According to the invoice from the rebuilder, I have a new VC-171 vacuum can on my distributor. I am getting 16 in manifold vacuum at idle. How much timing advance should this can provide?

 

VC-171 vacuum can:

starts at: 7-9"Hg
Max adv (Distributor degrees @ "Hg"): 8ş @ 14-16"Hg

remember that distributor degrees is HALF of crank degrees so when reading at the timing mark with a timing light it will actually give you 16ş of advance

 

Barry , considering the 2" rule isn't the VC-171 a bad choice since the OPs vacuum is only 16" ?

 

Roger, I wasn't replying on the "appropriateness" of the can, I was simply trying to answer the OP's direct question of the amount of advance the can supplied but I wouldn't disagree with your comment.

 

Well, it has no stop bushing and I am getting some slight chugging and a barely noticable popping in the exhaust at slow speeds. Going to investigate a little further to see what can is appropriate and available, then put in a stop bushing.

After break-in on the engine dyno, it pulled 408 hp with an HEI dizzy, set at 22 deg initial, 34 deg total on 93 pump gas.

The last time I had the timing set on this rebuilt original points dizzy, it was reportedly at 12 deg initial, 32 total with vacuum plugged. I was also told it still has the original springs in it. Going to check it myself tomorrow with a dial-back timing light and see where the total timing comes in at. Plan on setting it at 36 deg and see where the initial falls. A change of springs may also be appropriate...but it is hard for me to hear the death rattle over my loud side exhaust.

Who knows, by the time I get thru this fine tuning I may actually have learned something...hard to teach an old dog new tricks. Those papers by Lars are terrific!

 

Barry I apologize I didn't mean for my post to sound as if I was questioning you.
My post was a secondary question beside the OPs question. You answered his question without a doubt ,I was wondering if he should proceed with the 171 can or find one that will be "in" closer to 14"vacuum ?

 

Using a dial-back timing light, I found the VAC can was giving me 18 deg of additional advance on 16" of manifold vacuum. I get nothing, as expected, on ported vacuum. The guy who rebuilt my distributor is going to get me a VC-177 (B28) can and we are going to limit the travel with a brass bushing.

 

you can't get 18ş of vacuum advance from a vacuum cannister that is designed to provide 16ş. Those extra 2ş can't just appear from the can out of nowhere.
What may be happening is that when the vacuum can connected it's increasing your idle speed which is normal and expected when connected to full manifold vacuum - you can see this by unplugging the vacuum hose and watch the idle speed immediately drop and than reconnect it and watch the idle speed increase.
When the vacuum can is connected and increasing idle speed the centrifugal advance is starting to come in a little and that is most likely the reason for the "extra" 2ş of advance you are seeing.
That means that the centrifugal advance is actually coming in too soon and needs to come in a bit later which can be done by using slightly stiffer springs.
The centrifugal advance should not start coming in until approx 200-300rpm ABOVE idle speed.

Also, the B28 can isn't the best choice if you have 16"Hg of vacuum, it's designed for motors that have much lower vacuum levels and need the advance to come in fully deployed at those lower vacuum levels. Try another can such as a B20 or B26 can, which is the same as the VC1765

 

BarryK
Thanks for the explanation about that extra 2 degrees. It might also be my poor eyesight trying to watch the timing mark. Will take your advice on can selection, as well as the springs.

 

Just picked up a Standard VC181/B26 vacuum canister, which is more appropriate for my 16" Hg manifold vacuum. Also bought the Equus Innova 3568 digital timing light from Sears and it works great. Found out my centrifugal advance is coming in way too early and is somewhat erratic. Waiting on the brown truck to deliver a Mr. Gasket 928G advance curve kit with new springs and a brass bushing to limit the amount of vacuum advance. The guy that rebuilt my distributor did not change the springs and they are probably original. Once I get the distributor straightened out, I am going to fine tune the carb. This forum is the best and thanks for the help.