What is a healthy Idle vacuum reading for a stock 460 (with Automatic)? How low can you be and still be healthy? I am currently getting a reading of 14 inches of mercury (is that the right unit of measure?)


thanks

 

Ack,
Genrally speaking vacuum is measured in "inches of water column"(WC). 14" of W.C. is low for an idle but mercury has a higher specific gravity which means you really have about 16 ft.(192") of W.C.
Your units are off. I believe you mean W.C.
Set your vacuum for about 16" of vacuum (WC) and see what you get.
KingFisher

 

"set"??? Uh-oh...I am in over my head....the unit of measure on the gauge is IN Hg and registers in increments of 1 up to about 30-40

From the instructions that came with the gauge, it was strictly a plug and play affair. I am tapped into a hose that runs from near the electric choke, to the carb body immediately below where the filter mates with the carb.

I tried one place before that, but I think it was a distributor vaccum advance, because it read 0 until I hit the gas, then it registered.

So if I am supposed to set something....how do I do that?

 

Yes IN Hg is inches of mercury. All the guages I've seen have the units in IN HG. With mercury the highest theoretical reading would be around 30 inches. While you could measure vacuum with a water column and in some applications that would be correct, to be meaningful I think the units would be FT H2O. And the highest reading would be in the high 20s to low 30 feet.

As far as setting the vacuum level, you don't have much to adjust. First make sure you are on a manifold vacuum source and not a ported vacuum from the carb. If you are reading 14 in at idle then you probably are, unless the idle speed is way high. The adjustments you have are the idle mixture and distributor advance. Adjust the idle mixture screws untill you get the highest vacuum reading. You may have to readjust the idle speed as you adjust the mixture screws to maintain the idle speed spec. Once the carb is set you can then adjust the distributor advance to get the highest reading again.

With both cases what happens is if the adjustments are off the engine is not running at its most efficient rate. As you adjust the mixture and advance the engine speed will increase if an adjustment was needed. This increase in speed occurrs without a change in throttle blade settings. Since the throttle blades are in the same place (same restriction) and the engine speed increases (more air pumped through the motor) the engine pulls more vacuum.