Ignition Advance Question

[Lawrence C.]

I have a 1966 L79, 4spd coupe. The car runs great in all aspects except one:

It knocks on initial acceleration from most speeds. Depressing the accelerator seems to get through that area. I am using 93 octane Sunoco, with Lead Supreme to bring the octane rating up about 4 points. The vacuum advance cannister is new, and the distributor was disassembled a couple of years ago and cleaned and checked.

I finally got over to a friend's to use his setback timing light this past weekend, and this is our result:

Initial timing (vacuum disconnected) at 700 RPM: 8 degrees
Mechanical advance (vacuum advance disconnected) at 2600 RPM: 19 degrees
Total advance (with vacuum advance connected) at 2600 RPM: 43-45 degrees

I am not sure what total advance should be but 43-45 seems high. I am looking to get what the correct numbers should be, but also what route to take if 43-45 degrees is too high. Thanks very much.

[Duke W.]

Re: Ignition Advance Question

Your vacuum can is FUBAR. What vacuum can did you install - brand, part number, ID (stamped on bracket) and how did you happen to procure this particular part number for your L-79?

Do you have a CSM, AMA Specs, Motors Manual? The centrifugal and vacuum advance specs are published in those and other sources. It's obvious from your data that the vacuum can has WAAAAAAAAY more advance than the OE can.

Duke

[Lawrence C.]

Re: Ignition Advance Question

Thanks. Any idea of a source for a proper vacuum can?

[John H.]

Re: Ignition Advance Question

Larry -

You need to re-run the centrifugal advance check, unless you mean that the 19 degrees you mentioned at 2600 rpm is in ADDITION to the initial 8 degrees; if the 19 degrees you mention is actual observed advance, you either have weights sticking or you need to rev it up higher than 2600 to see where it stops advancing.

Also, "Total Timing" (which is the sum of initial plus centrifugal only) is checked with the vacuum advance disconnected; should be 34-36 degrees at the point where max advance is reached. The contribution of the vacuum advance is pretty much meaningless except at idle; check initial timing at idle with vacuum disconnected, then connect vacuum, and you should see an immediate increase of 15 degrees (i.e., from 8 to 23 degrees). Do you know which vacuum advance unit you have? Markings? It matters.

I don't have '66 centrifugal specs in front of me, but the '67 L-79 centrifugal curve adds 30 degrees, but isn't all in until 5100 rpm. They run much better with a "faster" curve, but we need to know when yours stops advancing (rpm) and how much advance you observe at that rpm.

[Jack W.]

Re: Ignition Advance Question

John, Duke - wouldn't that much total advance (if true) suggest something wrong with whatever controls the max mech advance, such as the "slot" or a missing / bent slot post or bushing?

[Richard D.]

Re: Ignition Advance Question

Larry -

NAPA has Echlin cans that are right for your distributor, VC1810 I believe is what Duke recommended in some previous posts for the L-79.

By the way, these are not the specs for a 66 L-79, but they are the specs for timing and advance curves for a 68 L-79, that I pulled from my 68 Chassis Service Manual that may be useful to you -

Centrifigal Advance
(Crank Degrees @ Engine RPM)

0 @ 950 RPM
14 @ 1400
20 @ 1800
30 @ 4700

Initial timing is 4 degrees BTDC

Regards,

Rich

[Duke W.]

Re: Ignition Advance Question

Could you provide the information I asked for in my last post?

Duke

[William C.]

Re: Ignition Advance specs

From the service manual, L-79 specs (Crank degrees @ engine rpm) 0@900, 15 @ 1500, 30@5100 Vacuum Advance (crank degrees) 0 @4 inches, 16 @8 inches Vacuum can 1116236 (stamped 236 16 on the support). This curve is very compatable with any decent gasoline, it is much less agressive than the '65 SHP curves.

[Lawrence C.]

Re: Ignition Advance Question

Duke,
If you mean the question from your first post regarding sourcing? I believe the vacuum can came from either Long Island or Paragon. It was a couple of years ago now and I am not at home (I'm at a client's site) so I can't check for the exact answer. The car is at home, too, so I can't check on what is stamped on it, but I will chack it this weekend.

Larry

[John H.]

Re: Ignition Advance Question

Jack -

The "total advance" mentioned in his post isn't a valid measurement - it's distorted by having the vacuum advance connected. WOT "Total Timing" only involves initial plus centrifugal advance; vacuum advance isn't part of that measurement (vacuum advance disappears when you put your foot down).