Your MS 360 12 is an original piece and is worth a few bucks so DO NOT throw it away. My 69 427/390 also has that vacuum can. Specs are 0 degrees at 6" vacuum and 12 degrees at 12" so if you are producing 14" of vacuum at idle you should be good with a 2" buffer. It will give you about 12 degrees total vacuum advance.

 

Thanks for the tip CanadaGrant! Great to know what my setup is and is capable of producing. I doubt the centrifugal weight springs have been tuned, but I will soon find out. I'll follow Lars suggestions with that if I need to get timing to come in at at lower RPM.

I also read up on manifold vacuum for the vacuum advance. I currently have the advance can plugged into ported vacuum, which means it currently isn't even doing what it is designed to do when the car is at idle.

I plan to switch the connection to a manifold source and readjust timing/mixture/idle speed.

 

I had to switch the timing on my 11:1 CR LT-1 by 8 degrees total to run pump gas, or 5 gal race gas. 28 degrees vs 36 degrees. Measured it at a 30HP difference! Very noticable. But it was still easier to run it on pump gas 90% of the time, and save the race gas tune for very special occasions. Your cranking compression with the new gauge now sounds about where mine was. So your experience (may) be similar.

Be sure to let us know how the test drive goes!

 

Wow that is a pretty significant difference! I still need to set timing off of manifold vacuum, but I suspect I will have to back off the timing and not be able to achieve 36 degrees. That's fine to make the car a driver, but it will be good to know my "fun" numbers for when I have some race gas on hand.

I'll probably do some more testing, and I will definitely share what I learn. I'll do my best to take good notes so that what I post here can be useful for others.

 

So in order to switch to manifold vacuum I need to solve a little puzzle. I have owned this car for a little while, and kept the vacuum connections just as the previous owner did. I never once gave them much thought.

The engine has a Holley 4150 style carburetor with 3 manifold vacuum connections. One connection is dedicated to the PCV in the driver side valve cover. Curiously, BOTH of the other manifold vacuum ports join together in a T-fitting near the distributor, and then continue on toward the check valve and vacuum tank.

This seems odd. If the vacuum tank accumulates vacuum it only needs a single source right? My intuition leads me to believe that joining two manifold vacuum ports with a T-fitting doesn't accomplish anything more than using a single port, does it?

Please excuse the varnish on my carburetor. I intend to take everything apart and clean it when I get the chance. I recently got out of the Navy, have a 2-year-old son, and I'm in a masters degree program right now so I don't have infinite time...but I do plan to clean it!

Note the vacuum connections in the picture: distributor vacuum advance is currently connected to ported vacuum; the other two visible vacuum hoses are both manifold (one from front, one from rear) and they join together with a t-fitting.

The vacuum t-fitting I described is not visible. It is located under the blue AN fitting leading to the fuel pressure regulator. The regulator is safety-wired because I'm a former helicopter pilot and it seemed like a fun thing to do at the time.

 

Something that’s been bugging me, and I am not at the level of expert as many on this forum, are you sure your cam timing is not off by a tooth, or the cam was degreed incorrectly?

 

You got bit by a "related thread" .

This thread is over 5 years old.