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'The Long-Winded One starts to blow again."
GENPIG, I believe Pat was referring to the synch linkages between the carbs. But yes, they all position themselves offa #3 as it has the idle screw.
(The blow-hard blows....)
Now then, the throttle butterflies. They are linked together by adjustment screws and springs to hold them in place. There is no slop in the system
(or a negligible amount) so I don't see this happening. Though I may be wrong, or reading Pat's intent incorrectly.
All the butterflies open in unison(by the linkage). What synch'ing does is to ensure that they all are in the same relative position. Once they are all the same, they will remain the same relative to each other. Vibration over time, and the twisting done to the carb bank when removing them and such, can get them out of synch.
Now then... concerning at what RPM to synch them. Two methods to synch carbs: static and dynamic. Static, as in using the bread tie, or drill bit method, or vacuum, which is easier and quicker, but actually less accurate. (The loss in accuracy is so minimal as to be ludicris, but needed to be mentioned so that others could disagree and provoke arguements)
I say bread tie is more accurate, as if done properly and exactly, you then will have achieved a perfect mechanical synch. (all butterflies open exactly to the same degree based upon their mechanical position, etc) But this is very hard to accomplish this way, as it's hard to gauge the adjustment drag of the bread tie from carb to carb, but from an engineering view, this would be the best method.
(The idea behind synch-ing, is to have all carbs, by the position of their throttle plates, allow the same exact amount of air/fuel to pass through them.)
The easy method(dynamic) is to measure the amount of vacuum generated (regulated by the position of the throttle plate) If the plate is closed a high vacuum will be generated in the manifold area... open the plate a little to allow more air to pass, and a lower vacuum is indicated. By adjusting the linkages between carbs, we get this vacuum even across the board and we say the carbs are synchronized. A good method, but with limitations.
Now for the crux of this arguement... at what idle speed to synch at; and herein lies the problem.
Vacuum synch-ing isn't so much aligning the throttle plates, as it is adjusting the vacuum. In a backwards way, the plates become adjusted as one fiddles with the vacuum measurement.
Vacuum is a reflection of general cylinder condition. Vacuum is formed by the downward movement of the piston. (Ok, technically, the piston doesn't draw in air... atmospheric pressure pushes it in to the open space, but we can fight that battle later)
You would have a fun time synchronizing carbs on an engine with low compression on one cylinder... like from a burnt exhaust valve. Due to the leaking exhaust valve, the draw from that cylinder (vacuum) would be lower than the others. You could mess with the adjustment screw and get the mercury lines all even, but the butterfly on that cylinder's carb would then be closed more than on the others. (The position of the butterfly determines the amount of fuel/air mixture allowed to pass) By restricting that carb, and raising it's measureable vacuum to be in line with the others, you have lessened the amount of fuel going into that cylinder, as compared to the others.
Ok... this is an extreme example just to wildly illustrate the point. But on a more subtler degree, let's deal with the valve train.
Valve clearances too tight/too loose determine how soon, or how late valves start to open, and how long they stay open. All these things determine vacuum. Differences between the cylinders also determine the amounts of vacuum. Inner cylinders run hotter than outboard cylinders, wear differently, and have different dynamic issues. So does engine speed. Again, all these things effect vacuum, as was noted previously that when one sets the synch at idle, it appears to be off when rpm's are raised. The mechanical synch hasn't changed at all, just the measureable vacuum has varied.
So... while the static way of synch-ing, (bread tie or whatever), is technically more accurate as far as throttle position is concerned, mechanically it's very difficult to achieve and is a pain in the rectum to fuss with.
I vacuum synch. (we use a digital gizmo at work which is about ten times more accurate than the mercury) And I vacuum synch at idle. Higher RPM's can change the flow dynamics of the clyinders. What also happens at higher RPM's is that the vacuum slide raises up a little which lessens the restrictiveness on the flow of air through the carbs which can also change your vacuum reading.
The idea again behind synch-ing is to get all the throttle plates synchronized... open to the same degree, not to regulate vacuum. The vacuum reading is a convenient method used to measure of the degree that the plates are open.
