1966 L72 running hot

[Duke W.]

Re: 1966 L72 running hot

I'm glad to hear the high operating temperatures are mitigated, but from what you've said it's not clear to me if you performed the procedure correctly. There is no such thing as total WOT advance with the VAC connected. To measure Total WOT advance the VAC hose must be disconnected and plugged because the VAC doesn't provide any advance at WOT.

Please measure and report the centrifugal start an stop points, and report what springs are installed. This must be done with the VAC disconnected and plugged. Format it the way the OE L-72 centrifugal is specified.

Start @ 900, 30 @ 5000.

Then you set total WOT advance (the sum of initial and full centrifugal) as high in the 36-40 range as the engine will tolerate without detonation a few hundred revs above the max centrifugal point you determined, which with the lighter springs should no be more than about 4000.

Alternatively, you can set initial timing but it MUST be done below the centrifugal start point you measured. If the distributor still has the OE 30 degrees max centrifugal then 12 is too much.

Connect the VAC and measure total idle advance at normal hot idle speed. Then, if you want, rev the engine to above the max centrifugal point and measure maximum cruise advance, which should be in the range of about 50-54.

Once you've got it set up and it passes a detonation road test go through the idle speed/mixture adjustment procedure in the CSM to get it fully dialed in, and check and adjust the fast idle speed to spec to as outlined in the CSM.

Duke

[Joe T.]

Re: 1966 L72 running hot

Sorry it was Total WOT with vacuum disconnected. Used the light springs (gold). The initial springs in the distributor were very heavy. They were larger in diameter than any in the 928G set. The centrifugal start was around 2200 RPM with light springs as best as I can remember with the new springs. The car is running very well with no detonation. I should of documented each step. I had a friend (mechanic) very familiar with carburetors and distributors helping using his digital timing light. I would think if it wasn't set up right, it wouldn't run as well as it is.

[Michael J.]

Re: 1966 L72 running hot

It's always great to get your car tuned right and running like it should be, like having a whole new car all over again. These cars should start easy, run smooth, not overheat, and have lots of power. The unfortunate thing is that many restoration shops concentrate on cosmetics and not mechanical perfection. They seem to think all a customer wants is something that looks good, they never seem to considering driving and enjoying it on the road. Many do not have the knowledge or equipment to tune these the way they should. Whenever I buy one, I and my friend, with all the old equipment, have to spend many hours getting it right. I have had few exceptions over the years of ones that came to me that were set up correctly and mechanically right. Enjoy driving.

[Joe T.]

Re: 1966 L72 running hot

Sure is great driving a 427 BB. It was a B21 in the car with very heavy springs. At this point in laymen terms. I have 12 degrees at idle (1000 RPM) with vacuum disconnect, 28 degrees with vacuum connected and 40 degrees vacuum connected at 2500 RPM. I'm not sure about WOT measurements. I used the light springs in the 928G set and a B28 can. The car does not overheat nor is there any detonation. It idles and runs very well.

[Duke W.]

Re: 1966 L72 running hot

[QUOTE=
The centrifugal start was around 2200 RPM with light springs as best as I can remember with the new springs. The car is running very well with no detonation.[/QUOTE]

That doesn't make any sense. With the OE springs the L-72 centrifugal start point is 900 and the max centrifugal advance is 30 @ 5000. With lighter springs it should start sooner and all 30 degrees should be in earlier. Did you mean that maximum centrifugal is achieved at 2200, but then how much? It's possible the distributor was modified for less or even more centrifugal.

I'm glad your car is running better, but I was hoping that this thread would be of educational value to others. Unfortunately the data you presented is incomplete in terms of characterizing your spark advance map in a concise and unambiguous manner.

For those who pursue optimizing the spark advance map in the future. Here's the basic work flow.

1. Get the OE specs from the service manual or AMA specs, and read my tuning seminar.

2. Determine the actual centrifugal start and stop points (VAC hose disconnected/plugged) and the maximum advance provided. It may have been previously modified, so it must be checked.

3. Determine if the car has ported or full time vacuum advance. Document the VAC ID data and determine the start and stop points in inches Hg. If ported, convert it to full time. Measure idle vacuum and use the Two-Inch Rule to select between the available 8", 12", or 15" VACs.

4. With the VAC disconnected and plugged, install lighter springs and determine the new start and stop points. The total centrifugal advance should not change from the baseline you measured in step 2 , but it should start earlier and be all in earlier.

