timing a carb'd 5.0

(imho)...

I would back the vacuum advance off about 10 deg and then add 4-6 degrees more initial advance (turn the dist)... for a 10-12 degree at idle reading.

You note (if I understand you correctly) that you have 6 degrees initial (reading at idle) and you have 29 total centrifigal in the distributor. That would give you 35 total advance at wot, which is probably about right... depending on how modern your heads are. (newer heads/better design make peak power with less total adv: 32-34, older/poor designs like more adv: 36-38-40)

In my exp, most motors (espitially ones with older heads) like more initial advance 10-12-14 degrees. The idle quality will improve and it will be peppier around town. But as you add more initial advance you may need to reduce the mechanical advance slightly, to maintain the same overall total adv. (34-36-38 degrees)

Also 30deg of vac adv seems a bit, um... optomistic to me. What I mean is, if you happen to get a load of bad gas, etc and you get out on the hwy and cruise you could be doing some damage and not even know it. (audible ping is about a 6 or 7 on a 1-10 scale so you can be doing dammage over the long term, and never hear it)

Your sig says you run a 4.11 rear gear, so I'm guessing your not in this for the gas milage... lol Right?

I would think 20 extra in the vacuum would be safer, and also allow you to go a little more agressive on the mech / initial timing for some more pep between stoplights / around town. (32-34-36 total, all in by 2500)

Also worth mentioning is heavier cars, or taller (highway) gears, etc... are less forgiving of agressive advance curves, on the flip side lighter, shorter gear (3.9-4.1-etc) cars are more forgiving of agressive advance because they can get "up on the curve" quicker and not lugg behind it...

Jetting also affects it... lean is less forgiving, fatter gives ya more fudge room, etc...

Bigger cams can also allow a more agressive curve cuz the overlap will bleed off some cylinder pressure at lower rpms, etc...

...so alott of things come into play, but as a general rule of thumb.

Hope this helps,
D

calpolymustang said:

I'm not a spark plug expert, but from I'm reading, this looks like pre-ignition?

Timing is
.....
55° total centrifigal w/ vac advance (if i hold throttle to 3k RPM)

Click to expand...

55° total would explain the pinging. That's about 15° more than it should be. I would aim for a total in the 35-40° range with 10-14° base timing. Base timing with the 5.0 HOs is 10°, fyi.

Maybe I'm wrong, but the plug gap set at .055 seems really wide. Stock range is about .035 and electronic CD systems like my Crane HI6 suggest .042-.045 range. Your spark might be too weak to get a full burn. What do your plugs look like?

calpolymustang said:

I'm not sure theres a way of adjusting the vacuum diaphram, but let's assume i have an aggressive vacuum diaphragm... How do I tone it down?

Click to expand...

Probably like this:
Vintage Ford Mustang Engine Tuning Aftermarket Vacuum Advance Adjustment Photo

With the ignition timing sorted out, you should be able to run 87. That's what I use and that's all that stock 5.0 HOs require, although I can run a little more advance thanks to aluminum heads. Stock compression is fairly mild at 9.0:1