Adjusting carbs above idle is adjusting vacuum, while misaligning throttle plates.
As always, I welcome comments that agree with my position.
they come off the stops and now rest against the top of the linkage between the carbs
(The blow-hard blows....)
Now then, the throttle butterflies. They are linked together by adjustment screws and springs to hold them in place. There is no slop in the system
they come off the stops and now rest against the top of the linkage
All the butterflies open in unison(by the linkage). What synch'ing does is to ensure that they all are in the same relative position. Once they are all the same, they will remain the same relative to each other. Vibration over time, and the twisting done to the carb bank when removing them and such, can get them out of synch.
Now then... concerning at what RPM to synch them. Two methods to synch carbs: static and dynamic. Static, as in using the bread tie, or drill bit method, or vacuum, which is easier and quicker, but actually less accurate. (The loss in accuracy is so minimal as to be ludicris, but needed to be mentioned so that others could disagree and provoke arguements)
I say bread tie is more accurate, as if done properly and exactly, you then will have achieved a perfect mechanical synch. (all butterflies open exactly to the same degree based upon their mechanical position, etc) But this is very hard to accomplish this way, as it's hard to gauge the adjustment drag of the bread tie from carb to carb, but from an engineering view, this would be the best method.
(The idea behind synch-ing, is to have all carbs, by the position of their throttle plates, allow the same exact amount of air/fuel to pass through them.)
The easy method(dynamic) is to measure the amount of vacuum generated (regulated by the position of the throttle plate) If the plate is closed a high vacuum will be generated in the manifold area... open the plate a little to allow more air to pass, and a lower vacuum is indicated. By adjusting the linkages between carbs, we get this vacuum even across the board and we say the carbs are synchronized. A good method, but with limitations.
Now for the crux of this arguement... at what idle speed to synch at; and herein lies the problem.
Vacuum synch-ing isn't so much aligning the throttle plates, as it is adjusting the vacuum. In a backwards way, the plates become adjusted as one fiddles with the vacuum measurement.
Vacuum is a reflection of general cylinder condition. Vacuum is formed by the downward movement of the piston. (Ok, technically, the piston doesn't draw in air... atmospheric pressure pushes it in to the open space, but we can fight that battle later)
You would have a fun time synchronizing carbs on an engine with low compression on one cylinder... like from a burnt exhaust valve. Due to the leaking exhaust valve, the draw from that cylinder (vacuum) would be lower than the others. You could mess with the adjustment screw and get the mercury lines all even, but the butterfly on that cylinder's carb would then be closed more than on the others. (The position of the butterfly determines the amount of fuel/air mixture allowed to pass) By restricting that carb, and raising it's measureable vacuum to be in line with the others, you have lessened the amount of fuel going into that cylinder, as compared to the others.
Ok... this is an extreme example just to wildly illustrate the point. But on a more subtler degree, let's deal with the valve train.
Valve clearances too tight/too loose determine how soon, or how late valves start to open, and how long they stay open. All these things determine vacuum. Differences between the cylinders also determine the amounts of vacuum. Inner cylinders run hotter than outboard cylinders, wear differently, and have different dynamic issues. So does engine speed. Again, all these things effect vacuum, as was noted previously that when one sets the synch at idle, it appears to be off when rpm's are raised. The mechanical synch hasn't changed at all, just the measureable vacuum has varied.
So... while the static way of synch-ing, (bread tie or whatever), is technically more accurate as far as throttle position is concerned, mechanically it's very difficult to achieve and is a pain in the rectum to fuss with.
I vacuum synch. (we use a digital gizmo at work which is about ten times more accurate than the mercury) And I vacuum synch at idle. Higher RPM's can change the flow dynamics of the clyinders. What also happens at higher RPM's is that the vacuum slide raises up a little which lessens the restrictiveness on the flow of air through the carbs which can also change your vacuum reading.
The idea again behind synch-ing is to get all the throttle plates synchronized... open to the same degree, not to regulate vacuum. The vacuum reading is a convenient method used to measure of the degree that the plates are open.
Adjusting carbs above idle is adjusting vacuum, while misaligning throttle plates.
As always, I welcome comments that agree with my position.
.
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