I had my '65 at a friend's house getting a frame swapped out. One thing led to another, 19 months later and it's quite a fresh looking car. The challenge is it's not running nearly as well as when I dropped it off. I am a fuel injection / electronics guy, I have no issues opening up a laptop and tuning a car. I know very little about the older, more mechanical setups and could use some guidance.

What is sending me down this rabbit hole is 2 fold; first, the car has always had an overheating issue while idling, which was "remedied" by a pusher fan. I'd really like to revisit this and am wondering if timing is an issue. Second, when I got the car back the vac advance wasn't connected to anything. Old pictures show it was connected to a port on the front passenger side of the Holley. It was mentioned to me that this was "port vacuum", which then got me thinking if this was all hooked up poorly to begin with.

Some basic questions / ideas I'd like to understand more:

1. Timing on these cars is set with your base setting at idle, mechanical advance, and vacuum advance. How is the mechanical advance set on a HEI distributor system?
2. Same basic question about the vacuum advance, how does this work?
3. It seems that an engine would idle "cooler" with more advanced timing. Wouldn't you want the vacuum advance portion to be active at idle? That would mean using intake vacuum rather than the port, correct?

Specifics about the engine, it's a rebuilt, 9.5:1 CR 350 with #7 heads, Edelbrock Performer RPM intake and cam, Holley 750 cfm carb. I have a shroud and electric fans on the way for it but I'd like to make sure I'm not applying a bandaid over some other issue.

 

Mechanical advance is controlled by the movement of the advance weights under the rotor. Centrifugal force causes these weights to pivot outward as RPM increases, and the shape of the weights causes the rotor to rotate relative to the distributor shaft, which advances timing. Mechanical advance is altered by changing the weights and/or springs. This is a trial-and-error process and it best accomplished with a distributor machine. If you have the stock distributor in a stock engine, there is little to be gained by playing with this. Mechanical advance is just about zero at idle and low engine speeds so has nothing to do with an idle overheating issue.

Vacuum advance is controlled by the vacuum advance canister. Engine vacuum sucks on the canister, moving a diaprhagm that rotates the breaker plate relative to the point cam on the distributor shaft and again changing the timing. Vacuum advance is greatest at idle and small throttle openings and goes to zero as throttle opening increases - typically at higher engine speeds where mechanical advance is maximum, vacuum is near zero and thus there is not vacuum advance.

The exception here is that some engines use ported vacuum instead of straight manifold vacuum for the vacuum advance source. Ported vacuum takes the vacuum source from a port that is above the throttle blades at idle, so there is no vacuum advance at idle. Once the throttle blades open enough to uncover these ports, then ported vacuum is the same as manifold vacuum. The problem is that without vacuum at idle, the timing is retarded and can cause overheating. Olds actually incorporated a thermal vacuum switch on certain cars that sensed engine overheating and switched from ported to manifold vacuum when that happened to help bring temperature down.

Since you have a bunch of aftermarket parts, we don't know how you have the advance connected, what the right advance curves are, or whether you even have the carb jetted properly. Lean mixtures will certainly cause overheating, for example. These cars did not overheat when new, so something in your modifications are causing the problem. You should not need electric fans to idle properly.

 

What are your timing settings now? You cannot use stock points settings on an HEI as the mechanical curve is different. You say you had an overheat condition but what were the temps at idle and cruise speeds?

 

Check out YouTube, lots of video on timing and distributor curving. Between the videos and the posts you'll be an expert in no TIME!

 

Something I didn’t know until recently that’s available for the older cars is Holley’s Sniper FI setup and how it can control your timing as well as fan operation, nitrous, etc. If you install a MSD magnetic distributor and lockout the advance mechanism, you can set all your timing parameters with Holley’s handheld device.

You say you’re into using a laptop to tune with, this puts you in that position after the initial setup.

I have tuned every street bike I’ve owned for the last 15 years and I’m looking forward to doing away with the carburetor and points distributor here shortly on my ‘69.

 

Being able to alter the timing map with a few keystrokes is much easier than the old school methods. Even I have to admit that. There are people who have used the distributor and computer from a CCC 307 to similarly allow timing control. You need the distributor, MAP sensor, and a computer with a custom PROM. Moates used to even make an emulator system that let you change the program from your laptop, so that once you got a map that you liked, you could then burn the PROM. I think people have also done this with the CCC distributor and a MegaSquirt computer. Probably not as user-friendly as the Holley, but likely much less expensive - especially if all you want to do is play with the advance map.

 

These are a good read,
http://www.camaros.org/pdf/timing101.pdf
http://www.crankshaftcoalition.com/w...ance_Specs.pdf

 

Thanks for all the replies. I've done some reading over the past day and the picture is becoming more clear. What's still uncertain is the source for the vac advance. Some people say to use port vacuum which does nothing at idle. Other's say to use manifold vacuum as it'll help with the heat issue at idle. What's your opinion keeping in mind emissions is of no concern and I'm running a (mild?) cam on the car (Edelbrock performer RPM)?

As for the heat issues, I use the term "overheating" probably too liberally. The car will cruise all day at or around 180-190. In sitting in traffic it'll go up to 210, that's where I start getting nervous. This was with a crappy shroud and an older pusher fan. It would only happen at idle or in traffic, airflow is certainly a concern but again, i want to make sure I'm not hiding and underlying problem by throwing fans at it.

I'll check base timing and see what it's set at when I have a moment, might be a while as the garage is now packed (backyard renovation means the garage is storage... )

 

Your temps are pretty normal with a warm ambient outside temperature and extended idling. The argument over ported vs manifold vacuum has been going on since the 70's, no one is really right or wrong, its what the engine likes is all that matters. Read the articles I posted above, it pretty much answers all your questions.

 

I've connected it both ways over the past 40 years and the current engine setup runs better on manifold vacuum. As said, try both and see which seems better for your engine. It's easy to switch back and forth, just change the vacuum hose connection and adjust the curb idle.