Stumped on Ignition Timing
 

Pontiac 455.
Aftermarket Stock Style HEI Distributor from reputable Source.
Total timing (Initial+Centrifigul) w/o Vac. Adv. = 36 Degrees BTDC @ 3000 RPM.
Initial at 750 RPM = 5 Degrees "AFTER" TDC.
Somehow I don't think this is right or desirable.
If not, what can I do to correct it so that with 36 degrees I+C @ 3000 RPM,
I can still have 8-10-or-12 Degrees BTDC Initial @ 750 RPM?

With Vac. Adv. disconnected, I have to turn up the idle screw just to keep it running.
Then, when finished, reconnect Vac. Adv. and turn idle down.
In both cases, idle is 750 RPM.

Thanks,

David

Have you taken a look at the weights and springs? I don't see how the motor will run at all with an initial timing of 5 deg. ATDC. If the timing is truly 36 deg. BTDC at 3000rpm then you would have 41 deg. of mech. advance. I don't know of any dist. that has that amount of mech. adv. built in. You are right that somewhere around 12 deg. initial plus 20-24 mech. is what most combos shoot for. I am assuming that you have had a timing light on this thing to check the #'s. In theory you need to rotate the dist. to give approx. 12 BTDC at idle and then set up the advance weights to bring in another 20-24 deg. by 3000rpm or earlier.

Agreed, something is off in the mechanical advance mechanism. Even if you had your timing set @5 BTDC it would still show 31 degrees mechanical advance. Time to pull the cap/rotor and find out whats up. A "Generic" tune would be initial between 10-12 BTDC, mechanical 20-22 degrees, and if your using a vacuum can another 8-11 degrees.

Go to this link, it will give you a detailed tutorial in setting up the HEI including pictures.
http://www.pontiacstreetperformance....distcurve.html

Check it out and report back! JD

I just went through something similar trying to use a 61 distributor to keep the engine looking semi original. If I set the timing at 36* I would be idling retarded. First I tried using a press on limiter underneath to knock out some of the advance. Then a thicker one. It got a little better but still not where I wanted it. When I pulled the distributor there was so much up and down slop in the main shaft I took it out. Dropped in another distributor from a 72 and the prob was solved.

Though that is a lot of mechanical advance, I have heard other Ponti guys complain about 30 degree mechanical advance distributors.

If your vacuum advance is adjustable it will save you here. Run manifold vacuum and add 12 degrees of vacuum advance for a total of 48 at your all in timing while cruising.

Your car will probably show a timing of 17 degrees at 800rpm now and it will idle, respond and run cooler.