Motorcycle Carb Sync Guide - Balancing CB750 Carburetors with Manometer | Part 30

Motorcycle Carb Sync Guide - Balancing CB750 Carburetors with Manometer | Part 30
I don’t think you can buy a new set of mercury manometers to use for motorcycle carburettor balancing these days, but there are mercury-free versions, or you can use a set of pressure gauges. Here are some I would use.
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In this video, I use a set of mercury manometers to synchronise the carbs on my CB750 K0.
I have already serviced the bike, setting the ignition timing, valve clearances, sonic cleaned the carbs, and set the float heights. Now I needed to set the carburettors so that the cylinders work together.
The gauges measure the vacuum in the carburettors when the engine is running. You start by first adjusting the idle settings to get each carb to show the same pressure.
Once you have the idle pressure set, you open the throttle about ¼ of a turn to speed up the engine, then allow it to drop back to idle. You want each carburettor's pressure to drop in the same way.
You adjust them by seeing which gauge shows the pressure dropping the fastest and adjusting the cable for that carb to slow the speed that the pressure drops down.
If however one seems to drop slower than the others you adjust that one to speed it up. By a mixture of speeding and slowing the pressure change on individual carbs, you get them to all work at the same speed.
In reality, you never seem to get it exactly the same. Once you get them close it is time to stop. If you then go back to look again, it will nearly always seem to have changed. I usually give it two goes, and then accept it. You can usually hear that the engine is noticeably smoother.
This process is for non-CV non-fuel injected bikes. For CV carbs you only need to set the idle pressure, the other setting is handled by the way that the carb works using a vacuum.
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[@ciig4604]

I love your videos I've been watching for awhile and ironically a CB750F From a client Who was moving, Gave me it title and everything it was that or a junkyard so glad I took it under only has 20'000 miles 1 Owner. a stuck front brake and a oil leak these videos have been very helpful so thank you very much

[@SPANNERRASH]

Thanks, glad they have been helpful for you! Enjoy your CB

[@terrycarter1061]

Very good explanation thanks.

[@SPANNERRASH]

Glad it was helpful!

[@28bikes53]

Brilliant and simple explanation, thank you!

[@SPANNERRASH]

Cheers Matthew! :)

[@john111677]

Brilliant I've a question at the top of the mercury balancer there are 4 pipes with 4 little black caps on do you remove the 4 black caps before putting tubes on?

[@SPANNERRASH]

Yes, assuming your balancer is like mine, the black caps are to plug the ends so that the mercury doesn't come out in storage. You also need to open the larger tube at the back before you start to equalise the pressure with the atmosphere.

[@john111677]

@@SPANNERRASH brilliant thanks mate 👍. Ive the same one was my grandads need to use it tomorrow on a 4 carb bandit 600 👍. So the larger cap at the back remove it and keep it off while balancing?

[@johnk7375]

This was super helpful, thank you! Quick question: if the bikes revs just fall too slowly across all cylinders, is there a way to remedy that? Should I just keep trying to match them evenly or should I make a uniform adjustment for all carbs (for ex. Quarter turn counterclockwise or clockwise on all cylinders)?

[@SPANNERRASH]

Hi John, no problem! I would up all 4 screws by a similar amount, then when the bike is idling, try dropping each one in turn until you find one that is set highest, repeating until you have the tickover at a reasonable speed. Then carry on with balancing them. Hope that helps!

[@johnk7375]

SPANNER RASH thanks for the response! I got a manometer on the way so I’ll try that when it gets here