E: Engine Breather Timing: A long description of a different way (Part 1)

[Nigel Spaxman]

Vincent Breather Timing



I am Nigel Spaxman from Richmond B.C. Canada. Having always admired Vincent motorcycles in 2001 I bought a rough collection of parts from which over about a 13 year period I built a Vincent to ride. It is mainly an early B Rapide but I have built it as a C Shadow. It has a C Shadow UFM, Girdraulics, and is painted all black. Every bearing, bushing and shaft in the engine is new as well as the pistons, liners, cams, and followers. I enjoyed the process of this build very much. I now have more than 10,000 miles on this bike and most of the bugs are ironed out. The bike has taken be back and forth to work, and also on some 1000 mile trips with a passenger and luggage. It is a beautiful machine to ride. It was my dream to build this bike not buy it. Anyway Robert Watson says “ you don’t really own a bike until you have held the crankpin in your hand”. I have always enjoyed motorcycles and most of my bikes have been bought as basket cases or projects requiring substantial work. Often I have found that by starting with incomplete basket case motorcycles I can build really nice machines, but I have never bothered to stick very much with standard specifications. The best way to get a completely standard bike is to buy one that is already that way. Often I find some interesting ways of doing things on my bikes. I have always been someone who can think outside the box.

With the Vincent I was less restricted than with my other builds, because I didn’t have to worry much about spending so much money that I could buy a similar bike cheaper. Vincents are expensive so I could spend more money. I stuck pretty close to standard specification because I think that the Vincent is one of the nicest motorcycles ever made. There are a lot of very typical modifications that local Vincent people advised me were wise. I fitted sealed intake valve guides, and an O ring chain, a McDogualator and electronic ignition. During the process of this build I relied a lot on the advice and some help from the late John McDougal. He built my crankshaft assembly for me and fitted new oversize sleeves in my cylinder muffs. I did many other of the proceedures using John’s advice. I sleeved my main bearing housings back to standard, and fitted all new spindles to the crankcases, using John’s advice and methods. Sometimes there were other methods I could have used but I stuck pretty closely to John’s ways most of the time. Some of the problems I had later were to do with not listening to John carefully enough.

While building this bike I read everything I could find about Vincents. I relied on Richardsons, Know Thy Beast and the parts book. I read everything in MPH written by Neville Higgins. I also read everything I could find written by Phil Irving. I have ended up knowing a lot about Vincents.

Some stuff I figured out myself from first principles and also from what I knew from my work on Triumphs, Nortons, Ducatis, Hondas and BMWs. During my career in Engineering I have often been able to find simple solutions to designs that no one else thought of. I think that is what I have done with the Vincent breather.

 

[Nigel Spaxman]

One of the things that is written about almost more than anything else on Vincents is crankcase breathing. That must be because a lot of people have problems with it. If you read about BMWs, Ducatis and Triumphs this subject is hardly mentioned. The best stuff I have read about crankcase breathing is on the Norton forum by Jim Comstock. There are numerous modifications or completely new systems that people use on Vincents. Even the factory changed the breather system on the series D machines. John suggested there was nothing wrong with the original system, that was what his bike used and it works, provided the engine is in good condition, so I decided to take his advice. I didn’t take his advice without taking into consideration all the other systems everyone else was writing about, and also keeping in mind the systems on other motorcycles. After riding the bike for about 2,000 miles I decided to experiment with my own timing of the standard breather. I am pretty sure that the timing I use now resulted in less leaks than the standard timing and I have no intention of using the standard timing again.

There are two articles in “40 years On” from old MPHs and also some information in “Into the Millenium” which inspired me to experiment a bit. I know I am not the first person to do this. One problem is often the very best mechanics are often not the ones who will write anything down. There are some exceptions, Phil Irving being the most famous one in the Vincent world. Also Neville Higgins has written a lot. The two articles of most interest to me was the one written by Carl Hungness “ Engine Breather Altering Timing of” 608/17 Also Neville Higgins comments about that article 608/19. It was Dick Busby who was making this modification decades ago, I am not sure what he was doing really. It has still not been properly described in MPH as far as I know. You can easily read these articles by doing a google search if you don't have 40 years on.

