recently rebuilt 350 olds.intial timing set 12 degrees, new HEI distributor. When reving engine in park, no issues whether vacuum advance connected or not. When driving, no issues when vacuum advance disconnected, but when connected to vacuum (ported) I get noticable pinging on throttle tip in while driving. Advance can is adjustable and i followed summit directions by turning can adjustment all the way clockwise (i had pinging when driving at that adjustment. I backed it off 2 full turns and still get pinging ) Should i continue turn vacuum can adjustment back or reduce initial 12 degree timing. No issues with starting car hot with current timing. Also engine temp sits at 180 degrees.other than this car runs/idles great and has good pep to it. I also run 93 octane fuel.
erson e540121 hydraulic cam adv. duration 292/292, lift .0478/.0478
Another thing. whiile out on test drive, i remebered I had disconnected the vaccuum advance hose. I pulled over to connect and saw i didnt have hose plugged (ported vaccum port) but it ran fine. shouldnt it have run bad with vaccum leak.
thanks eric.

 

The adjustment on the vacuum can is a rate adjustment not an amount advance limiting adjustment. You need to make a physical stop to limit the amount of advance the vac canister can provide.

 

 

You can make a stop or buy one from places such as Summit.

 

If you use manifold vacuum the vacuum advance should retreat as soon as you tip in. It'll be slower with ported vacuum.

 

I thought it should be ported
how will that affect initial timing
figured stock style hei on stock engine shouldn’t be a problem

 

Is 12 degrees too much initial-

 

Vacuum advance connected to manifold vacuum will give full vacuum advance at idle, and connected to a ported source will have no impact at idle but will advance as soon as the throttle is opened. Either way, restricting it to around 10 degrees is recommended. Some vacuum canisters give 24 or more degrees advance.

12 degrees initial is not too much. It might not be enough with an HEI depending upon the distributor’s amount of mechanical advance, but the engine should run OK, it just might not make maximum power.

 

I'm gonna make a timing sticky!
Part throttle or tip-in spark knock can indicate a lean spot in the carb and or wrong timing curve.

First verify no vacuum leaks are present including the oft over looked carb throttle shaft bushings.

Do you have a set-back timing light and a vacuum gauge?
Properly adjust the A/F mix either with an old scool vacuum gauge (adjust for the highest vac reading at idle) or even better get a wide band O2 meter. Take it for a ride. Then go at the timing.

You need to know what all three timing events are and at what RPM that total is "all in".

Use straight intake manifold vacuum on the vac canister.

Try 10-12° base at warm curb idle.
Limit the vac advance to 10-12° at idle....that = 20-24° at idle. Set the rate fast not slow.

Verify the engine speed (RPM) at which the total timing is fully advanced. For optimal performance, total timing should reach 34-36-38° at about 2800-3000 RPM.
Weights springs and stops are how this is adjusted if needs. You can use 2 different spring rates to.

Play around with all of this one at a time See what the car likes. Give it as much timing as it will tolerate.
It all depends on the engine characteristics, ambient conditions, sea level, gas, your driving habits et al...

 

The short version is that latter day HEI distributors, in factory default form, provide too much advance for Olds engines. Unless you create some kind of advance limit or dial the timing back (losing power), you’ll get knock/ping mid-throttle or maybe at wide open throttle (WOT.

The ‘net has tons of examples as to how to mechanically limit the part throttle advance to stop the ping. These work great and are a relatively inexpensive solution for the investment of a trip to the hardware store & a Saturday afternoon. Why not give it a shot?

I chased this issue for decades with quality HEI brands like MSD and Taylor. When they still pinged at mid-throttle, I tried slower-advancing non-Olds vacuum cans. My favorite was a big block Buick can for my Olds 455’s. And of course I tried the hex-key adjustable advance cans too. None of them got me maximum power with no knock/ping.

There was always a compromise. Either I’d have to turn the timing back & give up power to avoid the knock, or get max power + live with the knock. Or pour in $20.00/bottle octane booster voodoo sauce which sometimes worked and sometimes didn’t.

Back when Xylene and Toluene were legal in CA, I added a gallon or 2 to each tank here & there. That helped, but also cost $20.00/gallon. Plus - who really likes working with VOC chemicals like that which cannot be good for humans? Where I live they are banned now, so that’s out the window for me.

