BSA seem not to have suffered serious breather malfunctions on their singles until the arrival of the late big fin Goldies...Then it was found that the 'flap valve' type didn't know whether it was coming or going as maximum revs rose to beyond 7000rpm...A mechanically driven breather solved the problem on these models.
The flap valve breather functions OK at the less 'exotic' rev levels of the M20 though they sometimes need stripping and cleaning to maintain correct operation.....
There were problems with the flap valve breather when it was relocated to the timing cover, but this was not down to the basic design of the working parts which were not changed...
There are two versions of the flap valve housing which altered the volume of air passed by the breather...Ian
email (option): ian@wright52.plus.com
It's one of the reasons these bikes exist today, they leaked oil all over the place thereby protecting everything from the dreaded rust bug!
The flap valve breather functions OK at the less 'exotic' rev levels of the M20 though they sometimes need stripping and cleaning to maintain correct operation.....
There were problems with the flap valve breather when it was relocated to the timing cover, but this was not down to the basic design of the working parts which were not changed...
There are two versions of the flap valve housing which altered the volume of air passed by the breather...Ian
Actually they do not work any where near as well as one would think, even when new and when was the alst time any one of us replaced the badly worn body or even the totally knackered flaper disc ?
email (option): wariron@tpg.com.au
Well I guess if a measure of the effectiveness of the breather is how much oil it passes and how much the engine leaks through over pressurisation of the crankcases, then they work OK, or at worst, within practical limits...On a 2600 mile trip around Scotland on my B33 that included hundreds of miles of motorway I only had to top up the oil tank with a very small amount. In fact I gave my oil to my oppo for his Matchless which was getting through considerably more...My engine wasn't leaking either...just the usual oil mist build up here and there.
Overall I would say everything was fine...(all I had to do was adjust the rear chain during the trip).
Excessive oil consumption and leaking just hasn't been an issue over large mileages on different BSA singles I have owned.
I think I did find the limit of the valves capacity when I enlarged my B33 to over 700cc...Then it leaked. Fitting a second flap valve cured the problem though..
I don't think there was really a problem, even if the valve wasn't perfect. BSA didn't change it for over 25 years and more than 200,000 bikes(apart from the DBD Goldies). I think they would have if they really weren't fit for the task...Ian
email (option): ian@wright52.plus.com
According to the bumph they are supposed to create negative crank case pressure.
They do this really well at low speeds but as the revs increase the effectiveness drops off.
Just as with an exhaust pipe the length of the exhaust from the breather is critical to it's proper function.
Further more this "ideal" length changes as the frequency of the pulses changes.
Then there are mass inertia affects of the actual crankcase gasses themselves which get out of phase with the piston's motions.
Finally there is response time.
Weather your valve works well for you will be a function of these plus the condition of the actual valve parts and how "in tune" they are with your engine and riding style.
A spring loaded brake check valve worked much better at all speeds over about 1/3rd throttle ( on the test B44 ) while a vertical valve orientation with a heavy flapper worked best overall.
On the older very long stroke very slow reving engines it works much better than it does on a M20.
The fact that BSA never changed them till they introduced the timed breathers does not mean that they performed well. As we know BSA did not change much unless it had to and the valve worked well enough ( in the dealers showrooms ) for the time when oil dripping out of every orrifice was "acceptiable" to the general public.
And of course I have not mentioned engine condition.
Flappers depend upon a strong sharp pressure signal to "pull" closed as the piston assends and you will only get one of these if the rings & bore are in good condition and in particular the 2nd compression ring which is responsible for most of the compression.
It gets really confusing the deeper you get into it.
We ( well he really ) were working with a new model Bullet which was blowing a lot of oil out the breather.
The problem turned to be nothing more than the length of the breather tube.
The idea of creating a partial vaccuum in the cases sounds logical but in fact it was totally wrong and there was rarely much in the way of an operational vacuum there in the first place which is why the flow through design was adopted.
Oh and the whole shooting match came about because Rex had exactly the same problem with his B44, oil & crankcase gasses escaping from around an engine stud ( on the head in a B44 as they are through bolted ) and Rex was too lazy to pull off the barrels again, remove the studs , counter sink the stud holes and slip a tight fitting "O" ring down over the stud which has been the usual fix for this problem for as long as BSA had been making the unit singles.
email (option): wariron@tpg.com.au
Hi Keith...Take a ride to clear your head...and don't worry about the oil level in your tank or the slight bit of external preservative here and there...everything will be OK...Ian
email (option): ian@wright52.plus.com