what are the symptoms of a failed vacuum advance on a tvr ch
Discussion
Your idle and acceleration will both be adversly affected, but the car may still drive 'OK'.
It's an easy thing to test though. Disconnect the small black pipe from the plenum chamber and suck on it. If you can feel the resistance of the diphragm then all is well, but if you can suck air through it, you need a new one. Mine cost about £50 IIRC.
It's an easy thing to test though. Disconnect the small black pipe from the plenum chamber and suck on it. If you can feel the resistance of the diphragm then all is well, but if you can suck air through it, you need a new one. Mine cost about £50 IIRC.
Same symptoms of any other car with a vac advance unit and a mechanical distributor.
Poor economy on light throttle cruising.
The job of the vac advance unit is to improve economy.
The vac advance unit takes advantage of the fact that you can get a fuel economy benefit from advancing the spark during light throttle cruising.
The vac advance unit responds to the increase negative pressure in the manifold when the engine is running in its mid RPM range but the throttle butterfly is only part open.
This negative pressure acts on a diaphragm inside the unit which in turn pulls a rod, the rod is attached to the distributor base plate.
So when the negative manifold pressure reaches a given value it advances the ignition timing.
The vac advance unit usually fails in one of three ways.
1) The diaphragm splits so the negative pressure fails to initiate any movement
2) The mechanism seizes up, this is often the base plate seized not the vac advance unit itself, so to be sure you need to check correct operation of both in isolation
3) The vac advance unit rod becomes disconnected from the distributor base plate
The best way to test the unit is to connect a vacuum pump to the vac advance unit to simulate a negative pressure in the inlet manifold.
Using a timing light operate your vacuum pump and you should see the timing mark move on the crank pulley.
If you don't have a vacuum pump you could just try sucking on vac advance unit, but you need good lungs.
Using the suck method you will at least be able to tell if the diaphragm is OK or punctured.
See if the diaphragm holds pressure and with the distributor cap off see if the rod moves the base plate.
Poor economy on light throttle cruising.
The job of the vac advance unit is to improve economy.
The vac advance unit takes advantage of the fact that you can get a fuel economy benefit from advancing the spark during light throttle cruising.
The vac advance unit responds to the increase negative pressure in the manifold when the engine is running in its mid RPM range but the throttle butterfly is only part open.
This negative pressure acts on a diaphragm inside the unit which in turn pulls a rod, the rod is attached to the distributor base plate.
So when the negative manifold pressure reaches a given value it advances the ignition timing.
The vac advance unit usually fails in one of three ways.
1) The diaphragm splits so the negative pressure fails to initiate any movement
2) The mechanism seizes up, this is often the base plate seized not the vac advance unit itself, so to be sure you need to check correct operation of both in isolation
3) The vac advance unit rod becomes disconnected from the distributor base plate
The best way to test the unit is to connect a vacuum pump to the vac advance unit to simulate a negative pressure in the inlet manifold.
Using a timing light operate your vacuum pump and you should see the timing mark move on the crank pulley.
If you don't have a vacuum pump you could just try sucking on vac advance unit, but you need good lungs.
Using the suck method you will at least be able to tell if the diaphragm is OK or punctured.
See if the diaphragm holds pressure and with the distributor cap off see if the rod moves the base plate.
Edited by Chimpafrolic on Sunday 6th February 22:47
Gassing Station | Chimaera | Top of Page | What's New | My Stuff