Homemade Carbtune vs store bought?

Restrictors have two very important functions. They act as part of the dampening system to keep the instantaneous vacuum relatively steady so you can see the average vacuum. The other important thing is that they help isolate the manometer so the extra volume in the tubing does not significantly affect the vacuum in the intake tract. They don't isolate completely, other wise it could not get a reading, but it isolates enough so that the amount of actual airflow from manometer to intake tract is minimized.

One problem with a sealed type, that you have, is that it forces you to work very quickly or you will start sucking liquid into the engine. Another problem is that all of the channels interfere with each other such that if the restrictors are not identical, the reaction between channels can give a possible false reading. There is almost no way to make all of the tubing and restrictions identical. The wall rigidity and air volume will affect the reactance. It is better to compare each channel against a constant pressure, such as a typical mercury manometer setup does.

Here is a link to some notes on manometers:
home.comcast.net/~loudgpz/GPZweb/Manomet.../ManometerNotes.html

Here's the one I made:
The video compares it against a mercury type simultaneously connected to the same engine vacuum source.
kzrider.com/forum/21-tools/569922-latest...carb-synch-manometer

Unfortunately, without mercury being available, there is not a good, cheap solution available. Lighter liquids are problematic, from what I've read. The gauges work ok, but they just don't get a high precision.

In my opinion, four good, reliable mechanical gauges, that you can calibrate are probably the best way to go nowadays. But they need restrictors at the engine connection, and some means of comparing them against each other and normalizing them at the idle speed you are aiming for.

650 Ed had some photos of his setup, somewhere.

Here's mine.

I've had mine for over 20 years...works great