Good morning all,

Just looking for a little help interpreting the distributor test specs in my '66 Olds service manual. I'm having a hard time figuring out exactly what it's telling me on the vacuum advance side. The mechanical advance (per dist RPM) seems straightforward.......at 600 RPM you should be getting 0 to 2 degrees of mechanical advance. At 900 RPM you should see 3.5 to 5.5......and so on, got it.

The vacuum advance however, isn't quite so simple to read and I'm actually not even sure what it's saying. Maybe I'm just missing something. Is it saying that you get 9 degrees of vacuum advance at all times?

 

Looks like it's saying that vacuum advance starts to move (0*) at about 8-10in-Hg and all in max at 9* for 16.5-18.5in-Hg

 

You also need to multiply those numbers x2 because the crank turns 2x times to the distributor.

 

That is quite confusing. Does the vacuum advance go to ported or manifold vacuum?

 

It goes to ported vacuum, from the CSM.

 

It does not matter, those are the distributor advance specs.

 

This sounds right the more I look at it. Thanks!

 

Jeez, talk about confusing! I guess it's something that mechanics were much more used to seeing than someone like me who comes along 50 years later trying to figure everything out as I go.

 

When you look at timing on the crank with a timing light, that is crankshaft degrees. So, 9* of vacuum advance at the distributor translates to 18* on the crank reading. Same for the mechanical advance.

 

Well, I came across this great video showing how the distributor is tested on an old school type machine. Looks like the testing is right out of the Olds service manual as well. Specs are a bit different, I think but the procedure is pretty fun to follow (I thought so anyway). Clarifies all my earlier questions.