volition

So I finally got things dialed in and went to hook up the vacuum advance and was shocked at what it did to my timing light...

It looks like when I hook it to full manifold vacuum it's RETARDING my timing by seriously 50-60 degrees.

What in the world did I do wrong? It's a brand new Borg Warner unit too.

Peterbuilt

Full manifold Vacuum advance should give you 14 to 20 * at max, did somebody grind out the stop hole?

volition

The problem is it's retarding the timing, not advancing it.. and by a lot. I'm not really sure why.. maybe it's advancing so far that it's wrapping back around the balancer?

Which stop hole are you referring to? The canister itself is brand new

Peterbuilt

Do you have any sprigs in your distributor?
The vacuum can on the distributor should be a 'Zero' advance untill you hook up the vacuum line to the distributor, then you should get up to 16* of advance on the timing light.
What distributor are you using?

71coupe

How is the engine running? If it's ok, then your ballancer's outer hub is slipping

volition

Mechanical timing is already set ... 14 at idle 36 at 2900 that's all fine.

It's just when I plug in the vacuum that it get's wierd.

It's a GM HEI. This is when using full manifold vacuum of course. It does ok at idle if I use ported but it still advances the living crap out of the engine off-idle under light and moderate acceleration causing a bad ping.

The engine is absolutley screaming fast and silky smooth I'm just trying to get the vacuum advance to work right for the mileage improvement

73StreetRace

Maybe you could remove distributor cap, hook vacuum advance can to a vacuum source
( like another car with engine running for instance ) and see how the plate moves.
As the distributor rotor turns clockwise, the base plate should turn counterclockwise when you apply vacuum.

marz

Do you really need to hook up the advance? Mine did pretty much the same thing and I just left it disconnected and plugged it. Worked for me. I have a new GM HEI unit in mine. Interested to see what the end result is.

larrywalk

73StreetRace is right. After you remove the distributor cap, when you apply vacuum, the base plate should start to turn at about 4 to 7 inches of Hg and reach full advance at 12 to 15 inches of Hg. - the base plate should move about 3/16 inch counterclockwise, measured where the rod pulls the base plate.

__________________
1978 Corvette
406 SBC AFR heads Lunati roller Quadrajet TH-350 3.08
11.59 sec / 118.9 mph 1/4 mile

...Roger...

I doubt it is actually retarding the timing by 50-60 degrees--the engine would stop.

HEI ? Maybe it has a pickup coil from an Olds. I think that coil is backwards to the Chevy coil. JAT

The confusing part is you say manifold vacuum retards the timing but when you hook up to ported it "advances" the timing ???

73StreetRace

Vacuum advance is good for part throttle performance and economy.
Any street machine should use it.
Only race cars or drag racing cars don't need it, because they mostly use the gas pedal like an all or nothing switch...

myk7

I seriously doubt the vac advance is retarding your timing. It either advances timing or does nothing. If your timing is set at 14 at idle with the vac advance disconnected, timing should be 29 to 35 at idle with the vac advance hooked up.

Does the idle go up when you hook up the vac advance? If it does, that means it's advancing it. Revved up to 2900 (mech advance all in) with the vac advance hooked up, it should be about 52. Are you sure the mech advance is all in at 2900? Rev it on up to 4k and see if you get more advance out of it. It should not be pinging unless you are running crappy gas, that's why I suggest the vac advance is not all in at 2900.

bobs77vet

what if the mechanical arm is popped out of the slot? and totally misaligned......i bet that would do funny stuff

WhiteC3

Good point. Thats something to look at.

volition

Well I don't know for a fact that it's retarding it, but here's exactly what happens:

Unplugged with timing light on.. balancer is showing 14 degrees.

Hook up vacuum, suddenly my 0 degree timing mark on the balancer im watching goes the complete opposite direction about 4-5 inches away from the timing marks , in the retarded direction.

What is confusing is the idle does indeed go up as if it's advancing.

I just noticed this canister I picked up is stamped with a "51" on it. Now correct me if I'm wrong but isn't that number what indicates how much advance it pulls?

volition

Well, it doesn't actually advance my timing.. I should clarify. It's more like the timing mark acts normal under low vacuum (ported idle, part/heavy throttle) but at high vacuum the timing moves waaay back to the retarded side.

I went and picked up a new Accel cap/rotor/module/coil and put it on after reading your post just to make sure I didn't have the wrong one on there. Seems to still be doing the same thing

...Roger...

The part I was talking about was the pick-up coil.That's what the vacuum advance is attached to. You have to remove the dist and shaft to replace it.

volition

Is there a way to tell the olds vs chevy apart?

bobs77vet

i think the number on the cannister is a part number or reference number, i dont think you can draw any conclusions from it other then matching it up to the refernce guide....Lars has a refererence guide one that says what the part numbers mean in terms of advance and the the vacuum that it pulls in it. when you say retarded which way is it going clockwise or counter clock wise? it seems like we are missing something obvious here......

...Roger...

To be honest I can't remember. I was hoping someone else would chime in and confirm whether or not the wrong one would in fact retard the timing. I think the Olds dist installs on the other side of the cam. I'll look up the parts and see if I can see a way to tell them apart.