FBO-Est

Essentially, engine is air pump. Vacuum Test gives us a good overview of the engine trim. Now, however, to the point - the first test being carried out with the engine running.

1. Connect the "vacuum reader" into intake port and start the engine. In good condition motor the vacuum reader have to be between 15-22 Hg. Of course, camshaft profile and elevation from sea level may affect results. An example is that each of 1000ft (304.8m) above sea level change in the height of a reduction of 1Hg respectively. The same applies to highly modified engines!

If the reading is 15Hg or less it will generally be either worn or improper engine timing degree. Check timing and measure again. If the meter pointer "floats" there are usually reason unproper air / fuel ratio and mostly is mixture too rich. Adjust the idle speed screws or step on gas between 2000-2500 RPM, to see whether the result will change. If the reading is less than 15Hg may be a faulty gasket or lid block vacuum leak.

 The next step on gas pedal and release it . The engine speed should increase, pointer need to decrease a bit but need to remain in place after engine RPM-s are again came down. If, however reader pointer going to "wawe" you should check valve springs ( valve float). If you let off the gas, needle have to drop down to about 5Hg idle readings ,but need to be returned immediately into idle range. However, if a vacuum does not go so high there is probably the piston ring wear issue. If the vacuum reading is normal immediately after starting the engine, but rather falls to zero immediately and / or if you rise RPM-s smoothly to 2500rpm-s and the reading is the same, you have next suspect clogged exhaust.

If the engine is warmed up, try the cranking test. Disconnect coil wire and just spin them motor with starter. Check vacuum read at same time (you might need to take a friend for help if vacuum reader hose is not long enough) Vacuum read must remain constant. If not, start looking issues from cylinders. There might be issue with - camshaft, lifters, a leaking valves, piston rings, etc.. To find out which cylinder might have a problem, read next section - "Power Balance Test"

The following readings will not apply to turbocharged engines, or cars with a separate venturi for each cylinder (like Weber DCOE or Dellorto carbs). All readings are inches of mercury (in. Hg.).