How To Check A Vacuum Advance

How To Check The Vacuum Advance on Classic Cars

This guide captures ‘bygone’ advice on how to check a Vacuum Advance on Classic Cars

To see all my ‘bygone’ advice guides please visit Classic Car Maintenance DIY Guides.

Remember folks these are ‘bygone guides’ … useful but the safety and personal protective equipment measures are reflective of bygone awareness. Stay safe.

The Standard Vacuum Advance

Vacuum Advance
Cross Section of Vacuum Advance
Cross Section of Vacuum Advance
The structure of the vacuum advance unit

A vacuum modulator was introduced to later cars as emission controls became tighter. I will write about these separately and link it in here, but for now here is a picture to help you identify what type you have.

Vacuum Advance Modulator
A vacuum modulator used for emission control

Checking A Vacuum Advance

The vacuum advance system will only work if the piping from the carburettor to the diaphragm, the diaphragm itself, and the diaphragm chamber on the carburettor side are all airtight

View the pictures below then read the written instructions that follow:

How To Check A Vacuum Advance
To check the operation of the vacuum advance unit, first remove the tube from the base of the carburettor
How To Check A Vacuum Advance Image 2
Take the cap off the distributor and then suck the open end of the pipe. The suction should move the breaker plate slightly.
How To Check A Vacuum Advance Image 3
If the breaker plate does not move the pipe may be blocked. Remove the other end of the pipe and blow through it to clear it
How To Check A Vacuum Advance Image 4
The vacuum advance unit is attached to the side of the distributor and it is easily recognizable by its shape

Step By Step Guide

  • First, remove the distributor cap. 
  • Next disconnect the vacuum unit from the carburettor and suck the open end of the pipe. Move the base plate gently forward with a screw­driver. 
  • Remove the screwdriver and the base plate should remain in the new position as long as your tongue maintains the suction. Removing your tongue should allow the breaker plate to spring back to its original position.
  • If the breaker plate does not move, first make sure that there is no obstruction-all wiring should be routed free of the breaker plate. 
  • Next check that the piping is unblocked. 
  • To do this remove the piping from the diaphragm chamber and blow through it. 
  • If the piping is clear, unbroken and in good condition, the fault is most likely to be in the dia­phragm.
  • It may be a ruptured diaphragm, a jammed spring, or a faulty air seal. 
  • Alternatively, but rarely, the linkage from the diaphragm to the breaker plate may be broken or damaged. If any of these is the cause you will have to fit a new unit. 
  • The vacuum unit is held onto the distributor by a bracket and by a connecting linkage. 
  • Undo the bracket and fine tuning nut holding the linkage to the breaker plate. 
  • Take care to retain the spring on the linkage. 
  • Fitting the new unit is the reverse of removal.
  • Finally take time to reset the timing
To Remove A Faulty Vacuum Advance
To remove a faulty vacuum advance unit, undo the retaining bolts on the side of the distributor and lift out the linkage arm.

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