Home made manometer for synchronising carb, video showing how it works.

This Is a home made manometer cobbled together using two equal lengths of 30mm clear, plastic tube. The two tubes are joined together at the bottom with a short plastic tube thus allowing the fluid levels to mirror each other. The tubes are stoppered with rubber bungs top and bottom to prevent leaks and a long plastic tube is attached at the top of the tubes for connection to the inlet manifolds. There is a small jet, measuring .5mm inserted into the ends of the two inlet tract hoses to keep fluid movement steady and slow.
Here’s how it works: Notice when I start, the fluid levels are the same, as the idle mixture screw on number two carb is turned clockwise, the idle on number two is increased, thus increasing the pressure in the inlet manifold and number two tube via the plastic tubes connected to the manifold. The pressure in the two inlet tracts and plastic tubes is now different: higher in number two, lower in number one. the increased pressure in number two tube pushes the fluid down, through the small connection tube into the number one tube thus lowering the fluid level in number two tube and raising the fluid level in number one tube. Now, as the idle speed is decreased, so too is the pressure in the number two tube thus allowing the fluid level to rise again.
When the fluid levels are level and equal in both tubes, the pressure in the manifolds is equal, so now the carbs are synchronised as far as idle speed is concerned. However, given each carb is operated by a separate cable, synchronisation when the throttle is wound on may not be correct. Adjustment is made in the same manner, only the throttle is held open a little more from idle whilst the adjustments are made to the throttle cables. https://scontent.fadl3-1.fna.fbcdn.n...aa&oe=5E943E15
https://www.youtube.com/watch?v=SWPGzy3BzCM




How a fluid manometer works to synchronise carbs.
https://external.fadl3-1.fna.fbcdn.n...Daop6132a6eZZp



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WIN 20200302 15 13 21 Pro
How a fluid manometer works to synchronise carbs.

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Originally Posted by sacc51

This Is a home made manometer cobbled together using two equal lengths of 30mm clear, plastic tube. The two tubes are joined together at the bottom with a short plastic tube thus allowing the fluid levels to mirror each other. The tubes are stoppered with rubber bungs top and bottom to prevent leaks and a long plastic tube is attached at the top of the tubes for connection to the inlet manifolds. There is a small jet, measuring .5mm inserted into the ends of the two inlet tract hoses to keep fluid movement steady and slow.
Here’s how it works: Notice when I start, the fluid levels are the same, as the idle mixture screw on number two carb is turned clockwise, the idle on number two is increased, thus increasing the pressure in the inlet manifold and number two tube via the plastic tubes connected to the manifold. The pressure in the two inlet tracts and plastic tubes is now different: higher in number two, lower in number one. the increased pressure in number two tube pushes the fluid down, through the small connection tube into the number one tube thus lowering the fluid level in number two tube and raising the fluid level in number one tube. Now, as the idle speed is decreased, so too is the pressure in the number two tube thus allowing the fluid level to rise again.
When the fluid levels are level and equal in both tubes, the pressure in the manifolds is equal, so now the carbs are synchronised as far as idle speed is concerned. However, given each carb is operated by a separate cable, synchronisation when the throttle is wound on may not be correct. Adjustment is made in the same manner, only the throttle is held open a little more from idle whilst the adjustments are made to the throttle cables. https://scontent.fadl3-1.fna.fbcdn.n...aa&oe=5E943E15
https://www.youtube.com/watch?v=SWPGzy3BzCM




How a fluid manometer works to synchronise carbs.
https://external.fadl3-1.fna.fbcdn.n...Daop6132a6eZZp



YOUTUBE.COM



WIN 20200302 15 13 21 Pro
How a fluid manometer works to synchronise carbs.

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You could have achieved the same result by just using the plastic tube looped into a "U" shape with each end connected to the carbys.

Nope, the fluid in a small tube moves up and down too much and frequently gets sucked into the intake. Consider the amount of fluid in a small looped tube with the amount of fluid in those large tubes. If the carbs are sufficiently un syncronised the fluid will get sucked into the intake track on startup. This is a common and well known problem with looped tube manometers, as you suggest. And of course, that was my first try. Many use two litre coke bottles, a little over the top but dead safe. So, l couldn't have achieved the same result at all. My manometer is capable of sucking fluid into the inlet if the carbs are out sufficiently. I installed bungs in each tube with a 0.5mm hole through to slow down the action, giving me time to adjust the carbs before fluid enters the inlet tract. So, I couldn't have achieved the same result at all, but, I could have damaged the motor. Using small looped tube as you suggested is simply a fools errand!

Rough out the carbies with a piece of tube by listening to the intake noise and a normal U tube should not spill water into either carby after that. I used to use a stethoscope when doing multiple SU's or Strombergs and never bothered going to anything more fancy though there have always been systems around to do what the manometer does.

The amount of fluid in a 6mm length of looped tube is miniscule, just a tiny change in vacuum will result in the fluid level in the looped tube moving an incredible distance. to negate this you would need a very long 6mm tube. Carbs with out vacuum controlled slides can be tuned reasonably close using popsicle sticks, tuning by ear is great as long as you have good hearing in both ears. That is a method of no use to me as hearing is one ear is quite different to the other, an anomally that is not exactly rare. Many claim they can synchronise just by ear, I would dispute that and a manometer or vacuum gauge would agree if put to the test.

I think any old bloke, like me, has tuned bikes by ear, using tubes stuck in the inlet, stethoscopes, paddle pop sticks, drill bits, dead cylinder methods and so on, and even convinced themselves that that they do a really good job. Realistically, that's just rubbish, if it's worth doing, then do it properly and accurately, using either vacuum gauges or the more accurate manometer.
I may be old, but I'm not stupid enough to think the 'by ear' method is good enough for any engine I rebuild.