Vacuum Gauge

A vacuum gauge can be a very useful tool to check the performance or diagnose engine problems with your Model A. The condition of piston rings, timing, valves and other parts can be determined without tearing down the engine. The vacuum in a typical engine should be around 20” of mercury for a warm engine at sea level. For every 1000 ft. of elevation you should subtract 1”. The vacuum gauge should be connected to the intake manifold. Those with a vacuum wiper connection are set. If not, you will want to drill and tap your manifold. Consult the manuals for the correct position. Many “non-vacuum wiper” cars had porter manifolds, so you won’t be “wrecking your car” by doing this. Connect this gauge and allow the car to warm up. At a fast idle, the gauge should read around 20. If it is low, check your connections and also check to see that the timing is not retarded. If it is still low, check the spark plug gap, the breaker points and the ignition timing. If it is still low, adjust the carburetor. To do this, retard the spark and adjust the idle speed for the highest possible reading. Note that at low speeds the gauge will be bouncing all over the place. That’s OK…just go for the maximum reading. Next, advance the spark, set the throttle for a high-speed rev and make the adjustments for the highest vacuum reading using the high-speed dash adjustment. If this fails to give you the desired 20 reading, you still have a leak somewhere. Check, check, check until you find the cause. Your engine will love you for it. Carburetors are an often overlooked source of leakage, but are readily checked with a gauge. If you have a spare, swap it out and see if things improve. If not, check for leakage around the throttle shaft and the gasket. (This can be done by squirting some heavy oil on the leakage area and look for “suck-in” or the vacuum reading to improve.) Other sources of low vacuum due to the carburetor are: (1) bad idle adjustment screw; (2) blocked or incorrectly sized jets (by the way, the tape we have on the carburetor has a great chart for correctly sizing jets) will lead to poor high-speed vacuum. Always suspect the carburetor if the problem has been gradual in coming and suspect the ignition if it happened suddenly. Poor rings will give a steady low reading also. Likewise, late valve timing would give a low reading; suspect this if the engine has been running a bit hotter than normal. Remove the cover and check the gears. With the engine in proper shape the gauge should be rather steady around 20. If it is swinging wildly, you may have a problem. Worn valve guides or valve stems, bad valve seats, insufficient tappet clearance or a sticking valve will cause this. To differentiate between them, squirt a little engine oil into the vacuum gauge hose and reconnect the gauge. If the problem is gone for a short while, you have a sticking valve. To check the valve springs, rev the engine. The gauge should drop to around 5. Quickly return the throttle to idle; the gauge should bounce up to around 25 and then return to approximately 20. If when revving the engine the gauge does not go to 5 and stay there but rather bounces around, the springs are weak. (What you will typically see in this situation is the spread between the high and low reading will increase as the engine speed is increased.) If the reading does go to 5, but then slowly goes up from 5, chances are the valves are leaking. If when your engine is idling there is an intermittent drop in the reading, you may have a bad cylinder or a leak in the head gasket. By shorting out a spark plug, one at a time, you should be able to detect which cylinder is the problem. If so, check the gasket for leaks and check the torque on the head bolts (use the proper setting and sequence). If you find two adjacent plugs to be at fault, suspect that the gasket is blown between the cylinders. Just remember that the vacuum gauge is only one of many tools at your disposal. It’s a good idea to take readings from time to time so you know what “your” engine normally does. Because many troubles can cause similar vacuum reading, it is important that you use the other tools available to zero in on the right one. Common sense and the old process of elimination are sometimes all we have. This tool will hopefully help you get to the correct answer faster.

This tech tip was originally provided by Ron Sieloff and printed in the May 2001 “A” Quail Call.

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