Vacuum gauges are a cheap but effective method of ensuring that your classics are well tuned and problem free. The Vacuum gauge is a useful instrument to have on any classic as it can be used to tune for power, diagnose engine faults and for maintaining fuel economy.
This article will cover tuning, with articles below about how to diagnose common engine problems using your vacuum gauge.

Whichever gauge you choose whether its a test gauge or a permanent instrument it will need to be connected to the car's inlet manifold. Often there is a connection or pipe already but if not it may be necessary to drill and tap the manifold.

One your gauge is connected and seen to be registering a vacuum you can begin the tuning process

(normal vacuum at idle should be about 18-22In (4 Cyl), 19-21In (6 Cyl) or 15-18In on low compression engines (In is an abbreviation which stands for inches of mercury))

• Start by first warming the engine and note the idle vacuum reading
• Adjust the mixture on your carburettor to obtain the highest steady vacuum reading.
• Richen the mixture setting until the needle begins to fall. This will give you the correct mixture setting for maximum power.
• Slacken the distributor clamp bolt, and with the engine still at idle advance or retard the ignition until the highest steady vacuum reading is obtained.
• Retard the timing until the vacuum gauge reading drops slightly (½In).

Your engine should now be tuned to perfection. Tuning with the engine running compensates for wear in the timing gear and therefore will provide better results than the manufacturers ignition settings.

Fault Diagnostics with a vacuum gauge

Engine in good condition will give a reading of 17 to 21	When the accelerator is pressed the reading will drop to below 5 then go to 25 before returning to normal	A reading lower than normal could indicate worn piston rings	If the carbs are not well adjusted then the reading could drift between 12 and 16
When the reading drops 3 to 5 then this could indicate a sticking valve	When the reading drops by more than 7 it could indicate a burnt valve when that cylinder operates	When the reading drops 3 to 4 it could be due to a leaky valve whenever that valve operates	When the needle vibrates between 14 and 19 it could indicate loose valve guides
A low reading of below 5 could indicate a leaky manifold, manifold gasket or carb gasket	When the reading is between 14 and 8 this could indicate incorrect valve timing	A reading of 13 to 16 could indicate incorrect ignition timing	When the reading drifts slowly between 14 and 16 it could be the plug gaps are too close or the contact breaker points are not synchronized
Wide variations could indicate that there are weak or broken valve springs, increases with engine speed	After a normal reading which then drops to 0 could indicate a blocked exhaust	An occasional drop as the cylinder fires could indicate that this cylinders plug is not firing or open valve	If normal is 20 and reading is 14 check timing as spark could be retarded
	Wide variations could be a blown head gasket

Installing a vacuum gauge

A Vacuum gauge is a very simple and safe piece of equipement to install in your car and as well as being a great tool for tuning can also be used to help with your driving style to achive a smoother and more economical way of driving.

There are a number of different designs of vacuum gauge available as shown in the pictures below, from a pure readout of vacuum in inches of mercury, to a colourful "performace meter" which gives an indication of the type of driving you are doing. Finaly there is an electricaly opeated gauge but with a readout in Bar. All of these types are availble with prices ranging from a few pounds to about 30 pounds.

To install a vacuum gauge a piece of plastic tube has to be attached to the inlet manifold. This can be done by removing the manifold and drilling a hole, then tapping it out and making a fixing so that the tube can be attached. This can be a bit of a chore but the manifold must be removed otherwise debrie from the drilling will find its way into the engine and cause lots of expensive trouble.

The way I did on my MGB GT was to remove one of the tapped bungs that are fitted to the manifold (these tapped bungs fill in holes when things like a brake servo is not fitted). I then drilled a hole through the bung the correct size to be tapped out to take a bleed nipple used for the brakes. Before I fitted the bleed nippled I cafully drilled out the interenals (so that it just hollow), this allowed the pipe to be fitttd easily. It was just a case of fitting the bung back into the manifold and running the plastic tube into the car to the place where the gauge is to be fitted.

Once installed and running it can be used to tune the car, use it to help with economical driving or just to have as an extra gauge in the car (you can never have enough gauges!)

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