing chest which has a pipe connection to the oil tank, but there's nothing particularly wrong with that setup, as it appears to be an improvement over the two earlier breather arrangements:

http://www.oldbritts.com/n_c_case.html - 'Moving the breather'

Oil lines and breather: http://www.oldbritts.com/oillines.html
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If you subscribe to the theory (presumably dismissed by Jim's research into the CNW breather) that an open hole is more effective than a system that attempts to at least keep the crankcase pressure non-positive.
 
swooshdave said:
If you subscribe to the theory (presumably dismissed by Jim's research into the CNW breather) that an open hole is more effective than a system that attempts to at least keep the crankcase pressure non-positive.
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If owners wish to fit a one way breather then that's up to them if they think it is necessary, but I'm content to continue using the standard 850 arrangement, and my 850's engine has always remained 100% oil-tight, therefore there's been no great incentive for me to fit any sort of one way breather to it, so far.
 
Yes, the 69-70 timed breather that come off the end of the cam on the left side of the crank and had a tube that went to the top of the oil tank on a small circular device, what may be some sort of separator, and then it went through the oil tank to a hole that came out inside the air filter and oil would drip out of that hole down the front of the oil tank and onto the air filter and then on to the top of the gearbox. Not a satisfactory breather. I am still trying to figure out what to do about my 69 setup unless I break the crankcase in half and put holes in the timing side and a breather valve where the magneto used to be, kind of like Mic Hemmings.

Dave
69S
 
DogT said:
Yes, the 69-70 timed breather that come off the end of the cam on the left side of the crank and had a tube that went to the top of the oil tank on a small circular device, what may be some sort of separator, and then it went through the oil tank to a hole that came out inside the air filter and oil would drip out of that hole down the front of the oil tank and onto the air filter and then on to the top of the gearbox. Not a satisfactory breather. I am still trying to figure out what to do about my 69 setup unless I break the crankcase in half and put holes in the timing side and a breather valve where the magneto used to be, kind of like Mic Hemmings.

Dave
69S
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Is there any reason why you can't just put a plug where the timed breather goes? Then find an alternative breather (like the old mag placement)?
 
So the "improved" 850 breather is identical to an emissions-era auto breather without the PCV valve - primarily it keeps oil vapor out of the atmosphere, oil drips off the concrete while keeping the crankcase pressure mostly neutral by allowing it to "go" somewhere.

OK, so I can see how the attempt to add a "PCV" valve that can actually react quickly enough to the change in pressure would be able to provide some negative pressure in the crankcase. After reading about the different Norton breathers, I couldn't see how that could work with what I understood as the "timed breather."
 
MexicoMike said:
So the "improved" 850 breather is identical to an emissions-era auto breather without the PCV valve - primarily it keeps oil vapor out of the atmosphere, oil drips off the concrete while keeping the crankcase pressure mostly neutral by allowing it to "go" somewhere.

OK, so I can see how the attempt to add a "PCV" valve that can actually react quickly enough to the change in pressure would be able to provide some negative pressure in the crankcase. After reading about the different Norton breathers, I couldn't see how that could work with what I understood as the "timed breather."
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You would want to block off a timed breather if you used a valve breather.
 
You have to remember that as much volume change occurs below the pistons as above them and it takes holes the size of the carburettors and exhaust pipes move that volume in or out so a little hole like the one on the end of the cam with the timed breather is pretty useless. The way CNW does it is as good as can be done within limits, punching big holes in the crankcase would not be good for strength. Probably the best thing to lower crankcase pressures would be to use an offset crank.

Jean
 
It seems to me as the pistons continue their cycles there would be less air to move as it would start to work in a slight vacuum, This would decrease the amount of air that needs to move on the next stroke. Remember air can get out but not back into the crankcases. I think this is why even a small XS650 valve helps some, This is just how my little brain sees it. LOL
 
Hortons Norton said:
It seems to me as the pistons continue their cycles there would be less air to move as it would start to work in a slight vacuum, This would decrease the amount of air that needs to move on the next stroke. Remember air can get out but not back into the crankcases. I think this is why even a small XS650 valve helps some, This is just how my little brain sees it. LOL
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I guess it would work that way if air had no mass, but it does and in a small pipe, the air would move less and less as the frequency of the pulses went up so the idea is to place the valve as close as possible to the crankcase with the biggest pipe possible but not past the capacity of the valve.

Jean
 
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