5. With the VAC still disconnected and plugged, rev the engine a few hundred revs above the point of max centrifugal and set Total WOT advance as high in the range of 36-40 as the engine will tolerate without detonation. (Suggest you start at 38-40)

6. Connect the VAC to a full time vacuum source, conduct the idle speed/mixture adjustment procedure and measure total idle advance. (Also check fast idle speed and reset to spec.) Rev the engine a few hundred above the point of maximum centrifugal to measure maximum cruise advance. Subtract from this total WOT advance to determine max vacuum advance. For example: 54 - 38 = 16

7. Road test and listen for detonation. Load the engine up at low speed because detonation usually occurs at low revs and high load. If none is detected increase total WOT advance to 40 and retest. If detonation is detected reduce it to 36. If less than 36 is necessary to quell detonation, slow the centrifugal curve with stiffer springs and keep testing until your certain it's optimized, which is the ragged edge of detonation.

8. Permanently document the new spark advance map. Annotate your owner's manual, service manual, or your maintenance/repair file.

The following is an example of how the spark advance map data should be formatted.

Centrifugal: Start @ 700, 30 @ 3100 (max)

Vacuum advance: B26, start @ 6", 16 @ 12" (max)

Total WOT advance: 38 @ 3100-up

Maximum Cruse advance: 54 @ 3100-up

Total idle advance (14" @ 900): 26

Note that in this case the total idle advance is more than the sum of initial and full vacuum. That's because it starts at 700, which is 200 below the 900 idle speed, so two degrees centrifugal is added at idle speed, and this is NOT a problem. You can measure initial advance, but it must be done below the 700 RPM centrifugal start point. SHP cam engines may not idle stably below this point long enough to measure, but that's not a problem.

The important thing is to get the total WOT advance optimized. If you measure 38 and know the max centrifugal is 30 then initial is 8, and the sum of initial and full vacuum is 24. Since total idle advance is 26, not 24 and you know that the sum of initial and full vacuum is 24, then you know that 2 degrees centrifugal is present at idle.

Duke

[Joe T.]

Re: 1966 L72 running hot

Sorry I am not an Automobile Engineer and did the best I understood. Used the light springs (gold). The initial springs in the distributor were very heavy. They were larger in diameter than any in the 928G set. The centrifugal MAX was around 2200 RPM with light springs as best as I can remember. I didn't have your paper with me to follow each step. I had a friend very familiar with carburetors and distributors that used his digital timing light. I then went back after reading your post and took readings with my old timing light and vacuum gauge and this is what I currently have to the best of my knowledge.

I have 10 to 11" of manifold vac at 1000 RPM. New B28 vacuum can comes in at 4" and all out at 7". Old B21 can that was in the car came in at 9" and all out at 20". I used the B28 can and light springs. Timing as follows: 12 degrees at idle with vacuum disconnected, total idle advance with vacuum connected 28 at 1000 RPM, total advance with vacuum connected is 40 at 2500 RPM. The carburetor was set up with a vacuum gauge.

I would think if it wasn't set up right, it wouldn't run as well as it is with no overheating or detonation. I was unable to take any other readings.

[Duke W.]

Re: 1966 L72 running hot

I have 10 to 11" of manifold vac at 1000 RPM. New B28 vacuum can comes in at 4" and all out at 7". Old B21 can that was in the car came in at 9" and all out at 20". I used the B28 can and light springs. Timing as follows: 12 degrees at idle with vacuum disconnected, total idle advance with vacuum connected 28 at 1000 RPM, total advance with vacuum connected is 40 at 2500 RPM. The carburetor was set up with a vacuum gauge.

.

NO! NO! NO! Total WOT advance has to be set with the VAC HOSE DISCONNECTED AND PLUGGED at an engine speed a few hundred revs above the point of max centrifugal.

I've stated this multiple times in this thread. At this point I have no idea what you have, but if you're satisfied with the performance, lets' leave it at that.

Duke

[Joe T.]

Re: 1966 L72 running hot

I have exactly what I identified in my last post and the car is running fantastic. I did not take a WOT reading with vac disconnected and plugged. My advance was 40 degrees at 2500 RPM with vac connected. My concerns were those identified in earlier post and those that I understood, along with having an individual that has an ignition and carburetor shop helping. He also has a 502 Camaro set up for racing and a 1966 396 350 HP Chevelle. Again, my idle advance with vac disconnected (12), idle advance vac connected (28) and advance at 2500 RPM with vac connected (40). I will take a WOT with vac disconnected and plugged to provide information for this thread and for my record.

Thanks again for all the help I have received.