Breather Timing

Nigel Spaxman

Well Known and Active Forum User
VOC Member
I know that this topic has been covered many times before and there are quite a few ideas about this. Carl Hungness, and Neville Higgins talked about it a lot.http://www.voc.uk.com/net/docs/3.5f/3.5-608-17.pdf Reading all of that and also reading lots of stuff about reed valves and PCV valves and non timed breathers, this stuff is all written about a lot and not just to do with Vincents. It is a very popular topic for Nortons many British machines and also Harleys.

It does seem as though most of the current thinking favors reed valves although most people probably use the original breather. I have the original one on my bike and it is timed as per Richardson's or Higgins by blowing in the breather and timing it by the closing point. I did a lot of thinking about this recently as I doubted my original settings and checked the breather timing again.

Yesterday though I was reading something in Chapter 20 of Roland Pikes Biography
http://beezagent.blogspot.ca/2009/01/roland-pike-autobiography-chapter-20.html that made me think that the standard breather timing is wrong! It is like what Carl was talking about, but the way he describes it in his old MPH article I am pretty sure that his idea is correct although reading the whole thing through the first time I tended to believe Neville. I love reading Neville's articles and tend to trust him absolutely. In Roland Pikes autobiography apparently on the MC4 bike that Roland was developing the timed breather worked poorly for racing until the timing of that valve was retarded 70 degrees from it's standard setting. (he doesn't say what that setting was) Set like that apparently there was a slight vacuum in the case at all engine RPM and loads. (exactly what we all want) This bike was a single 250cc. I believe like Neville Higgins that the V twin behaves a lot like a single but with the bottom dead center position of the single being comparable to the crank position at the bottom on the V twin.

The thing is that everyone is thinking of the engine as a pump so the breather needs to open at the TDC position and close at the BDC postion more or less. The breather is not a pump though it is a vacuum pump. Air will only go out the breather when the pressure inside the crankcase is above the pressure outside the engine. Really the valve needs to be open when the pressure might be high inside the crankcase and should close when the pressure is lower. That is exactly what a reed valve does. In order to do that though the timed breather should really be open all the time when the pistons are down, or in the lower part of the travel and be closed when the crankcase volume is high when the pistons are in the top half. This timing would be about 70 or 90 degrees different than the standard timing mentioned in Richardsons. It might be better. Carl and quite a few others think so although they can't seem to express it as well as Neville, but I think they are right and I am going to try it!

Please tell me what you guys think and what you have already tried.

Nigel
 

roy the mechanic

Well Known and Active Forum User
VOC Member
My rapide (built by Furness + Searle) has no breathing "troubles". My Norvin hot-rod runs standard breather timing and does not display any faults. For my money most breather troubles the the result of poor bores/rings or just plain wore-out + tired old motors.
 

mercurycrest

Well Known and Active Forum User
VOC Member
I agree with Roy.. Our outfit with Alphasud pistons, started blowing oil out the breather after years of a dry breather. Upon tear down, I found some pretty tired rings and replaced them with a new set of Hastings rings with a 3 piece oil ring and a chrome top ring, End of problem. Maybe it's time for a new set of rings?
 

vibrac

Well Known and Active Forum User
VOC Member
Yes I believe the engine condition is paramount. However our racing twin had shuttle valves on both exhaust cap pipes as I reasoned that a partial vacuum under the pistons could grab a BHP or so. A Season resulted in a tablespoon of oil in the mandatory 500cc catch tank so the engine was in good condition.
 

greg brillus

Well Known and Active Forum User
VOC Member
I had a problem with a Shadow that I restored that was suffering quite heavy breathing problems, but the main issue was hard starting and very difficult to obtain a stable idle. I worked out that the only difference with this and another Shadow I had also restored with the same problems, was the installation of Terry's MK 2 cams set as per his instructions. I ordinarily used his MK 3 cams with no problems, but he had run out of stock, so I used the MK 2 's instead. End result was that I changed the cams for factory spec MK 2 cams and this transformed both engines. They now start easily and idle nicely........It will be interesting to see if the breathing on the First bike settles down now. These bikes both had brand new 229/289 carbs and either new BT-H or rebuilt Lucas mag's. I also checked the cam timing before I removed them from the engine, and found that the inlet valves were closing at 85 degrees BTDC this is way too late a closing time for a road engine. I concluded that these cams are a "Race" cam only. The owner of the first bike has just spent last weekend at Broadford race circuit in Victoria (The annual Broadford bike bonanza) where he exercised the bike around the track, and reported that it performed flawlessly.
 

davidd

Well Known and Active Forum User
VOC Member
The thing is that everyone is thinking of the engine as a pump so the breather needs to open at the TDC position and close at the BDC postion more or less. The breather is not a pump though it is a vacuum pump. Air will only go out the breather when the pressure inside the crankcase is above the pressure outside the engine.

