67 427/435 L71 Vacuum advance.

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

Duke:
The OE specs are:
Centrifugal advance: 0 degrees at 900 RPM

30 degrees at 3800 RPM.
Vacuum advance is: 0 degrees @ 8 inches.

15 degrees @ 15.5 inches.
Ignition timing at idle(750)RPM: 5 degrees.

Engine compression is probably less than original spec.of 11-1. maybe 10-1.

Here are the vacuum readings I get from the manifold connection with the vacuum can connected:
At idle(750)RPM: 13 inches.
AT 1500 RPM: 19 inches.
At 2000 RPM: 21 inches.
At 3000 RPM: 22 inches.

And with the vacuum line disconnected and plugged:
At idle(600) RPM: 10 inches.
At 1500 RPM: 17 inches.
At 2000 RPM: 20 inches.
At 3000 RPM: 21 inches.

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

I also have a SUN Electric Corp. Specs sheet for the 67 L-71 and it shows;
Ignition advance at 2500 RPM: cent.vac: 28-36 degrees..cent.only:15-19 degrees.
Do I need to have the weights and springs changed and the distributor recurved?
Thanks.

[Duke W.]

Re: 67 427/435 L71 Vacuum advance.

Thank you for the specs. As you can see your centrifugal curve appears to be quicker than OE. You need to slow it down.

It's still not clear where you are measuring manifold vacuum. Please measure it from the choke vacuum break line as this is full manifold vacuum. You vacuum advance signal source appears to be "ported" and does not show full manifold vacuum.

I'll just cut to the chase. Here's what I recommend.

8-10 initial timing
OE centrifugal curve
VC1765 vacuum can (0@6", 16@ 12") connected to a FULL MANIFOLD VACUUM SOURCE
Idle speed 800-900
Total idle timing 24-26 degrees (8-10 initial, 16 vacuum)
Estimated manifold vacuum at idle speed with above setup: 14-15"

From this starting point some adjustments in initial and/or rate of centrifugal may be required to achieve detonation free operation on unleaded premium.

The other question I have regards the cam. Are you absolutely sure it is the OE mechanical lifter cam?

Duke

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

Yes, I'm positive it's an OE spec.cam. Sorry for the inexperience here, but the only place I can connect my vacuum guage to the manifold is on the passenger side where a small hose goes from the passenger side of the manifold into the passenger side valve cover and that's where I took the last vacuum readings. Is there another place to connect?

[Duke W.]

Re: 67 427/435 L71 Vacuum advance.

My vacuum gage has a length of 1/8" rubber tubing to the base of a tee. One side of the tee cross bar has a short length of 1/8" rubber tubing.

To take a vacuum reading from any 1/8" vacuum line, I disconnect the vehicle rubber vacuum tubing at a convenient location, place the disconnected end of vehicle rubber tubing over the open end of the tee on my vacuum gage, then place the short length of tubing onto the nipple from whence the disconnected end of vehicle tubing came from.

This provides a "parallel" connection to the vacuum line that allows you to read vacuum without affecting the operation of the vacuum circuit.

I would like you to affect this connection to the choke break vacuum line on the center carburetor, so you get a true manifold vacuum reading at idle. The choke vacuum break line is a short piece of rubber tubing that routes from a nipple on the carburetor to a small plastic diaphragm that is connected to the choke linkage.

Once you have this reading affect the same connection of the vacuum gage into the vacuum advance signal line. If the vacuum reading at idle at this location is less than what you read at idle from the choke vacuum break connection, then you have a ported vacuum advacne source.

In this case fabricate a new vacuum harness that tees the vacuum signal line into the choke vacuum break line. Remove the existing lengths of rubber tubing from the vacuum can and choke vacuumn break and keep them. Make a complete new harness. This will require a foot or two of eighth inch vacuum tubing and a tee.

Duke

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

Duke:
Thanks for the telephone call and the one on one answers to my questions. I really appreciate your time and interest in helping with my timing problems. It seems 1967 427/435 cars operate on "ported" vacuum. I now realize that "ported" means partial....not full manifold vacuum. I installed the VC1765 vacuum can and connected it via a newly constructed line to the choke vacuum brake port on the center carb. I also capped the old vacuum port. I then set the initial timing as you suggested. I could not get the car to run without continuous/constant detonation even with the initial later set at only 2 degrees. It looks like I need to remove the distributor and have it recurved to OE specs. Later in the day and just as an experiment, I installed a 360 vacuum advance can (in place of the previously tried VC1765 and 1810 cans)and connected it to the original vacuum source on the center carb.(not the choke brake vacuum port) I set the initial timing at 3 degrees. The car ran fine, without detonation....however, when I stepped on the accelerator to open the other two carbs the car seemed to misfire a bit before the two other carbs kicked in....any thought?

[Duke W.]

Re: 67 427/435 L71 Vacuum advance.

I think if you just get some stiffer springs or some sets of progressively stiffer springs and keep installing stiffer springs until the detonation stops with about 6-8 degrees initial timing and the VC1765 hooked to full manifold vacuum you will find a setup that will work. It's a lot quicker to change springs with the dist. in the car, and you can immediately test the results.

The OE centrifugal curve I have from Corvette News for L-71 is 0@900, 30@3800, which is quicker than '66 L-72. I cant' recall, did you measure the actual centrifugal curve as it exists now?

