Calibrating Vacuum Gauges

I'm curious as the best way to calibrate your vacuum gauges correctly. Before I synched my carbs I hooked each gauge, one at a time, to the carb #1 intake port and calibrated each one so that the needle measured the same on each gauge. What's kind of concerning is that when the bike was turned off all the gauges were now at a different starting point (see pic). Was I correct to have calibrated each gauge to the first intake port to make sure they all measured the same?

 

Technically, to callibrate those you need to set zero and span, repeat until done, or as near as the repeatability of the gauges will allow.
You won't be setting your vacuum at zero vacuum, so I wouldn't worry about it.

 

That is a lot of the reason I like using a home-made tubing style device for syncing the carbs. You are not measuring actual numbers anyway. You are just adjusting relative to the other carbs. Balancing the levels of fluid. Cheap too. I use ATF in mine and if it gets drawn in the cylinders you get mosquito discouraging smoke for a moment.

 

My gauges all register slightly different , but when running I have fish pump restricter so I open or close as required this keeps then fro bouncing around . But I also have the Morgan tune , this much more accurate and I prefer using this . Soon will be giving this a work out on my Johnny Cash bike .

 

What does it mean to "set zero and span"?

 

Quite simply to callibrate at (or better, near zero), and at full scale (or even better, near where you're likely to be reading them).
To do this you have to take the gauges out of the casings to access the mechanism, zero is a tweek of the linkage (or moving the pin around), span is a stretch of the bourdon tube to change it's leverage. You really need a mityvac type hand pump, and it sounds complicated, but really it isn't. I did a set I had lent to me in about half an hour.
The alternative is to note the differences in callibration around where you are using them and compensate for it, or move the needles around and ignore the zero error - just like you did in fact.

 

Okay. Awesome. But for clarification purposes let's say I have two gauges set to "0" and I connect the first gauge to the first intake port and it reads 0.7. Then I connect the second gauge also to the first intake port and that one reads 0.4. To "calibrate" the second gauge to match the first I adjust it to now also read 0.7. Does this mean that both gauges are equal and that I can now perform a proper synch?

 

Yes, that would be fine, but you should do a final check once you've finished with one gauge. This is because you don't know how far out the span callibration is between each gauge - ie one may read 0.6 and the othe 0.57, the correction you made only works for that point, and they both might be smack on at 0.7..

 

Ugh! I wish there was a Youtube vide explaining what you're talking about. This is all Greek to me. What is a "final check". I thought what I did was a final check. What is "span callibration"? "Bourdon tube"? "Tweak of linkage"? I'm not getting any of this.

 

you should have thought about making a two bottle bubble rig
no calibration and more accurate than your gauges will ever be

 

Maybe you should take @Polock s advice?
Or do a google search on callibrating pressure gauges, then re-read the above.

 

This guy does a pretty good job explaining. But yes, I agree, should've made my own.

DIY Carb Sync Gauge Set Calibration - YouTube