Inliners International Forums General Discussion Engines DON'T WANT TO START AN ARGUMENT, BUT...

IGNITION TIMING: Ported or Manifold Vacuum.? "Modest Centrifugal and High Vacuum Advance", and everywhere I research I find conflicting 'Expert Opinions'. Now my brain hurts...!

I've got a stock 250 with a 32/36, Langdon headers and large cap HEI. I'm running 17 degrees intial, 28 degrees BTDC max without vacuum and a total of 50 degrees with Ported Vacuum. The engine starts instantly, hot or cold, and is responsive without any detonation. I understand 50 degrees total is good, but I wonder if my 11 degrees of centrifugal should be increased and/or intial & vacuum advance be decreased as well as moved from Ported to Manifold.

I've been told don't fix it if it works, but my 'Tinker Gene' is acting-up...

To test: shut off the vacuum. Don't use full throttle.
Rotate the distributor to advance the spark 5°.
Is the idle speed up?
Is the response better?
Is light throttle vacuum higher at the same MPH?
If all are yes, you need a bit more initial (and perhaps some change in your centrifugal total).
If results are inconclusive (not always the same, or too small to be sure), leave it alone.

Test 2:
Shut vacuum off, advance spark 5° and run at WOT with current advance curve over known distance to reach high speed (at least 60).
Does it knock continuously (not just as you nail it)?
If yes, stop and go back 2-3°, and re-try.
Is MPH higher?
If yes, your total is short.
If no change, back off and re-try at +2-3°
If worse, try 2-3° less than stock.

Test 3:
Using best initial spark lead from above, record cruising vacuum at 60 mph.
Advance spark 5°.
Is vacuum higher?
If yes, try another 2-3°.
If yes, back off initial adjustment to best setting, and add test amount to vacuum arm travel.
If vacuum is higher but it knocks on light tip-in, adjust the spring in the vacuum can stronger (turn the screw out, record how many turns).

Hey Panic,
I've arrivied at the current 17 degree intial timing after much trial and error. At this setting, on rare occasion, I will bump against the starter on those Hot Summer Runs, so I can't advance the timing. But I wonder if retarding the static timing and adding more centrifigual to reach the same 18 degrees BTDC at 1000 rpm would be better. With no vacuum advance at idle using Ported Vacuum, could I retard intial and advance the timing by switching to manifold vacumm...? I guess I'm wondering if my 250 would benefit from less intial advance and have centrifugal & vacumm coming in earlier..?
Thanks for your thoughts.
Danny

Rusty,
I think your hunches are right.
I run about 10 initial at 600rpm, 32 total mech at about 3000-3500 all in. The curve starts quickly at about 1000-1200rpm.
Good for stock cammed 8.0-8.5cr or so 250 and 292, both of them turn in good times.

Adding ported vac, the total adv is 50 or so.

Panic is right though, best is to try it and clock the results. In cool weather you could kick up the initial to get 21 initial and 32 total mech, just to see if it runs better. If so, then I'd fiddle with the recurve to get about 10-12 initial with 32 total.

Your own testing will be better than any prediction.