Hello Together,

I have a bad ideling, Everything around ignition is already checked and in good condition. Last option would be theVacuum Advance . Symptom is below around 2000 rpm the engine works very badly and dies. Could it be a worn Vacuum Advance ?

Is there another way to check if it works correctly or not ?

Thanks in advance

Frank

 

Its easy to find out if the Vac Can is doing anything. Just simply disconnect the hose at idle from the carb and plug the carb port. You should notice a reduction in RPMs.
It may want to quit running.

Another test is to purchase a mity vac hand pump tool. With the engine running and carb vac port plugged, apply vac to distributor with the tool. This time you will note an increase in RPMs, or should.

So how much advance timing is the canister providing? That test is a simple subtraction. Using a timing light, note the before & after of the timing marks on the dampener. With vac. Then w/o vac. Simple.
Also, remove four plug wires from dizzy cap on either side. Lay dist cap over on its side. Apply vac w/ tool and verify the advance mechanism is truly moving without interference or binding.

You want about 10-12 degrees worth of Vac Adv. You can purchase any vac cans to suit your needs. There is a GM list of the vac cans about a mile & a half long showing when the can vac begins & ends in inches.
Or, you can purchase an adjustable unit. You will need to know what your engine is pulling for vac in inches. Always use a known manifold vac port. Not "timed" port.

Too much vac adv results in a term known as "trailer-hitching". It feels like one minute a trailer is pushing you, then pulling you back. All this takes place on level ground at different RPMs or different vacuum loads.

 

A timing light, vacuum gauge, and Mity-Vac are critical tools for these cars.

@HeadsU.P. covered most everything. Perhaps this image of bench testing an adjustable vacuum can helps.

Also, you may immediately see an idle improvement if you make sure the vacuum can is connected directly to a manifold vacuum port on the carb, not ported. You can check with your finger, or a vacuum gauge.