I have an almost bone stock 1972 455. The only non stock thing is an HEI distributor. It seems every forum has a different idea of how much initial timing you want. Right now I have perfect air/fuel ratio, no vacuum leaks, and it shakes at idle. Before you tell me what to do, know this: All new plugs and plug wires, in the correct order, and initial timing is set around 8* btdc. Thanks in advance, Andrew

 

Terms:
Inital timing: Where the timing is set at idle.
Mechanical advance: How much centrifugal advance the distributor has built into it. You can change this.
Total timing: Initial+mechanical. Yours, as an example, has 8 initial and maybe 17 mechanical - giving you a total of 8+17=23 degrees.

Your initial advance is dictated by your mechanical advance, in the sense that you want your total to be around 36 degrees at full mechanical advance (vacuum advance disconnected and the supply hose capped/plugged). If your distributor has a long mechanical advance curve, maybe 28 degrees, then 8 degrees of timing at idle might be right.

But if your total is only in the 20s, then you need to advance your timing. Your total timing figure ensures that your ignite the mixture early enough at higher RPMS to take full advantage of the pressure of combustion at the opportune crank angle degree.

An additional, and simple test: If your vacuum advance is hooked up to ported vacuum, move it to manifold vacuum, and see if the engine idles faster than before. If this is the case, your car wants more advance at idle.

I hope that made sense.

 

If it's a factory HEI (read original GM) and not modified, then you want the initial at 20° @ 1100RPM.

 

Randy is correct - the factory HEI distributors had a short advance curve, and thus wanted a lot of initial timing to achieve the correct total timing.

 

Seff you did great until the vacuum advance. The idle will go up if the vacuum advance line is moved from ported to manifold vacuum no matter what because it goes from near or 0 vacuum to full vacuum.

The factory points timing settings are n/a since your not running points. I would start with 18 degrees btdc at idle and connect the vacuum advance to manifold as Seff suggested. Do you have a dial back timing light?

 

You're right, it's most likely to go up in this case. But had his initial been at 35, moving it from ported to manifold might very well have showed down the idle - indicative of too much timing. That's what I've seen, anyway.

 

I do! Thanks for the help. I really appreciate it.

 

Yes seff that makes perfect sense to me, thanks for explaining!
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Just a note, make sure you total with vacuum advance connected is not more than 50-52* or you may run into part throttle ping.

 

I already have a part throttle ping at 36, any suggestions?

 

Disconnect and plug the vacuum advance hose and see if it goes away.

 

I am surprised you have pinging with a 8 to 1 455 with such low timing. I would use a top end cleaner down the carb. I have ran around 60 degrees part throttle. Get an adjustable vacuum advance canister and a dial back timing light. Most of the factory cans have like 25 to 30 degrees advance. The Crane adjustable can has 20 degrees full out. I had mild bucking on my 9 to 1 350 with the 30 degree can, just by putting the Crane in at full advance fixed it.

 

Well I'm not positive it's pinging, it's just a little tick, but not a lifter clatter or rod knock

 

When does it make this noise? Have you disconnected your vacuum advance and taken it for a ride?

 

I have open headers so I can only hear it at idle, I will take it for a drive without vacuum advance and let you know

 

You won't get pinging at idle.

 

I set mine to 8 ^ BTDC at 800 rpm idle. I run 92 pump gas, have no pinging or dieseling issues, and passed emissions.
Mark 1976 Olds 98 Regency Brougham'

 

Define "Perfect air/fuel ratio". If you're letting an A/F ratio gauge tell you how to tune the idle, shaking at idle is the predictable result.

If you're using a vacuum gauge, and adjusting the mixture screws for highest vacuum then turning them just a bit lean, the only thing I'd suggest is to perform a power-balance test to see if you have one or more weak cylinders. If there are weak cylinders, a cranking compression test, and a verification of vacuum leaks into the intake runner feeding that cylinder are in order.

 

Umm.....1976 Regency 98 should have an HEI. Your base timing should be 20° BTDC @ 1100 RPM, not 8° BTDC @ 800 RPM (it's not a chevy). There just isn't that much mechanical advance in that distributor and you are leaving both a lot of power and fuel economy (I know...what economy with a 455) on the table.

 

I'd also point out that there's no need for 92 octane fuel for a 1976 455 with 8.5:1 compression ratio.