I have a 77 with a 350 and a Rochester. Carb , distributor, and motor (along with all new fuel lines cause the car was rebuilt) have been rebuilt. The car will run at 1000 rpm once it’s started then after driving for a couple minutes it wants die so I give it some gas (get it to about 1000 rpm) and then it starts to climb by it’s self and it idles at 2000 rpm till I put it back in drive and then it cherps the wheels and drop back down to 700 rpm until another stop.(I put it in neutral at lights or when I stop because it has a really big cam, my auto teacher said that’s what you are supposed to do). I’ve asked so many people what it could be and no one has any ideas, most vacuum ports are closed off and the choke was “adjusted” so was the carb, I checked if the weights were sticking on the distributor but it’s was all fine. The engine and carb ran good before being rebuilt but we rebuilt the engine the ran it with carb(before we rebuilt it) and it did the same thing so we rebuilt the carb and still. Anyone have any ideas I’m running out of summer time.

 

You wrote: most Vac ports closed off. So I assume the dizzy is still receiving some vacuum. And I think that is the key word; some.

Only thing I can think of is the Vacuum Canister is playing games. Its applying some advance, then RPMs drop, canister compensates, RPMs back up, etc.
That's the fun of having a big cam.

Do you have a vacuum pump tool w/gauge and a vacuum gauge?
I think the first step is getting some Vacuum data;
How much vacuum is the engine providing?
With the distributor cap removed, a vac pump attached to canister port, observe how much vac is needed to "start" moving the canisters advance arm?
At what point does the vac arm "stop" moving? (limit)
And last, does the canister even hold vacuum?

If your modified 350 is only producing 8-9 inches and it takes 12-15 to move the vac adv, then you know that is not going to work.
There are dozens of canisters on the market. Some are adjustable w/allen wrench for tweaking.
And there are adv limit plates that affix under the screw of the canister. Those have four choices of limiting vac adv degrees depending on orientation of the unit in the distributor.

 

What do you mean by the canisters advance arm? I have no experience with these canisters

 

The vacuum canister is attached to a small arm that moves the timing plate. It is just a part of the vacuum system, nothing to worry about. I would get a vacuum gauge on your engine first and see what it is doing. What you are experiencing sounds like it could be related to your distributor. It is worth a look if you are comfortable with removing the distributor and re-installing it when ready. I would not recommend trying to take apart the distributor while on the car. The advance mechanisms were known for getting gummed up and not working properly.

I finally removed my factory T.I. box and distributor and boxed them up. I am using a MSD Distributor with vacuum advance and a mechanical tach drive as used in my 1968 C3. Running high compression in a 427 BB my engine is picky about timing. The MSD billet distributor has many options for different curves and limits and in their instructions you learn how to do all these things. I might even suggest you take a look at the MSD instructions as they helped me understand the whole concept better. Here is a link to the MSD instructions so you can see how the advance curve and the setup of a distributor is done. I have no experience with a Rochester carburetor, sorry.
https://documents.holley.com/8572.pdf

Good Luck, it won't be long and you will be cruising again!

Best regards,
Chris

 

I took the advance arms off and cleaned them off but even before I did they moved very very freely but I still took them off and cleaned them and tried to see if it would change, it didnt fix anything

 

I'd suggest disconnecting the vacuum advance line from the distributor, plugging the line, then set the idle speed so it's steady and reasonable. Then take the car for a test drive....

 

Now if I do that will that tell me if the vacuum advance canister is bad or...?

 

That's all fine & dandy that you cleaned them. But you never ran a test to see what range the canister is operating at. if you don't have a hand vacuum tool, they can be found on Ebay cheap enough. You need to know what's going on with the Initial Timing, Mechanical Advance Timing and the Vacuum Timing.

A vacuum gauge,
a vacuum pump tool,
a dial-back timing light,
are all a must if you want to do any tuning yourself.
One trip to a shop to have this done ($75-$100) would otherwise pay for all the tools you need.

And I am not insinuating the canister is bad. It just might not be the correct unit for you. Consider your original canister maybe designed for 15-22 inches of vac. You have a big cam. You will be lucky to get 7-10 inches.

 

Let’s just say that my stock won’t work with my cam (it is a big cam so I wouldn’t be surprised if the vaccine is off) where can I get a canister that’ll fit my application?

 

Yes, it will tell you if your canister is bad, or not the correct one for your specific application. AC Delco used to make dozens of different canisters, set to operate at different vacuum levels, and they were tailored to each individual engine that GM produced. There was a number stamped into the mounting bracket of the canister that identified this parameter. However, since GM eliminated the vacuum advance function somewhere in the mid-late 80s, I doubt that they still provide the canisters.

However, the aftermarket still provides them, and the best thing is that the aftermarket units are adjustable, to allow you to tailor it's settings to your specific needs. Crane Cams was the first company to offer them, beginning in the mid-80s, and I do believe they still sell them. I also believe that Mr. Gasket and possibly Moroso offer them. Jegs or Summit Racing would be your best source for one.

 

Why don't you want to do a vacuum test? I bet autozone or whoever even lends those tools out for free.

 

True

 

I hooked up a gauge and the car ran better without the vacuum advance installed, and it was running at about 10hg. So now my question is how do I adjust one of those adjustable ones(found it on summits site)? Like I install then how do I know how much to turn it?

 

The aftermarket units come with complete instructions, however the "cliff notes" are that the advance is initially set at "zero", with the adjustment screw set at the end of the stop, as it comes out of the package. You install the unit, insert the Allen wrench, and give it a half turn, or so, if I remember correctly, counter clockwise, then go for a test drive. Essentially, by "trial and error", you keep giving the car more advance, until it either "pings" or possibly in your case, starts running rough. Then you gradually start turning the adjustment screw the other way, until you find the "sweet spot".

 

Let me acknowledge you that a new adjustable canister can be installed while the distributor is in the car. Tricky, but doable. By removing four of the plug wires from the cap on one side, the cap can be laid off to the side. Place a large towel or sheet under the dizzy. If you drop a screw it will stay on the top side of the engine bay.
Another forum member said that if the canisters arm refuses to release from the advance mechanism, you can move the advance plate to allow removal of canister.

As far as how much Vac Adv to dial in? Trial runs. Lars said no more than 10-12 degrees. That's where the vac pump tool will show you movement of the arm, when it starts and when it stops. My MSD has a limiter plate that has four choices of maximum degrees allowed by the canister. But I never saw another one for sale.

A dial-back timing light will also show you how much Vac Adv has been added.

 

So I ordered one off summit last night and it came in the morning, I installed it took about 5 minutes everything went smooth. Now I turned the car on and it still did it but sooner so I turned it down and it kept delaying more and more every time I turned it down. Until it I think it reached it max for turning (It didn’t bottom out it just kept kinda like kept clicking). The car seems to run better but now it won’t stay idling and the problem is still there when it’s in neutral it doesn’t do it anymore when driving. So I am running out of ideas. I tired turning the idle screw but it still just wants to die. Any new ideas?

 

Where does the vacuum hose from the distributor attached to the carb?

 

It’s ported, and I did some research and found that it might be the springs for the vacuum advance

 

I think you might mean "springs for the mechanical advance".....

 

Remove hose from "smog" port, plug that, and reattach hose onto full manifold Vac port.