crankcase pressure etc

[KeithJ]

Having some pressure problems after 200 miles with my A10 S/A with a completely rebuilt engine, I found the posts on the Bunn breather.  What I don't understand, among many other things is, if it requires to have a valve to allow air in and one to allow air out, why not just have a tube with no valves on it?  Is it because at some time there is a "dwell" when both valves are shut?  I thought there was a period where the engine required a specific time to be in either positive or negative pressure, can't recall which.

Appreciate some pearls of wisdom.

Keith

[rocker21]

many years ago Eddie Dow used to sell a pair of rockerbox  covers that were finned and the rear one had a breather in it and you fitted a pipe to it and took it out to the rear of the bike, problem solved

[KeithJ]

Had "vent tubes" on my engine for years before I rebuilt and it did leak some oil form the engine breather but I think that was due to worn bores.  Will probably go back to some breather on the rocker box and see how it works.

ATB

Keith

[Tomcat]

Ive read about the Bunn breather too, and came to the same conclusion.  Triumph have a huge breather pipe into the primary case which I assume is to neutralise the pressure and vacuum.
What about racing engines? Heavy breathing must be a serious problem when you thrash the f*** out of a vetical twin? Rocket Racer and Muskrat???

[KiwiGF]

I'm not sure where the allow air in bit comes in, but the basic principle I think of the breathers on our type of engines (e.g. with pressure fluctuations in the cases) is to allow air out when the pistons have compressed any gases at around bdc, and to prevent air entering the cases when the pistons reach around tdc, and could potentially suck air in through the breather outlet.

The standard timed breather theoretically does the above and hence a vacuum develops in the cases after a few revs after start up, once the vacuum has developed the breather will just let any gases from piston blow by out at bdc, these gases should be minimal....all this can be seen by putting a tube in the breather outlet, with one end in a jar of water and starting the engine.

Creating a breather in the rocker box (which should share the same vacuum as the cases) would seem to negate the design principal behind the timed breather, but it seems to work for some.

From what I've heard it's very difficult to make a non return valve (which is what the bunn breather essentially is) that can operate reliably at high speed (which I guess is why BSA designed the timed breather) the bunn breather claims to operate at the required speed, but there are differing opinions on whether it works well in practice (and can replace or supplement the std breather) or not.....or is it solving a non existent problem  or making things worse

I've no experience of a bunn breather myself but a friend had one fitted but he eventually reverted back to the standard breather after a major investigation into his continuing his "blow by" and crank case pressure problems, this meant addressing blow by from all possible sources (Pistons, rings, bores, valve guides etc).

[BSA_54A10]

back in the days when engines had no oil seals and peak revs of 4000 rpm engine designers used the pumping pressure under the engine to create a partial vacuum in the crankcase thus at all of the places with oil slingers and all of the shafts with scrolls sucked air in thus reducing the amount of oil leaks.
No one bothered to challenge this idea for decades so it was thought you need a low pressure zone in the cases,
BSA originally used an unsprung flapper valve and these work fine on low reving long stroke engines with modest compression ratios and slow acceleration rates
By the time BSA started making Gold Stars, it became apparent the simple flapper could not cope, in fact flappers end up working backwards at high speeds and get totally out of phase with the engine, they also spew out oil vapours.

Thus BSA changed the position of the flapper on the singles and went to a timmed breather on the twins and latter unit singles.

However air has mass and there fore inertia, it will also compress before it starts to move so again the port ends up being out of time with the flow of gas.
BSA did a fairly reasonable job considering the amount of money available for this type of research remembering it was slide rules in those days.
No thermionic cameras and no computers and no one doubted you need to have a low pressure in the crank case.

However it was found that a flow through systems actually works better and remains in time with the engine pressure pulses.
Thus the Bunn Breather, a system that had been used in cars for decades but no one thought it would be beneficial on motorcycles.

The next problem was finding a valve that did not resonate anywhere within the operational range of the engine and that was hard.
On an A 10 you close off the timed breather in favour of two non return valves, one lets air in once a specific low pressure signal has been achieved while the other lets air out.

On some engines the valves can be the the same but on a lot of engines the valves need to be different and on others still no valve is needed on the inlet, it is all engine specific.
Then we come to the outlet.
Outlet tubes need to be tuned, same as an exhaust pipe so the diameter, length & stiffness of the outlet tube all need to work together to get the best results.
He did get up to 2 Hp increase out of some engines.
Royal Enfield & Harley both endorsed the system and originally  offerred them as a factory approved dealer fitted accessory.
Enfield then modified the breathing on subsequent new models based on the work Rex did for them.

If you normally ride sedately you may or may not derrive a benefit from fitting one.
BSA's own breathing seems to work reasonably well at speeds between 30mph & 60 mph on most models with standard gearing.
However I can testify that a Bunn will work better and a lot of previous leaky bikes have become oil tight through fitting one.
With A 7 & 10's the big advantage is the end of rocker box weeping.
Some will find a pair of over the counter check valves work just as well as the Bunn, it is factor of the actual bike and the way the owner rides it.
However the Bunn will work with everything because the resonance of his valve is outside the operating speeds of your bike.

