The Stovebolt Site Forums The Shop Area Engine Shop Vacuum question (DUAL CARBS)

QUESTION:
I have a dual carb setup on my 235cui (two 1bbl rochester carbs) Fenton intake manifold. I have the vacuum advance line going to the front carb (closest to radiator) and the other is plugged.
I bought (2) vacuum guages of the same brand and plugged them into the vacuum ports on the carb. At idle, they read zero. Am I doing this right?
Someone told me there is an old guage that was used to set carb vacuum on dual Volkswagons that can be put on top of the carb. What is this instrument? Has anyone every heard of it?
I cant get my carbs tuned right...please help.

Make sure your gauges are not on a ported vacuum port, look for a port under or even with the throttle blades. You won't gain any information from two gauges as the vacuum will read the same no matter how you adjust the carbs.

You need a air flow meter like the Uni-syn as linked above. They check air flow through the carbs which is set by throttle opening.

Its a simple gauge that blocks the airflow just enough to create a vacuum in the attached tube. In this tube is a small ball. To use the gauge just set on the first carb and adjust it till the ball is 1/2 way up the tube gauge, then with out moving the adjustment set it on the second carb. Adjust the throttle till it reads the same as the first carb. It takes a few back and forth adjustments till you get the just right and get the idle speed just right. Unhook the linkage before starting and recheck after you rehook the linkage. Once set, don't turn the idle screws with out using the Uni-syn again.

All this is so each carb sees the same air flow and if they are a matched pair of carbs, the fuel mixture will be the same. If they are not match 100%, the air flow can be the same but one mixture could be way off from the other.

Joe

If it is hooked to a ported spot, you should get vacuum when you rev it a bit. If that is the case, it should advance your timing correctly.

When you use a vacuum gauge, you want to measure manifold vacuum. Connect it to the manifold, where the windshield wipers connect.

As for tuning your carbs,
Are they both rebuilt
Are they matched tags
Are the floats set the same
Horay, if so, you are half way there.

Set both mixture screws to the factory setting, which I think is one and a half turns out, check the shop manual.

Set both throttles to about three quarters of a turn past closed. You can see and do this easiest on the bench. Or, set them a little more open. You want the truck to run when started, and the throttles to be close.

Fire it up, and let it warm up fully. Buy a uni-syn. Read the directions.

You can also sync them by listening with a piece of vacuum hose. You want them to make the same sucking noise. After the throttles are in sync, you can slowly and evenly reduce your idle speed. After you are idling where you want, check that the carbs are still "synced".

Now you can turn the mixture screws, again, you must turn them evenly, to get the vacuum gauge to read the highest. The factory setting should be really close.

Now you can hook up the throttle linkage. And you will likely need to set your timing ahead. California Bill's book explains all this to a "tee".

Hey, Joe H is right on, and faster on the draw.

This might (or might not) help:

http://www.thecarburetorshop.com/Dual1barrelcarbs.htm

Jon.

Lots of good info above but before you get into any of that, make absolutely sure the throttle plates open at the exact same instant. If one plate leads the other by a nat's eyelash you're gonna have problems getting them in sync. Just as important be sure your linkage from the foot feed to the carbs is solid and smooth as silk, sloppy linkage will not get it done.

I learned this the hard way after screwing with the unisyn gauge for a week, a local 50s hot rodder had this all sorted out in about 15 minutes.
Good luck and also make double sure you have no vacuum leaks!!!