Vacuum advance
 

While giving the engine in the C8 a run with the engine covers removed, I can see the distributor rotate with the vacuum advance.

This is good read: http://www.eevac.org/A-5-Motor%20Vacuum.htm

At idle my 216 doesn't rotate the distributor at all. I have a old echlin vacuum gauge on the windscreen wiper pipe and it sits on 20 at idle and about 17 on a steady throttle.

Does the dizzy shift as you start accelerate?
 

Hi Mike

Seeing the timing marks on a Stovebolt has got to be one of the hardest things to do tuning a Chevy CMP. So it is real hard to see the input of the vacuum advance. Couple with this the mechanical centrifical advance adds to the fun. Would add to the discussion that checking the function of the vacuum actuator is one of the first checks to make. Have encountered more than one that no longer worked.

Checking that the centrifical advance works is important. Have found these stuck, springs missing etc. Without one of those fancy bench distributor test units that allows you to spin the shaft without the cab on and see the advance move and of course check the advance curve we are pretty much limited to testing the timing with a timing light, with the vacuum advance disconnected.

Will have to look through my engine test stand videos see if there is any good video showing the vacuum gauge and the distributor at the sametime.

The link to the article is an intersting read.

Cheers Phil

PS I have a small vacuum gauge mounted on my Pat 13 C60S permanently good indicator of engine health. For example vacuum starts to drop or worse starts to flick time to check the valve adjustment and quick. The space above the windscreen is a great place to add extra instruments without cutting into the dash. On my C60S have vacuum and extra oil pressure gauge with the 261 the dash oil pressure gauge is pegged most of the time. At times have also had extra electric temperature gauges mounted up there to understand engine temp from front and rear of the engine under load.

vacuum
 

Hi Phil

Yes tuning these old motors is somewhat of a guessing game , with most of us having only basic test equipment . It pays to stick to the same type of fuel ( octane ) , and then you can go on from there and fiddle with the adjustments.

With the Lucas distributor on my Land Rover , I replaced the perished diaphram in the vacuum advance, I managed to pry the disc open and made a new diaphram using some Nitrile rubber sheet. With a simple Test - sucking on the pipe , I could see the base plate rotating so it all seemed to be OK.