I have a carbed 350 in my 86 Camaro and I am having some timing issues with it. It wants as much advance at idle as I can run. Right now I have the base timing set at about 12* and the vacume advance hooked to manifold vacume for a total of about 22* at idle. Is this normal? If I hook the advance up to ported vacume and the base at 10* - 12* I have to turn the idle screw up and the idle gets pretty rough and choppy. It also doesn't have much vacume like that, about 10 inches. The harmonic balancer is accurate as far as the timing mark because I was able to check that when I did the head swap a few months ago. Compression is about 9.25 - 9.5 to 1 and I have never had problems with detonation. I only run premium in it though. The cam is an XE274 and I am running a stock HEI distributor which should get upgraded the winter. I doubt the distributor has a very aggresive timing curve. It runs good the way I have it hooked up now, I'm just curious if it is bad to have that much advance at idle.

Thanks for and help.

 

what's your total advance? base doesn't really sound out of line and every engine combo is differant

 

I don't know for sure. I need to get a timing tape. It looks like another 15* or so with the timing light when I rev it up to about 3000.

 

That could be the problem. The cetrifugal advance mechanism should give about 20* more advance over the initial setting, all-in by about 3000. Stock springs inside the distributor are usually too strong and won't give full advance until about 4500.

Vacuum advance canister should give an additional 12-16* (most stock cans give a lot MORE than this. So much so, that once you get the initial and centrifugal really "dialed in" to make best WOT power, you will encounter part throttle detonation). Crane makes an adjustable vacuum advance canister for only about $15.

A good "starting point" for an HEI ignition curve is......

12* initial
20* centrifugal, all in by about 3000 (therefore 32* total... 12 + 20 = 32)
16* vacuum advance all in by whatever your idle vacuum is while cruising on the highway at a constant speed.

 

I GENERALLY like to hook the vacuum advance to a ported vacuum source- NOT having vacuum advance kicking in at idle. If you're going to hook it up to full manifold vacuum just make sure you're pulling enough vacuum at idle to FULLY pull in the vacuum advance. You don't want it "floating" at a vacuum level where the vacuum advance is not fully in. You'll get a "rolling" idle if that is the case.

The XE 274 is a relatively big cam. Idle with only 12* advance will certainly require opening up the throttle slightly and it will be choppy. At 22* it would idle much smoother and with less throttle opening. As long as you are NOT having issues with "dieseling" or "run-on" at shut-down then hooking it to a full vacuum souce may be a better way of going with your combo.

It's a try-it-and-see thing in my expereince.

 

Thank you, that helps alot.