Disconnected Vacuum Advance??

Hi all,

When replacing the rocker box gasket earlier today (thinking it might be a source of a small oil leak I am trying to find) I noticed that the vacuum advanced tube wasn't actually connected to its port but zip tied to the side of the rubber bung. The port itself was sealed off.

I have since reconnected this and the car started better than ever before, although of course that could just be coincidence. I have two questions:

1) Is there any reason why the advance would have been left disconnected? The previous owner had the ignition upgraded from contact to optical, but I can't think why this would stop the need for the advance.

2) I have been experience some backfiring when idling at speed. Someone here before mentioned that this may be related to the advance. I didn't get the chance to do much on the car since then until today, hence I didn't notice that the pipe wasn't actually connected! Would this be a symptom of having no advance?

I plan (weather depending) to take the car for a short run tomorrow to see if I can notice any change in performance with this now reconnected. But wanted to check here also to see what issues there might be. Oh and in case it makes a difference she's an overdrive-less '73 GT.

Many thanks,
Peter

73.

ok

but change your profile to tell us what car you have.

there's no real reason to drop the vac advance. Other than myth. And the myth is 180 wrong. The vac advance, if it is correct, is a good thing.

There's also no real easily noticable problem in most cases for not using the vac advance. Just works better if you do.

Thanks for the quick reply... also for the slap on the wrist for not yet having put the car into the registry (now done!).

- Pete

one reason they get disconnected is they are bad, check and see if yours holds vacuum

Both Pete and Mack are on it.

The reason that racers don't use a vacuum is that they operate at relatively consistent high revs and don't worry about low end timing as it might relate to starting or economy.

Disconnect the line at the carb end and suck on it. If it holds a vacuum, pull the cap and watch the contact breaker plate for movement as you apply said sucking action. It should move through at least a few degrees.

If you can't hold vac replace the vac pot. If you can and the plate doesn't move, yank the dist and find out why. It could be something simple like the plate rotating surface being just dirty or scummed up. Clean it, lightly lube the innards of the dist, make sure that the cam rotates a little on the main dist spindle, test for movement of the plate again while the unit is out, put the whole thing back in and set the timing.

BTW: If for some reason you have HS4s, do you have vac at a manifold port or out of the rear carb body?

Jack

Also keep in mind that if your vacuum advance is the correct unit with the vacuum port off to the side of the cannister, it will be pretty difficult to generate enough vacuum by mouth to move the breaker plate. That unit is designed to work with manifold vacuum, so it takes a high vacuum level to generate advance.

So what does this mean Jeff? If you have the later vac pot you really have to suck! LOL

Jeff is right. If you have one of them you might well need a MightyVac to do the sucking for you.

Jack

I think I'd check the timing after connecting the advance. If it is working, it may be putting too much spark advance into the system. If the PO had "advanced" the static timing such as not to need the advance, then, with it connected, if it's working, you may have too much total advance at road RPMs.

I have a 73 BGT that the vac advance has been disconnected on. It has HIF4's.

The vac line from the pot comes straight out, not on side or at a 90 degree angle.

Where is the correct place to hook the vac line, carbs, manifold???

Thanks,

Bryan

If you refer to the picture below, the 41491 distributor used in '73 and '74 used the vacuum advanc eshown on the left. The earlier models used the unit shown on the right. The best vacuum advance to have for manifold vacuum is the 10-15-5 spec unit shown on the left, Moss part #163-670, or I rebuild them like the one shown.

Thank you all for the replies...

I have the distributor on the right hand of the picture, and as far as I can tell everything is working ok there. I did however, horror of all horrors, discoverer that there is oil leaking into my back cylinder from somewhere! Sooty is not an accurate description of how the plug looks!

OK nose back to the grind... this report unfortunately will not write itself.

Cheers,
Peter

Peter, look closely at the valve cover gasket. Its common fo rhtem to leak downintot he plug well on the outside of the head, causing the oil-fouled plug when you remove it.