1966 L72 running hot

[Duke W.]

Re: 1966 L72 running hot

Even with ported vacuum advance and a few degrees of initial timing 9" at 850 is indicative of a cam with WAAAAAAAAY too much overlap. Vacuum readings at higher RPM are of no consequence.

The Airtex 4V1053 should have the B28 ID on the part. Test the stop and start points. The engine might pickup some additional vacuum at 850, but set the idle speed higher if necessary to achieve 2" more manifold vacuum that the amount required to pull the VAC to the limit.

Set initial at about 8 and when you connect the B28 to full manifold vacuum it should add about 16 degrees, so total idle advance will be about 24. You will need to go through the idle speed/mixture adjustment procedure, and reset the fast idle speed to spec.

Once the above is dialed in you should by the spring kit to get the centrifugal in faster than the lazy OE 30 @ 5000, but measure it first because it may have been modified in the past.

The tuning seminar has guidance for starting point modifications to the spark advance map for all OE engine families.

Duke

[Joe T.]

Re: 1966 L72 running hot

I am a little confused. I got 9" at 850 Rpm with the vacuum hose disconnected from the distributor (manifold vacuum) and plugged into my vacuum gauge. The distributor can was not plugged. My initial timing had been set at 12 degrees at idle. As far as a spring kit goes, the distributor is from Chicago Corvette (1111093). Not sure what spring they use. How do I check start stop points and if it is coming in fast enough? Will initial at 8 along with the B28 solve my overheating issue? Sorry for all the questions..

[Joe T.]

Re: 1966 L72 running hot

I figured out how to check start stop points in inches for the new B28 can. Called Chicago Corvette and they will let me know the spring used. He said it should be stock.

[Leif A.]

Re: 1966 L72 running hot

I figured out how to check start stop points in inches for the new B28 can. Called Chicago Corvette and they will let me know the spring used. He said it should be stock.

Joe,
What spring and where? There is no spring in the vacuum advance canister.

[Joe T.]

Re: 1966 L72 running hot

The spring in the distributor for centrifugal advance. As you can see in this thread, when checking my manifold vacuum I got 9" at 850 RPM. Duke's article on a big block during idle at 900 RPM it should be 10". I thought 9" was good for my 850 RPM. Perhaps I have way to much over lap on my cam.

[Duke W.]

Re: 1966 L72 running hot

The vacuum gage MUST be connected with a tee to a source of full manifold vacuum. If the engine has full time vacuum advance you can tee into the VAC signal hose. If ported, tee into the choke vacuum break hose.

Normal SHP big block idle behavior is 14" @ 900, not 10".

As stated in the tuning seminar, along with a photo, the Mr. Gasket 928G spring kit includes three different sets of springs.

You need to test the start and stop points of the centrifugal advance to determine if it is still the lazy OE spec as stated in the CSM or has already been modified to be more aggressive.

Duke

[Joe T.]

Re: 1966 L72 running hot

I really appreciate all of the help... The article I was reading "Vacuum Advance Principles and Applications" identified the range on C1/C2 engines is the very high overlap 30-30 cam pulls 10" @ 900. Sorry for my misunderstanding or perhaps I am reading the wrong article.

I connected the vacuum gauge at the choke vacuum break and disconnected and plugged the hose at the can.

I don't have the tools to check the spring start and stop points. I will find out the one used from Chicago Corvette today. Does the spring have any effect on advance at idle/low RPM and my overheating issue?

[Duke W.]

Re: 1966 L72 running hot

All you need to characterize the centrifugal advance is a dial back timing light, which costs about 50 bucks or maybe you can borrow one. In the tuning seminar look at the "tools of the trade" slide. Any vintage Corvette owner needs to have that equipment or know someone who does. The total cost is likely less than a "tuneup" from some commercial shop, and there is no guarantee that they will actually do it correctly.

Don't rely on some Corvette parts house to tell you "what springs". They likely have little or no idea of what's actually installed or what centrifugal curve they yield.

As I've said before, compare your measured centrifugal curve with OE specs and go from there.

Post you results.

Duke

[Joe T.]

Re: 1966 L72 running hot

Thank you very much for all the information you and others have provided. I have a timing light (old school). Never thought I would purchase another Corvette at my age. (retired Nuclear Engineer). I also located my vacuum gauge. Have all the tools needed from past cars. Never tied "over heating" with the distributor in the past. Now it makes sense, especially with all the other improvements I have made to this 427.

[Joe T.]

Re: 1966 L72 running hot

I have the parts as identified. B28 vacuum can. Checked the B28, starts at 4" and all in at 7". Mr Gasket spring kit 928G. 2000 to 2800 full mechanical advance.

