Vacuum Guage Reading

[john lokay]

This past weekend, after I took my 1966 425hp coupe out for a ride, I decided to take a vacuum guage reading. I hooked up the guage to the hose that runs off the bottom of the carb and connects to the vacuum advance unit. It is my understanding that this is a manifold vacuum source and not ported vacuum. I prefer to take a reading from the opening on the top side of the intake manifold but I can't get the plug out. Anyhow, the reading I got was a very steady but low 8 inches of vacuum. Given the cam in the car, which is a Sealed Power replica of the factory original, I thought I should be pulling around 14 inches of vacuum or higher. According to the vacuum guage operating instructions, a low readings of 8 indicates a vacuum leak and/or late valve timing. Any thoughts out there! Thanks

[Duke W.]

What were the idle revs?

The manifold vacuum will vary with idle speed. Eight inches sounds a bit low, but may be okay with the idle down at 700 or 750.

Take vacuum readings at several idle speeds - say 800, 900, and 1000. Also, give us a feel for the idle quality. Chevrolet often listed unrealistically low idle speeds for Special High Performance engines. If memory serves, I think your L-72 idle speed is specified at 750 or 800, but 900 or 1000 should yield much better idle quality and more vacuum.

Also, did you plumb the vacuum gage into the vacuum advance line with a "tee". If you disconnect the vacuum advance to hook up the vacuum gage, the readings don't mean anything. The tee allows you to check engine vacuum without disturbing the idle setup.

Duke

[Chris H.]

Re: Vacuum Guage Reading

That reading will be a lower reading because the tap is upstream of the throttle plate. The highsest vacuum reading will be downstream of the throttle plate, i.e. the vacuum tap for power brakes.

[John H.]

Re: Vacuum Gauge Reading

Although not "correct", I use a vacuum fitting on the manifold behind the carb with an extra tap for the vacuum gauge, and plug it with a rubber cap when not in use. Gives a solid vacuum reading irrespective of weird vacuum tap arrangements on carburetors ("ported" vs. full, etc.).