Updated Oct 20, 2022 ...
Demystifying The Tool Calibration Process
In the past, whenever I'd see the word "calibrate", I'd set a given task aside and move onto something else. A recent effort to synchronize v-twin carburetors on a 1988 Honda Hawk GT inspired me to move beyond this tendency. With our recent purchase of four carburetor synchronization vacuum meters, I felt it an appropriate opportunity to experiment.
“A bit of ingenuity allows for all of them to be stressed and calibrated simultaneously, greatly simplifying the process.”
Before I elaborate on the matter, note that synchronizing twin carburetors can be done without much difficulty using only one meter. First, plug one carb's outlet adaptor with your finger and then connect said meter to the other before taking a mental record of it's reading. Next, switch which outlet is plugged versus metered and synchronize accordingly, repeating the process until you hit common ground per the instructions in your service manual.
The only information I collected for multi-unit calibration is that each individual meter should be stressed under an identical load before adjustments are made. A bit of ingenuity allows for all of them to be stressed simultaneously, greatly simplifying the process. Below is what I came up with.
Three T-connectors and a bit of vacuum hosing keep the Mightyvac hand pump pressure(s) completely balanced. Four outlets, each equidistant from each other, receive the same draw via a vacuum connection at the direct center. Now it's just a matter of assembly.
For this specific set of vacuum meters, a knob on each must be rotated clockwise or counter to adjust the sensitivity. Loosening everything before slowly tightening is somehow a more accurate way to land in the "butter zone" for our Mightyvac. As the photo below points out, each dial is slightly off it's mark, so calibration is definitely necessary.
The service manual for our Hawk GT states that the carburetors should be within 40mm of one another with regards to vacuum pressure. I prefer more exacting figures, my end goal being a near-identical reading. This means I'll need my meters to be equally accurate, if not more.
My vacuum meters have a front side adjustment screw that's visibly located behind the lens. A twist of the wrist and all are exposed so that I can set things to an (inaccurate) zero before applying a vacuum and adjusting with more scrutiny. Pumping the Mityvac and tweaking the meters at different levels takes a bit of patience/precision but the end result is well worth it.
Balancing friendly outside advice against my own limited understanding of vacuum geometry, this slightly compulsive approach to calibrating four carburetor synchronization meters simultaneously has turned out to be both efficient and effective. While my gut tells me that mercury based tools are easier to work with, I like that these meters are useful in other ways (one tool, multipurpose). For complete details on how I went about all of the above, see the following video ...
How Do You Calibrate Your Accuracy Tools?
Vacuum meters are just the beginning. Many other tools involve a more elaborate process for calibration. What tools have you calibrated in the past? Was the process difficult and if so, how? Your input is invited. Leave a comment!