The OE L-71 vacuum can spec is 15@15.5" (per Corvette News), which is the same as '65 L-78, but NOT the same as '66 L-72. I don't have enough data to correlate these specs to a Delco part number. The VC 1765 is close to the '66 L-72 spec. Your idle vacuum is not enough to keep a 15" can locked on full advance at idle, which is why I recommended the VC 1765 It will will be locked on full advance at 12", which is less than your idle vacuum.

Is the detonation worse under part throttle or full throttle acceleration?

Is the idle overheating situation better with the VC 1765 signaled by full manifold vacuum?

GM completely retuned the L-71 ignition map relative to L-72 eventhough both engines are essentially the same with the exception of the induction system. L-72s respond well to just converting the OE vacuum can to full manifold vacuum signal with no changes to the the OE initial timing recommendation and OE centrifugal curve. The OE L-72 ignition map is (Corvette News) 0@900, 30@5000 centrifugal, 0@6", 15@12" vacuum (which is essentially the same as the VC 1765), initial timing 8 nomiminal 8-14 range.

Duke

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

Well, I just took it for a spin and it stalled six or seven times while cruising at 60 to 70 MPH.....just stalled dead while cruising....I put her in neutral and turn the key and she starts right up again. Several very loud backfires out of the drivers side exhaust...then a couple out of the passenger side exhaust during the 3 mile drive. When I shut her off at home in the garage she dieseled for a few seconds....things are getting worse...I think I need to see a doctor....

[Duke W.]

Re: 67 427/435 L71 Vacuum advance.

Okay, what is the configuration of the ignition map? I'm having a tough time keeping up with all the installations and swapping of the different components and vacuum sources.

You said you installed the original 360 can and it backfired. Did it do that before? Maybe there is some other issue.

Also, when you change the total idle timing (by switching to ported vacuum advance) you must go through the idle speed/mixture adjustment to optimize the fuel mixture for the new manifold vacuum and total idle timing.

My recommendation it to install the VC 1765 with a full manifold vacuum source. Set the initial at about 6-8. Install some STIFFFFFFF springs on the centrifugal, then set the idle speed and mixture, verify that the VC 1765 meets spec, and measure the centrifugal curve.

It might be best if you call so we can sort it out.

Duke

[Richard S.]

Re: 67 427/435 L71 Vacuum advance.

On Saturday there was a short, very brief hesitation with the 1765 can.....but it came back to life almost immediately and didn't occur again. I couldn't get it to idle very well either.....very low idle at stop signs after setting idle at 800 RPM before I took it out. I think there could be a carb issue as well....the center carb is quite black....and today the fast idle would not work. I appreciate your trying to keep up and your patience.....but I think I'm at the point where I need someone experienced to baseline the distributor and tri power carbs....I just don't have enough experience to know where to start.

[Mark Zediker]

Re: 67 427/435 L71 Vacuum advance. Duke

I had substantial backfire, detonation and other problems with my 66 -L72... I sent my distributor and TI ignition amp to Dave Fiedler at TI Specialties. He found several disbtributor problems immediately... recommended repairs. I paid... miracle of miracles all of my problems disappeared.... I run on premium gas and I add a 2 octane number (not points) octane booster to get me to 95 octane, 12 degrees at idle through ported vacuum. No overheat, no detonation, good power... Fully stock L72 engine. Runs rough without octane booster, have to retard ignition to keep combustion problems at bay wihtout octane booster... live in a hot humid area, so I worked hard to get it to this point... note octane boosters like 104+ add up to 7 points so your octane number is increased from only 93.0 to 93.7.

I highly recommend Dave, he will look at your distributor for a nominal fee and tell you where you are at.

[Everett Ogilvie]

Re: 67 427/435 L71 Vacuum advance. Duke

During the past five years I have set up 3 L72's, 1 L78, and 1 L89 with similar but more aggressive specs, all with ported vacuum and no overheating nor poor idle. Because of my high altitude I can get away with (and recommend) more advance - I remove the dist. springs and time for the total timing method at 42 - 44 total mechanical advance (here again, high altitude specs). This yields 9 or 10 advance at 600 rpm when the springs are reinstalled, and 14 to 17 at 1000 rpm with springs installed (all numbers are mechanical advance only). You can see how much mechanical advance is coming in at 900 to 1000 rpm. The difference is due to various styles of advance weights which are used in these distributors. There are weights with holes and without (approximately 500 grams vs 475 grams if I recall), and the 093 distributor during the '66 model year alone used both (according to Dave Fiedler). As has been said many times, springs can be swapped to change the curve, but one should also be aware of the installed weights. Note that I run straight 110+ octane leaded racing fuel with these specs to eliminate any dieseling or detonation problems.

[Mark Zediker]

Re: 67 427/435 L71 Vacuum advance. Duke

Hey Duke,
Wow, 44 degrees thats aggressive... I estimated my advance at 38. Do you have any dyno runs at 44 degrees? What kind of horsepower are you seeing? Would this work at sea level where I am at...

Great info..

Mark

[Duke W.]

Re: 67 427/435 L71 Vacuum advance. Duke

No! As Everett said, he lives at high altitude (Alberquerque, which is at least 4000 feet IIRC), and he is using straight 110 octane leaded race gas. Under those conditions he could probably run as much total timing and as quick a centrifugal curve as he pleases and will not get detonation.

At more common altitudes and using the best available pump gas, a BB will be limited in the amount of total WOT timing and the rate of centrifugal advance to the OE specs if not less without running a blend of race gas and pump premium.

Duke

[Mark Zediker]

Re: 67 427/435 L71 Vacuum advance. Duke

Got it... Thanks.