Distributor timing range too narrow

[Formerly Steve Wilkinson]

The optimum ignition timing for my engine (3.4-liter SC, twin plugs, S cams, PMOs, headers) is five degrees BTDC at idle (950 or so) and 26-27 degrees BTDC at 6,000 rpm. If I establish the correct idle timing, at 6,000 rpm it's at about 18 degrees BTDC. If I instead establish the correct 6,000-rpm timing, the idle timing is at about eight degrees retarded (ATDC). I think I've got that right...in any case, the timing range of the distributor is too narrow. I can establish one point but not the other. The advance is entirely centrifugal--vacuum disconnected.

Anybody else experienced this and know how to correct it?

Stephan

[CamB]

Ummm, I'm getting myself confused.

So if you set it to 5 deg BTDC it advances by 13 deg to 18 deg BTDC.

If you set it to 8 deg ATDC it advances by 35 deg to 27 deg BTDC.

How can it advance 13 deg with one setting and 35 the other?


You sure that you don't mean 5 deg ATDC in the first one and 8 deg BTDC in the second (giving about 20 deg of advance either way)?

(edit) Important disclaimer - I don't really know what I am talking about...

I just checked my MoTeC data and it looks like the dyno guy set me at about 20-25 deg advance, with about 30ish deg BTDC at 6000rpm. Engine is a 3.2 short stroke with S cams.

[dean]

I think it is more important to have the total advance correct. You could then put a vacume adv. unit on for cruise. I think

[304065]

5 btdc
18 btdc
= 13 degrees advance from idle to 6k

26 btdc
8 atdc
= 34 degrees retard from 6k to idle

alternate hypothesis: 26 btdc retards to 8 btdc = 16 deg retard (about the same)

Ok, I see the issue. If it's really 8 ATDC I would think the distributor advance mechanism is sticking somehow, because the range of motion of the advance mechanism should be the same regardless of where you set it. Maybe clean and grease the advance and repeat the test?

Man, Slick magnetos don't have ANY of this stuff!

[Formerly Steve Wilkinson]

That's why I said, "I think I've got the numbers right..." Obviously I don't--hard to remember which side of the TDC mark the numbers are falling, particularly with headers singing away at 6,000 rpm. But the basic point is that I can get one timing parameter set correctly and not the other. The range is too narrow and I don't know what is causing that.

Stephan

[CamB]

I think you are ok if the total advance is out in the upper 20s (due to twin plug) and the idle advance is in the low single figures. Maybe someone who has the full 'S' timing curve can post it.

[Jdub]

Is your distributor moving freely? Do you have total advance via vacuum or is the diaphragm in the cone dying?

You might consider pulling the dist. and pulling it apart for a good cleaning. Warren has posted on this so you can check Search.

I was pretty impressed when I saw how much gunk was in there, but more importantly I checked the spring set, weight bobs and pivots (light mahcine oil) and the plastic protectors on the spring pins (one was missing). Point is this is a worthwhile weekend of work regardless.

John

[Formerly Steve Wilkinson]

No, it has only been 4,000 miles since Steve Weiner went over the whole thing converting it to twin-plug. I'm sure it's clean and at least should be functional.

Stephan

[RPMClassic]

My understanding of SW work is that the twin plug set up (single 12 distributor) is set at 10 degrees advance(idle) and then a max advance of 26 degrees.