Neville Higgins did a great job of describing how the breather is supposed to work, but I don’t agree with everything he says, maybe Dick Busby had it right although Carl didn’t quite describe it properly. Neville, to describe how the breather works decided it was simplest to describe the twin in terms of the crankcase volume as being like a big single with a maximum volume when the crankpin is pointed straight up between the two cylinders and the minimum volume when the crankpin is pointed straight down. I agree with that idea. The standard timing for the breather, opens the valve roughly when the crankpin is 45 degrees past the point of maximum volume, and closes the valve roughly when the crank pin is pointed straight down (minimum crankcase volume). Then he goes onto say that this is as it should be. As the valve is open during the period of reducing crankcase volume. Then he goes onto say that in Carl’s article Carl suggests advancing the timing by 30 degrees, which he doesn’t agree with (I don’t either). What Carl says says is a bit mixed up, but the important part is he says to pull out the breather gear and turn it two teeth anticlockwise. That is advancing the breather timing about 30 degrees. What I have found is that retarding the timing of the breather is what needs to be done to get the breather to work better.

The thing is the breather doesn’t need to open whenever the pistons are moving down or the crankcase volume is decreasing (as Neville suggest). What the valve needs to do is open whenever the pressure in the crankcase is higher than atmospheric pressure. If the valve is open when the pressure is lower than atmospheric air will go into the engine instead of out. The goal of having a timed breather is to achieve an average crankcase pressure lower than atmospheric pressure, buy letting air out. With the stock timing when the valve opens at the 45 degree position after maximum crankcase volume, it is likely that the pressure would still be quite low. The volume in the crankcase at this point is around 85% of the maximum. Even if the crankcase average pressure was at atmospheric the pressure would not come up as high as atmospheric until the point of crankcase volume of 50% (that would occur at 90 ATDC). The highest pressure is sure to occur at the position of minimum crankcase volume, but this is the position when the standard breather closes! It closes way too soon. The valve should definitely be open for a bit longer around the bottom of the stroke. Probably the standard timing setting lets air into the crankcase (instead of out) during the first 45 degrees of it’s opening.

I think the best timing might be to have the valve open for the standard duration of about 140 degrees but with that timing centered on the bottom dead center position of the crankpin. I don’t think with this timing there is any advantage in increasing the duration of the opening either. It might even work better with a bit less duration!

On my bike I set the timing by using a degree wheel and blowing in the breather pipe. I set the degree wheel so it read 0 when the rear cylinder was at 25 ATDC ad the front 25 BTDC I set the opening of the valve at about 90 degrees ATDC and the closing ended up at about 50 ABDC. This is about three teeth retarded from the standard position and really four teeth might have given an even better result, maybe I will try that next year. If you want to use settings from the TDC setting of the rear cylinder the figure to use would be 140 ATDC opening of the valve. Setting the timing of the breather in this way will make it act a lot like a reed valve would work. I have talked to some of my Vincent friends about this and sometimes mentioned it on some forums but no one seems to believe me! It does work. For some reason the factory seems to have gotten this detail wrong, and this has caused a lot of trouble since then. It may be because of this mistake that the factory changed to the D system. The D system shouldn't really work better than a properly timed breather. There is one other mention in MPH of this type of breather timing modification, it is “Engine Breathing” Sid Bibermann 756/10. He mentions a Steve Hamel built Comet race motor with breather timing opens 65 ATDC and closes 25 ABDC. (which is a slightly less radical change than I made)

If you have a Vincent that is like Carls was and you can’t get the oil to stay inside I suggest you set the breather timing as I have and try it out. I have not succeeded in keeping all the oil inside my Vincent, but I am getting close. I am sure this breather timing helps. The timing is not overly critical, but the original timing is probably at least 45 degrees away from optimal. I don’t believe in just moving the timing a few teeth as Carl did, as you never know what you are starting with. On my engine for example I have the breather spindle installed with the slot pointing straight down instead of forwards as it is supposed to be. The standard marks won’t work on my engine. You have to measure it as Neville says. It seems as though it is difficult for people to understand it but please try.

I know a lot of people resist my reasoning and they believe that the breather has to be open all the time while the pistons are descending. It might seem that my reasoning only works when it is assumed that there is very little blow by past the rings as is the case on my bike that has completely un worn bores round bores, new liners, and low expansion pistons, fitted with 2 ½ thou. clearance with thin moly sprayed rings. What about an old engine with Specialoids in worn bores, old rings and substantial blow by. That bike might work better with an elephants trunk or a reed valve system. However if it is going to run with the standard breather it will work best with the timing I specify. The reason is that the breather is only open for 140 degrees, that 140 degrees needs to be when the pressure is highest. That higher pressure is going to occur while both pistons are near BDC. Probably this change will make more of an improvement on a machine with a lot of blow by than on a machine with hardly any blow by.

