# Help needed vacuum advance



## bondobill (Oct 24, 2011)

68 GTO 400 4spd.
We got Brents car home from garage, our mechanic says the carburetor should have vacuum advance. This is the way it was setup when Brent bought it.
What's missing, what do we need.
Like I said before I am no mechanic, I can swap out parts but the technical stuff I shy away from. Here some photos of the way carb is setup.
















































Brent says the car pings really bad on acceleration.
The mechanic asked us to do some research.
Any help would be appreciated guys. Have more photos if need be.

Thanks
Bill


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## BearGFR (Aug 25, 2008)

Well, if it's pinging my first questions are:
What's the compression ratio?
Which cam are you running?
What's the initial (and total) advance set to?

There is no vacuum advance on any carburetor, the vacuum advance is the diaphragm that the line connects to on the distributor (your 3rd photo). It will take a vacuum source that's either "ported" (doesn't activate until the throttle is opened slightly) or non-ported (active any time the engine is at less than full throttle). There's a great debate on whether you "should" use a ported or a non-ported source for vacuum advance. I'm of the opinion that you should try both and then use whichever one your engine "likes" the best.
Vacuum advance isn't going to have an effect on pinging one way or the other regardless, because ping is going to happen when the engine is under load - and when it's under load it's not going to be making any vacuum. If it's pinging under even light throttle, then your compression ratio is probably WAY too high and/or your ignition timing is set way too high.

Bear


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## bondobill (Oct 24, 2011)

Thanks Bear
I am going to talk to the mech myself to find out what he was talking about, I got it second hand thru Brent.
Brent said she only pinged at higher rpms.
As far as what the compression is we don't know.
P.O. said the engine was rebuilt to Pontiac specs for 350 hp 400c.i. Who knows :confused
Before they pulled the drive train they checked out the engine did a compression test and said it appeared to be in good mechanical order. They installed a new oil pan, replaced all the gaskets ect. They never tore the engine down. 
P.O. did say they put a different cam in it later but couldn't remember what.
When we get her back together the shop whats it back to fine tune it.

Bill


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## rickm (Feb 8, 2012)

just a suggestion. if you love the car and plan on keeping it to enjoy, invest in a shop manual. it will answer many of your questions and you will gain knowledge about it in many ways. even if your not a mechanic.


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## jmt455 (Mar 26, 2012)

Bill,
68 had a unique distributor with both advance AND retard functions.
I can't tell from your pics if yours is a 68; if it is, you need to make sure the vacuum lines and thermal vacuum switch are properly connected.

Chances are that your distributor has been replaced or modified and only has the advance unit, but you'll need to make sure what you have before any damage is done.


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## bondobill (Oct 24, 2011)

jmt455 said:


> Bill,
> 68 had a unique distributor with both advance AND retard functions.
> I can't tell from your pics if yours is a 68; if it is, you need to make sure the vacuum lines and thermal vacuum switch are properly connected.
> 
> Chances are that your distributor has been replaced or modified and only has the advance unit, but you'll need to make sure what you have before any damage is done.


The block is a 69 so who knows about the distributor. Would a electronic ignition be the way to go. Problem with that is we replaced the engine harness and it's not wired for a electronic ignition 
I installed electronic ignition in my 69 Vette better than 20 years ago, been real happy with it.

Bill


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## BearGFR (Aug 25, 2008)

If you want to convert to something like an HEI, it's really simple. Go get yourself a generic headlight relay, mount it someplace convenient (mine's on the firewall behind the HEI), then use the original ignition lead to the coil to instead "turn on" the relay. Wire the 'other side' of the relay so that it gets a direct unimpeded 12v, perhaps straight from the battery and through a fuse for protection. This way you get a nice strong 12v feed direct to the HEI (or other electronic ignition) which is usually what they want. You don't have to alter your original wiring harness or circuitry at all, so if you ever decide to go back to stock with the resistor wire feed to the points you'll be all set.

Bear


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## jmt455 (Mar 26, 2012)

bondobill said:


> The block is a 69 so who knows about the distributor. Would a electronic ignition be the way to go. Problem with that is we replaced the engine harness and it's not wired for a electronic ignition
> I installed electronic ignition in my 69 Vette better than 20 years ago, been real happy with it.
> 
> Bill


A GM HEI conversion is simple and inexpensive, but you have to make certain the advance curve is appropriate for your engine. You also need to run a 12V feed from the ignition switch (Start & Run) to the distributor. It might work using the resistance wire, but it could lead to problems due to the low voltage (only about 9V) being fed to the HEI unit; Bear's relay recommendation is an excellent solution!

I can see in the photos that there is a vacuum advance canister on your distributor. I would test it to see if it's working; there are a couple of common problems to look for. When these things get old, the advance plates can stick (old, crusty grease) or the diaphragm might be torn/leaking. 

To test it, disconnect (at the carburetor) the hose that goes from the top, front of the air horn to the distributor canister. Use a vacuum pump (with gauge) to apply vacuum to the canister (at least 18" Hg) and watch to see if the vacuum level stays steady or leaks down. If the vacuum level drops off, you either have a bad advance canister of a leaking vacuum hose; find the culprit and repair or replace it.

If the canister holds vacuum, remove the distributor cap and rotor. Release the vacuum and watch the advance plate (where the points are mounted). The plate should rotate smoothly back to its stop. Apply and release vacuum a few times to verify that the advance is working properly and that it returns to the "home" position every time.


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