Nigel,

I have heard folks on this forum say several times that the breather should open at TDC and close at BDC, but I don't think that has ever been the case. If that is what the BSA disc was set at, then when Roland retarded it 70° he set it at exactly the same opening recommended by Neville. It seemed to me that the retardation of the opening and closing was due to the momentum of the air being displaced. But I would agree that a reed valve would probably best show the state of pressure in the crank case. If you could plot the opening and closing of the reed it would probably tell you exactly what the timing of a mechanical breather should be.

My beef with ball check valves is that they are slow to react. This is fine around idle, but at 7000 RPM, where I spend a lot of time while racing, the piston is going up and down 58 times a second. I don't think a ball can keep up with this, but I am not sure a reed can do it either, even if it does better than a ball. Further, I am not sure you need a breather at 58 times a second. As a result I have a PVC valve on the Egli and a reed valve on the racer. I also raced with a timed breather and I had the same results as with a reed valve.

It would be interesting to do some data collection, but running a reed valve is a good second.

David
 

Nigel Spaxman

Well Known and Active Forum User
VOC Member
I haven't really ridden my bike yet. I have about 10 miles on a fresh engine. It leaked a bit of oil out one of the pushrod tubes and I had to dismantle it and sort out an alignment problem. It was surprising how quickly the oil gushed out before the leak was fixed. That is why I double checked the breather timing. It was set properly. It at least showed there was plenty of oil in the timing chest. I think there may have been quite a bit of blow by at first on initial start up before the rings had seated at all. The engine seems much better now after a short ride. It idles fairly well already; at first it wouldn't. At first it was hard to kick over and start. Already it has settled down it mostly starts first kick now, hot or cold. It has 8:1s and Terry Prince MKII 105 cams. I still have to get the carburetors set properly etc.

The thing is though the standard timing even though it does work, does not really make sense. There is no reason for the valve to open when the volume of the crankcase is near maximum, but it should stay open while the crankcase is near minimum, not close when it will still be near minimum for another 30 or 40 degrees. The valve should try to do the same as what a reed valve would do. The valve needs to open when the pressure would be highest.

It may be that with the standard timing opening the breather valve at 45 degrees after crankshaft TDC that the pressure might be low enough in the crankcase that air would actually be allowed to flow into the crankcase through the breather.

It might be if the timing was set a bit later, that the engine would be able to tolerate worn rings for a bit longer before the re build or new rings would really be necessary. I think even Neville Higgins might see the logic in this. For some reason it does seem that it has worked for a number of people.
 

Magnetoman

Well Known and Active Forum User
VOC Member
It would be interesting to do some data collection,
I'm not volunteering to do this because I don't have the time, but it would be very easy to instrument a Vincent engine with a pressure transducer (or a half-dozen in different locations) and collect the data. 6000 rpm is only 100 Hz so any microprocessor made this millennium could process the results with one hand tied behind its RAM, and 1 Tbyte flash drives are so small and cheap the stores practically force you to take them ("buy a ream of printer paper and get a 1TB drive free...") so storage of the data until ready for download and analysis isn't an issue. Given this, someone must have done this already(?). Are they keeping the results secret so no one will know why less oil dribbles from their bike than from others?
 

greg brillus

Well Known and Active Forum User
VOC Member
Also the original breather used a very short length of pipe/flexible tube exiting under the engine, whereas these days we run a long length of line to the rear so as not to get oil on the rear tire. Therefore the breather has to work harder to exit any built up pressure due to this longer plumbing. Although this topic has been surrounding Vincent's since their beginning, it is quite fair to say that their breather system is in reality too small for such a capacity engine, and at elevated speeds you will soon find the shortcomings of the original set up, obviously this is all exacerbated with worn rings and scored/out of round bores, as is commonly the case. But my point above described another symptom that had nothing to do with a worn out engine.
 

Nigel Spaxman

Well Known and Active Forum User
VOC Member
I just re read Carl's article. He is recommending advancing the timing not retarding it. I can see how retarding it might help, and also I can see how the standard timing works. It is hard to see how advancing the timing would be any use at all, unless the timing was wrong in the first place.
 
Top