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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Breather Timing: A long description of a different way (Part 1)
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<blockquote data-quote="Nigel Spaxman" data-source="post: 103154" data-attributes="member: 2907"><p>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. </p><p></p><p>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. </p><p></p><p>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. </p><p></p><p>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. </p><p></p><p>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! </p><p></p><p>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) </p><p></p><p>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. </p><p></p><p>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. </p><p></p><p>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.</p></blockquote><p></p>
[QUOTE="Nigel Spaxman, post: 103154, member: 2907"] 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. [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Breather Timing: A long description of a different way (Part 1)
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