All of this experimentation lead me to Progression Ignitions HEI distributors. Upfront; I’m not associated, but I’ve bought 2 units used on 2 different cars and they’re working fine for 3 or 4 years now. The big deal with the distributors is you can build your own timing maps using your mobile phone and upload them to the distributor. So, when you hear a knock, dial back the advance in that RPM range _from_your_phone. Dang. That’s cool. Or keep separate timing maps for different gas at different times of year you might use. Progression pitches a bunch of other benefits like no advance <900 rpm (easier starting) and no starting if your phone lock it out (reduce auto theft since the engine won’t start). But the big deal is allowing each (educated) driver to define their own timing maps for the cam they have, the compression ratio they have and the gas they buy… To me, this was an enormous win. Read up on the possible downsides, like the company dies and whatnot, but I’m happy & I’m sticking.

Hope this helps,
Chris

 

Gotta map your advance curve (usually takes 2 people) and it would help if you have A:F numbers. The Progression Ignition distributor is one way take care of the curve side of things, but it comes at a price. The A:F will show you if/where you might have a consistent or transient lean condition.

What is the cam duration @ .050? Cam timing? Actual compression ratio? These can all play into issues like you’re having. Decades ago I had a 350 that I fought a situation like this and solved with a bigger cam. It had too much compression with too small a cam. I think that “too small” cam (a small hyd rllr) eventually ended up in another 350, 9:1 and it’s been happy since 2009.

Limiting vac advance may help as well. There used to be a vac adv limit cam available for about $5, but they disappeared early in, or just before Covid. Below is an ugly, but effective piece that I whipped up to limit vac on an HEI that required such measures. There are vac advance “stops” that eliminate movement of the linkage that could likely be modified to use as an adjustable limiter.









…

 

Really? Every adjustable vacuum advance canister I have used has about a maximum of 20 degrees and every turn adds or reduces advance. I currently have mine set around 10 degrees. Is it a Chinese aftermarket HEI? Otherwise most factory HEI have less mechanical advance and need more initial timing. I currently have my Chinese HEI at 15 base at 1000 rpm with 10 degrees vacuum advance ported and 38 total in around high 2800 rpm. These new Edelbrock heads do not like timing like iron heads, it doesn't even like 25 degrees at idle. The low compression Olds V8 are snappy around 60 degrees timing part throttle. At 12 base, depending at the rpm, you are probably in the 35 to 38 range total before vacuum advance. If an aftermarket vacuum can, it is counter clockwise to reduce the vacuum advance, about 2 degrees per turn.

 

Basically from what I understand, these vacuum cans use spring pressure to overcome vacuum to lower the advance. There is no mechanical stop inside to raise or lower the amount of advance provided. The end result is with a lopey cam you get timing dither/fluctuation because there is no hard stop.

 

That may be true of some variance but it does limit the maximum amount of vacuum advance. But they do fail and go full canister advance. A mechanical limiter is the most permanent solution. A stop on the mechanical advance on an HEI is a good idea as well, they add a few extra degrees in the 4000 to 5000 rpm.

 

What year was the engine and what compression ratio is it supposed to have?

I believe it was mentioned elsewhere but I'm pretty sure your distributor vacuum should be manifold and not ported.

 

1969, stock compression- says hook up to ported - was set up that way before rebuild w old hei

 

He should be about 9.6 to 1 for the high compression 4 barrel engine. That cam is close to BBO Edelbrock Performer flat tappet cam. For part throttle pinging, manifold or ported won't make much of a difference at part throttle, it should already be at full advance. Idle quality would improve. He should pull 10 degrees of vacuum advance out and run manifold vacuum.

 

Besides gasoline octane being too low or a vacuum leak causing it to run too lean, assuming your carburetor has stock jetting the only other thing I could think of is try a points distributor, then re-time it according to the manual. HEI didn't come about until the mid 70s and has different curves, it's not what the engine would have had originally.

 

What carb is he running? Is it a factory or aftermarket HEI? Depends if it is a junkyard or aftermarket Chinese HEI? My Chinese HEI has a points like curve. I would have 35 total with his 12 base. His vacuum advance should be adjustable, if an aftermarket HEI just use an Allen key.

 

I’ve backed off adj vac advance and seems to have improved but still slightly there. Called summit and they suggested adj edelbrock carb secondaries either richening or coming in sooner. I’m going to call edelbrock tech and talk w them

 

A wideband is very handy to tune a carb. If the AVS2, the secondary air door is easy to adjust. But otherwise, you are guessing. My carb was much richer than I thought it would be.

 

That's a good point, it didn't even occur to me it might be a genetic aftermarket carburetor.