n100

I've built a mild 350 engine. CC xe262 cam. Vortec heads. Hei distributor. Now I go to time the engine and I get it all in at about by 2800 rpm at 36 degrees advanced. This puts my initial timing at around 15-16 btdc. Now I go to connect my vacuum advance (full manifold) and my initial timing shoots up to around almost 40 and I get a lot of pinging. My vacuum advance is adjustable however I can turn the Allen screw in it all day long and it doesn't seem to adjust or bottom out.

Is my vacuum advance broken and if so what kind should I get to put my initial timing at a reasonable level with no pinging? Or should I be retarding my timing more?

DUB

If you apply vacuum to the vacuum pod and the distributor cap is off so you can watch it move...does it move??? If it does not...then I would have to possibly consider it is damaged.

Also I have seen that the amount of 'throw' that the rod of the vacuum advance pod can move can be incorrect. The amount of movement may need to be limited due to the square cut out in the advance pod is longer...thus adding more advance.

Personally... I do not get caught up in the 'numbers' when I am timing an engine. There is no 'holy grail' number that a motor has to run perfectly on...because each engine.... due to variables... can cause it to be set differently to run the best possible.

I use a vacuum gauge and get it set and then when I am done.....I use my timing light so I can write down numbers on paper for the owner. Because I have had some engines where the amount of advance is NOT as what many people feel it should be....but yet the engine will peel your eyelids back and not ping....and IF the engine is set to the 'numbers' that seem to be what it should be set at...you can tell that it is not running as crisply as before.

DUB

71VetteLover

It's pinging because you have too much mechanical advance for the octane of the gas you're using. Back the initial off to around 10 degrees and see if that cures the pinging. 36 degrees is just dandy IF you're running 91 to 93 octane but it's usually too much for 87 octane.

HeadsU.P.

I did not read anywhere what octane you are using. Actually, with vacuum hooked up you should be at 52* total. Careful with that allen wrench. Instructions warn not to turn screw all the way out, will damage unit. H-m-m-m-m-m. Maybe you or somebody else already did.
You really need a vacuum pump tool (EBay $20). Then you can apply vac to the canister and know exactly where you're at. And do you have a limiting (stop) plate on the vac adv? Max should be limited to 10*- 12*.

n100

I am currently running 87 octane but have no problem running higher octane if that will help.

I have a vacuum pump. What should I be looking for? And I'm fairly positive that is what happened to my vacuum advance I have no stop on the VA

I will also regard my initial to 10 or so but would that not drop my mechanical or my "all in" timing so it occurs later in the RPM range?

Bloodzone

I could not get the allen wrench can to work with manifold vacuum. I switch to the AR31 can and it worked great for me.

DUB

As far as I know there has to be a stop made into the vacuum advance.

DUB

HeadsU.P.

Yes, dropping your initial will affect the total because you are turning the distributor to do so. If you want less initial & more total, that's where the color coded bushings come into play: allows more mechnical adv or restricts the adv.
The color coded springs determine at what rate or how fast the adv kicks in.
Then the Vac canister adds even more adv on top of that at cruising speeds. Such as 36* to 52* total for better gas mileage, cooler temps and better throttle response.

A Vac Adv stop plate is mounted under a screw that the Vac canister is attached too. The plate has four settings for adv and is somewhat adjustable. Here is where that vacuum pump tool will not only show when the Vac canister starts to move but also when its being stopped or limited by the plate. If I have you totally confused, then I did my job.

n100

No that actually clears it up pretty well thankyou! And thankyou everyone else for the responses. I guess I got hung up on trying to get my total timing to 36 degrees per the Lars timing specs. I settled on about 12 initial which puts me at about 32 total at 2500 ish rpm. Took the car for a spin without the VA connected and holy cow it's like a new car! And I get no pinging at all!

I do go back to hook up my VA though and I get a nasty rattle still at that timing. I know there's a huge debate about running or not running a VA, but for now I'm going to run without one I suppose. I would like to get one hooked up though. How can I find the amount of advance I need with a VA without pinging as much as I do?

Bloodzone

This is how I got manifold vacuum to work for me.

Car pulls 11-12ish vacuum per gauge.
Used AR31 VC connected to full manifold vacuum.
Can starts at 4ish, all in at 8ish with 16* of Vacuum.
Initial is 12 (with VC connected it gives me 28 initial at idle.)
Mech adds 22, so thats 12 initial + 22 mech for 34 Total.
Add in the 16 VC at cruise and I get 50.

My car really likes manifold vac and the high 28* initial it gives at idle.