Tuning

Vacuum gauges are a cheap yet effective method of ensuring that your classic is 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 as well as helping to increase and maintain fuel economy.

Whether the gauge you choose is a test gauge or a permanent dash mounted 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.

Once 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 for inches of mercury))

• Start by first warming the engine and note the idle vacuum reading
  
  
• Adjust the mixture on your carburetor(s) 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 using the manufacturers ignition settings.

Fault Finding

Use this hand chart to diagnose engine problems quikly and easily.

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 by 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 suggest a blown head gasket