Anyway maybe some of you can read this a few times and try to understand it. If there is a part you don't understand then ask me and I will try to clarify it. I know a lot of bikes run ok with the standard breather settings, but this made an improvement to mine. Maybe you can convince me I have gotten it wrong but I doubt it.

 

[Marcus Bowden]

Thank you Nigel my handsome, a lot of food for thought, nearly ten years I went after a rebuild in 1990 with out a wet patch along joints and consumption of oil, a pint of oil to a 1k mile yet I rode a borrowed bike in 2007 had the furthest distance travelled (it should have been Dan Smith & Robert Watson but they decided to leave for Tasmania before the rally finished) any way I returned back to Kapunda South Australia and had to put in nearly two pints after more than 5 k miles. so it is possible to achieve good results with various methods, during that trip we had a talk at one stage from a chap that woffulled on and on Idian Royal Infields picked up on his ideas he said.
Proof of the pudding is in the eating but the majority are little used.
bananaman.

 

[vibrac]

I think the logic is right the pressure is unlikely to ever be more than atmospheric so why open early?
However I cannot see the point in mechanically controling a breather when simple ball valves from the front inlet and rear exhaust in inconspicuous pipes leading to a small bottle resulted in a tablespoon of oil over a racing season.
As was mentioned a lot of the conversation was generated in the sixties when 10 to 15 year old motors were passing wind after a hard life.

 

[Vincent Brake]

I will open my timing cover and install it the way you do Nigel.

Thanx for your thoughts and experience here.

Cheers
Vinnie

 

[Black Flash]

@Nigel Spaxman
What you write makes perfect sense to me. I opened up the slot in the bush as described by Phil Irving and set it up a little less radical than you in my comet. Never had anything than slight mist coming out.

 

[Monkeypants]

Hi Nigel
That idea "You don't own it until you've held the crankpin in your hands" is a high bar to get over.
I end up only owning 3 out of 12 and 2 of the three are two strokes, hardly count!
Then I know a man who would say" big deal, you put a pin made by someone else in a hole made by someone else"
Thank God for Transfer Papers and Vehicle Registration!
I ll try moving the breather timing as you suggest if I have the timing cover off for some reason in the future.

Sounds promising!

Glen

 

[Nigel Spaxman]

I have 10 motorcycles. I have only had the crank pin (crankshaft out) on 6 of them. Anyway that standard of ownership was Roberts not mine. When you have had the whole bike apart it does give you a certain mastery that you can only have that way. Anyway it will be really interesting to hear if some of you get some positive results from this. It makes sense in theory and I have tried it and it works. It may not make a noticeable difference on a bike that is already working well. When I first did it I was hoping it would stop the leak from my decompressor mechanism, it didn't. In the end to stop that I installed an additional spring to put more compression on the rubber, that stopped it. I think my bike probably drips more oil than some of the ones with the standard breather set up, but I think the leaks I have have little to do with the breather. Last year the problem I had was with my McDougalator leaking! Also until I put in the Cometic gasket my primary leaked. I do get some white emulsified oil dripping out of my breather when I park the bike. I notice very little oil consumption at all. I like the idea of a reed valve breather. Jim Comstock did experiments on Nortons with different reed valve and timed breathers using a very high speed pressure sensor attached to an oscilloscope. From that he designed a few different reed valve breathers for Nortons. Probably you can't do any better than what he has done.

 

[Monkeypants]

My OZ Rapide was quite oil tight until I took it apart this winter.
I took the top end apart to find out where all the compression had gone and to fix a very slight oil weep at one cylinder stud.
I figured that after lots of hard miles it would need a rebore and new valves etc.
Happily, I was wrong about that!

Got the compression back by lapping the valves ( carbon deposits on valve seats) and fixed the leaking stud. Also created about six new leaks!
Now the one by one leak elimination process begins.
I seem to go through this every time I touch something on the bike.
Can't complain, 55,000 miles covered since the top end was last apart. John McDougall put all the good stuff in it at that time and there is still virtually no wear in there. The valve stems( Kemp's tuftrided and stellited ) measure and look as new.
The cylinders ( Terry Prince supplied sleeves) haven't any ridge at top, still a bit of cross hatch showing and measure less than 1 thou total taper.
Hastings the ring people say it needs a rebore at 12 thou taper. Ive got some riding to do!
Honda Chrome rings in there too.


Glen

 

[Michael Vane-Hunt]

Well the North American Rally in Illinois comes to mind. Good short run like that should find out if you have fixed your leaks.