Is there any reason I couldn't just buy an antique vacuum gauge on ebay for 5 bucks and connect it to the ported vacuum on the back of the TB via some vacuum hose as opposed to screwing with a water manometer?

i could just adjust each side until it has the same vacuum reading

 

So, I'm confused...

How do you adjust each side of the TB? That port is vented to the manifold so you'd be getting the same reading from both sides.

Am I missing something?

 

My understanding is that the center port on each TB is connected to that small rectangle right above the throttle plate, and therefore the vacuum from it is directly proportional to the air flow past the TB at idle.

 

On multi cylinder motorcycles for a long time people struggled to synchronize the carburetors. There are tools that used up to four tubes full of fluid (in the old days it was Mercury). You simply connected a vacuum line to the particular carb and then you could "see" the differences and correct them easily.
These Carburetor Synchronizers are available online but be careful, you don't need a expensive one for once in a while usage. If you look you might find one with just 2 tubes for a more reasonable price.
I use mine when adjusting the four point idle screws on my Holly carburetor. It makes it a lot easier to have the right tool for the job!

 

Well it would be pretty easy to test the gauge connected to the same location and compare results. A manometer is easy to make from tygon tubing.

Sorry I'm not familiar with TBI and it's required vacuum measurements. But it's a common mistake with carburetors to measure vacuum at a "ported" vacuum port which takes measurement in the carb venturi throught. At this location vacuum increases with increased airflow when throttle blades open instead of decreasing as manifold vacuum does. I honestly don't know which you are trying to measure but thought you should be aware of how a ported vacuum will respond.

Hope this helps.

 

I bought a motorcye vacuum gauge set for a v twin that worked pretty good.
i did find i got better results by playing with the rpm, but i have a very smooth idle currently.

 

I have an 82 XFire so I'm dangerous with TBI. From your avatar I guess you have an 84?

You really don't want to individually adjust the TBI; the goal is to get them in synch with each other.

Your MAP (Manifold absolute pressure) sensor, TPS (Throttle positioning sensor) and coolant sensor will have the biggest effect on how your engine runs. Once dialed in damn near perfect and your engine will run like a maniac.

Once, synched up, the computer will adjust the fuel flow and timing.

Just put your vacuum on the rear of the manifold (not the TBI) to get the read on how your engine is running. The TBI ports are there to pull a constant (small) vacuum to your intake heat riser and the other side to the ERG. There might be variances in the TBI ports but the manifold IS the measurement to take.

 

I work on these engines AND YES....using a manometer is the way to go due to how precise the manometer is when getting the 6 inches of water. Do not to re-invent the wheel here. Sorry for beign so blunt...but it is the truth. You also MUST use the specific plugs to block of the air passages for your IACS....without them you are wasting you time.

Connecting the manometer to the correct port on the backside of the TB..and raising up the rear of the car and putting the car in drive while the rear wheels CANNOT turn.

There is a SPECIFIC procedure it the factory service manual. I follow it to the letter and I have yet to have a Cross-Fire engine NOT run right. And that is assuming that the throttle shafts are not so worn out I am having vacuum leaks which cause for an inconsistent idle from time to time.

I know some people use a piece of paper to set the throttle plates. But I can tell you this...that IF you use a manometer and see what it can do...and then think about it. The 'paper method' is something I would do ONLY if I was stuck on the side of the interstate in the middle of the desert.

And YES....I am aware that IF a person has there TB's bored out and larger throttle plates installed.....then using the paper method may be the only choice because the port needed has been blocked off and no longer available for the manometer to work.

DUB

 

Wow! Some valuable information there Dub and thxs for sharing here. I didn't know we are are trying to measure such little pressure here - 6"H2O.

I'd like to add that a manometer can easily be made from tygon tubing mounted to a small board and adding food coloring (any color) will help reading the vertical distance between levels under vacuum. A bubble level here will keep you honest in the vertical direction/distance. Also you can move the tubes out to 45° of vertical to greatly increase reading accuracy.

Good luck with it.