Vacuum Port Size

I was planning on making my own carb sync tool and bought some 1/8" ID tubing thinking this would be large enough to go on the vacuum ports on the carb holders, but it is too small to squeeze on. Anyone know the correct inside diameter tubing that needs to be used?

The bike is a '78 KZ650 D1 running Mikuni VM22SS carbs.

Thank you

On my mercury sync. set up the tubes are 1/4" O.D. and the vacuum fitings on my bike are metric but 3/16" I.D. tubing can be made to fit. I went to the local hobby store that sells metal (K&S is the brand I used) purchased some brass tubing and a small tubing cutter, and made adapters to fit what I needed to sync the carbs/injectors.

Thanks MFolks. 3/16" sounds like it may be right for mine. Thanks again.