I is exactly the same principle as resonance in reed valves ( which are a one way check valve ) fitted to 2 stroke inductions.
What works on one bike and makes a huge difference can actually make a different bike run worse. 

[nimrod650]

rocker box breather in past years i drilled a hole in rear cover to take a brake or clutch cable adjuster two fibre washers and length of petrol pipe job done

[muskrat]

+1 for the Bunn breather. Unfortunately no longer available.
Cheers

[mugwump]

Early on in this post NIGES stated he used USED pistons and new rings. How USED were the pistons I wonder. If the ring grooves or lands were worn this could cause the rings to 'pump' oil into the comb. chamber.

[Klaus]

Find some time to whrite some lines about cankcase pressure.
All my engines had the genuie timed valve with no extra holes and other breathers.
At any time I had no problem with oilleaks or heavy oilconsumption, doesnt matter if i ride road or racetracks.
There is only a little more than a pint air in the crankcase that will pressed and expand by the up and down from the pistons.
This air will find his ballance in the first seconds after fired up the engine.
There is only one small hole in the timingvalve giving the full port open some hundreds of a second.
Imagine you driving with a 4000 revs it is fantastic that a pint of air will find his way out.

Oilleaks causing in bad gaskets, loosen bolts or nuts. Oil out of the breatherhole is a result from worn out corkwashers fixing the breathervalve, and blow by from the pistons, that give a heavy preasure to the crankcase from the combustion.

Check the pistonring gap, and be sure that the rings with the right side up, the are signed.
I trust no dealer with aftermarked spares I check it all. If parts are scrabby I send it without exception back to demand a good quality or money back.

I have done with a mate a two and a half week trip to Ireland and back to Germany with a full milage of a little more than 4000km.
Both bikes (A10 engines) had an oilconsumption from only 1 litre oil, all the way.
Ok after this trip the rockerbox gaskets was worn out, but this was the only one was leaky.

cheers Klaus

[KeithJ]

As far as I know, I have assembled the engine correctly.  I did check the points you mention and I should be OK.  As I have not run in the bike, should I be hopeful, it will get better?
ATB
K

[Topdad]

Keith,I vented the rocker box and it stopped the rockers weeping, really easy bit's to use are free standing gas fire pipe connectors approx. diameter qtr inch hole drilled washers each side and tighten cost less than a £5from local p/merchants done in an hour,.

[t20racerman]

Hi all

A very interesting thread. My A10 has long had oil leaks from the top end, but after a rebuild, copper rocker gaskets, surface preparation etc, its fairly oil tight... UNTIL you hold high speeds for long periods of time. If you hold 80mph (about 4000rpm on mine) for along distances it starts to leak from the top end - as well as out of the tacho cable connection, the inner timing case tacho cable fitting boss and even the oil tank filler cap!. Clearly seems to be a high revs only crankcase breathing problem, as described by many. I have a tube on the engine breather leading to the chain, and very little (if any?) oil comes out there since I sorted out the pistons, bore and valve guides (to which I added seals on the inlets).

As this is a higher revs problem, I was going to try the rocker cover mod, but am unclear from this thread if that badly affects the main timed breather, or just helps sort the problem out. I wondered:

1) Does anyone use the 'road draft tube' idea? This would be a pipe running from the valve cover down to an open end facing down and located in the vehicle's slipstream so that (ref Wikipedia):
 "When the vehicle is moving, airflow across the open end of the tube creates a draft that pulls gases out of the crankcase. The high location of the engine end of the pipe minimises liquid oil loss. An air inlet path to the crankcase, called the breather and often incorporated into the oil filler cap, meant that when a draft was generated at the tube, fresh air swept through the crankcase to clear out the blow-by gases."
As a physicist, (but not an A10 breathing expert!) this makes a lot of sense

2) If not the above, do you need a collector, or just vent to the air/chain etc via a tube

3) A PCV valve was suggested - is this necessary, and has anyone tried this? I don't have an airbox though, so it seems this would not work correctly.

I'm happy to modify and experiment, but am only looking for rocker box mod advice as everything else is spot on.

Just for background info, my A10 is heavily modified, runs 9:1 pistons, has gas flowed head, spitfire cam etc.

Thanks
Adrian

[edboy]

there are lots of different problems mentioned and probable causes of oil leaking. my rockerbox doesnt leak oil as i prepare joining faces with a file and wellseal on all the joints. no air is drawn in and no oil is blown out. simple.
before engine strips i would clean and reset the PRV so oil can go where it was designed to go. bsa recomended every 1000 miles i believe. however i also have a nice weepy oil surface on the crankcases which deters corrosion.

[Topdad]

I used a second hand gas fire fitting ,fitted to the rear tappet cover and joined to rubber tubing running down towards the rear mudguard didn't cost a penny and works.