The vacuum can in the car now, starts at 9" and all in at 20". The mechanical advance pulls in at approximately 4000 to 4500. My car is running 9" at 850 RPM.

The B28 and springs should have a major effect with stopping my overheating problem. I plan to set the initial timing at 10 degrees at idle. I will update on the changes next week.

Any comments or other suggestions?

[Leif A.]

Re: 1966 L72 running hot

I really appreciate all of the help... The article I was reading "Vacuum Advance Principles and Applications" identified the range on C1/C2 engines is the very high overlap 30-30 cam pulls 10" @ 900. Sorry for my misunderstanding or perhaps I am reading the wrong article.

I connected the vacuum gauge at the choke vacuum break and disconnected and plugged the hose at the can.

I don't have the tools to check the spring start and stop points. I will find out the one used from Chicago Corvette today. Does the spring have any effect on advance at idle/low RPM and my overheating issue?

Joe,
Glad you got the B28 can. The one that was installed sounds like a B1 can...useless for your application as indicated by it's start and stop points. When you get the B28 installed, you're going to want to run some pulls with the new can and with the current springs. The springs determine how quickly the mechanical advance comes in, the vacuum can affects the vacuum advance. These are two different measurements and the combination of the two is what you'll be working towards. I haven't read Duke's paper in a while but I bet he has the parameters in his paper that you will be wanting to attain. Keep us posted and we'll continue to walk you thru this process. For me, when I was doing all this several years ago, the process reminded me of a college freshman course I took 50 years ago called "Logic"...the most illogical course I ever took in my life...it was like Chinese until one day the "light came on" and it, suddenly, made complete sense. We will guide you to the light...persevere.

[Michael J.]

Re: 1966 L72 running hot

Yes, it is all very mysterious to start with. I first went through it all back in 2010 with my '67 L71. Duke's articles and coaching were great. I am now lucky enough to have a friend with the old Sun distributor machine, and it goes quickly now. Only took about an hour when my newly acquired '63 L84 needed to be set up right, and I have kept a stock of B28 and B26 cans, just in case. It was restored about 10 years ago, and it was obvious whoever set up the engine didn't have a clue about any of this. Now it is like new, 180 all the time on the temp gauge. Now if I could just fix that annoying whine in the rear end as easily........

[Duke W.]

Re: 1966 L72 running hot

I have the parts as identified. B28 vacuum can. Checked the B28, starts at 4" and all in at 7". Mr Gasket spring kit 928G. 2000 to 2800 full mechanical advance.

The vacuum can in the car now, starts at 9" and all in at 20". The mechanical advance pulls in at approximately 4000 to 4500. My car is running 9" at 850 RPM.

The B28 and springs should have a major effect with stopping my overheating problem. I plan to set the initial timing at 10 degrees at idle. I will update on the changes next week.

Any comments or other suggestions?

The B28 VAC is good. The 928G spring kit has three sets of springs. They can be used together or mixed and matched to achieve max centrifugal at various engine speeds. So what combination provides maximum advance at 2000 and at 2800?

2000 might be a little too aggressive, but you can try it. If the OE centrifugal is still 30 degrees (check it), then set total WOT advance (VAC disconnected and plugged) in the 36-40 range, as high as possible without detonation. With the lighter springs you only need to rev it a few hundred revs above where the advance is all in to do this. Don't worry, the fan or flywheel will not explode.

So your final results should look like this:

Total WOT advance (VAC disconnected) xx @ yyyy RPM and above

Total idle advanced (VAC connected) aa@bbb RPM @ cc manifold vacuum.

Initial timing - whatever. You can measure it if you want at some RPM that is below the point that centrifugal starts, but that can be tough with an aggressive centrifugal curve and a big cam since the centrifugal may start below the RPM that the engine will idle stably long enough to measure it.

Document your results and keep them where they can be accessed for future reference.

Like Michael said the guys that typically set up these distributors are TOTALLY CLUELESS about the spark advance map that is best for the engine that the distributor will be installed in.

Duke

[Joe T.]

Re: 1966 L72 running hot

I plan on installing the new parts and making adjustments this weekend. Will keep all of you updated.

[Joe T.]

Re: 1966 L72 running hot

Thanks to all of you with a special thanks to Duke, my over heating problems are resolved. After a new radiator, hoses, thermostat and timing changes, the final solution was a B28 vacuum can and a 928G spring kit. Timing as follows: 12 degrees at idle with vacuum disconnected, total idle advance with vacuum connected 28 at 1000 RPM, total WOT advance with vacuum connected 40 at 2500 RPM.

Car performs fantastic. Driving in town along with stopping at 6 stop signs and sitting at 7 traffic lights it did not get over 210 Degrees. I would have been boiling over long before with the B1 can and stock springs.