HonestDave

More weirdness. I just remembered the carb guy said there was no proper vacuum port on this carb for the advance.
There are two on the front, one upper, one lower. They both seem to have full vacuum at idle. Actually, also one on the side that is going to the choke assembly fast idle thingee. I just checked with my gauge.
When I check the numbers 17082283 (Rochester) the ID charts say 1983 but no "8" for seventh digit. One chart had an "8" and it said Mercruiser Marine Carb.
I believe the vacuum for the distributor should be very low and increase with RPM.
Is this not correct for a ported vacuum?

Right now my initial timing is 14 degrees, and add 12-14 for the advance when I plug it on. I suppose it's dropping when I give it gas. This is just too much. The carb guy says he can put a proper port in for me, but I have to remove the carb.
Always more to ponder. Never less, always more....

rjohnson442

Vacuum DECREASES as RPM rise! What kind of carb Mfg is it from? If you don't have a ported vac inlet on the base plate of the carb you should have access to one on the manifold itself. I Think your "carb guy" is taking you for a ride.

HonestDave

It's possible I'm misunderstanding him. I'm pestering him whilst he's working on other people's cars, and trying to remember what he told me last fall.

For the port going to the vacuum advance, what should the reading be at idle, and roughly what should it do as RPM increases.
It seems over the years that some cars have low advance port vacuum and it increases as RPM increases. Other ones have more vacuum and it decreases as RPM increases. I go back and forth from GM to Ford to Import, and start to loose track. I want this thing running perfectly, and my carb guy is off sick for weeks now, so I'm getting adventurous and impatient.

stan 65 cutlass

gm is somehow different in that is has vaccuum even at idle. but i know what your thinkin' dave, its usualy the lowest port, below the throttle plates

brown7373

Ported has zero vacuum at idle. It comes on at light throttle, and all but disappears at wide open throttle. If it is a marine application, it may not have a ported vacuum advance. Vacuum advance of either type is designed to give you lots of advance at "light" throttle, and is more for better drivability and increased fuel mileage.

HonestDave

I like this answer the best. It kind of sounds like what my carb guy was trying to tell me......possibly.

I think with my carb plugged into the port, I'm probably sitting at a timing of around 27 degrees at idle. If I unplug it (and cap the hole) I'm at about 14 degrees.
The ports on the front both seem to add about 13 degrees when I plug into them. I think I need a new, correct port installed. Pretty sure full vacuum added to the advance, would be wrong.

MDchanic

Yes, but how does it run?

- Eric

HonestDave

Runs not too bad, actually, but I'm striving for better. I also want it to be correct, if possible.

I'd like to have the smoothest idle possible, with the best acceleration the motor will give me. That's why I want to have the advance working properly. I don't think I'm quite there yet.

MDchanic

The idle will be smoother on manifold vacuum than it will on ported vacuum.

Ported vacuum will be zero at idle, and will rise to be equal to manifold vacuum once the throttle is opened just beyond "a crack."

- Eric

joe_padavano

Sounds like it's time for a carb vacuum tutorial.

There are THREE different vacuum signals available on a carb.

Manifold vacuum - comes from below the throttle blades of the carb. High at closed throttle then decreasing to zero (atmospheric pressure) as the throttle blades move to fully open.

Ported vacuum - this comes from a port in the carb bore just ABOVE the closed throttle blade. Because it is above the throttle blade, ported vacuum is ZERO at closed throttle. As soon as the throttle blade opens enough to uncover this port, ported vacuum jumps to equal manifold vacuum and is essentially identical to manifold vacuum from that point to W.O.T.

Venturi vacuum - this comes from a tap at the narrow section of the venturi. Venturi vacuum is directly proportional to the flow through the venturi. It starts out at zero with the throttle blades closed and gradually goes up to maximum at W.O.T. This is caused by the Bernoulli effect (go ahead, Google it. I'll wait). This is why airplane wings generate lift. It is also why airflow through the venturi sucks gasoline up out of the float bowl and through the main jets.

HonestDave

Perhaps I should just leave my vacuum advance the way it is, then. It's sucking about 15 on the gauge at idle, as are all the ports including manifold ports. I can't adjust my fuel mixture screws because they don't seem to do anything no matter how low I get the idle.

I usually have more luck with carbs. Maybe I just expect too much. Was thinking if I got it on the right port, maybe the other things could be played with (timing, fuel mixture) and I could get it running dead smooth.

Nothing wrong with dead smooth is there? Doesn't seem like too much to ask of a rebuilt 455 with the wrong carb on it. 1982 model, according to the numbers. Still like to know why an "8" as the seventh digit.

oldcutlass

Generally when you can't adjust your idle A/F and your getting vacuum through the ported vacuum tube, its because the carb is not in the idle circuit. Look down into the carb and note the position of the lower butterfies, see if they car in the correct position.

If your getting a total timing number of around 50 BTDC @ 3500 rpm with your vacuum advance connected and no pinging or surging under light load cruising up a hill, your good.

HonestDave

I'll take a look at that in the next few days. Tomorrow I have to go look at a 59 Pontiac 2 door hardtop and a 41 Chev 2 dr sedan. A nice fellow in the valley needs loot, and is trying to get me to help out.

HonestDave

I took a look at the lower butterflies. Hard to see, but they look flat. Don't appear to be partly open.