Idle & vacuum questions

[bwilhite1]

Hi all,
I've got two separate questions that I'm struggling to answer. I've got an 86 half ton with a 350 crate motor in it with a mild cam. I installed an Edelbrock 1406 with an electric choke about a year ago on it, but don't have much other info on the motor.

First: When I swapped carbs, I hooked the distributor up to ported vacuum because I didn't know what I was doing. The truck runs fine and I haven't had any issues. Last week, I read an article called "Timing and Vacuum Advance 101" that says that since there's no emissions control, it should be hooked up to manifold vacuum, so I swapped it. With the timing advanced at idle, it seems (smells) like it's running really rich and I can't see why you'd want it that way. Haven't had any issues running hot and the mileage seems to have suffered in the little bit it's been driven while on manifold vacuum. Seems like the thing to do would be swap it back, but just wondering what I'm missing?

Second: When I'm driving around, the engine seems to be warm, and stop at a light, my idle jumps back up to at least as high as it is when the choke is on. Looking at the carb, the choke plate is straight up and down. I can step on the gas in neutral to disengage the choke and sometimes it works and goes back to low idle, but sometimes it doesn't. Anybody have an idea why? I rotated the choke toward the cab and it seems to have loosened the choke plate up a little but I haven't driven it yet to see if it will help. Any advice is appreciated.

 

[74 Shortbed]

Going to manifold vacuum won't make it run rich, did you adjust the mixture screws??, sounds like you have a weak return spring or linkage sticking, choke don't have anything to do with it after it's hot, undo your cable and make sure the carb works smooth then cable movement it should move freely back and forth.

 

[bwilhite1]

I did try and lean it out after hooking it up to manifold vacuum, but it was still putting off some smoke at idle and it smells like gas. It doesn't do that when it's on the ported side, so I thought it had something to do with the timing advance.

Thanks for the heads up on the return spring. I'll take a look at it tomorrow. I probably should have mentioned it before, but it occasionally diesels as well when I shut it off. I idled it down a little, which seemed to help a little, but didn't take care of it completely.

 

[Georgeb]

Take some time and look at this thread.
http://www.gmsquarebody.com/forum/showthread.php?t=16563&highlight=Failed+emissions

 

[rich weyand]

Read this thread so you understand a bit more about timing and mixture:
http://www.gmsquarebody.com/forum/showthread.php?t=14508

Basically, ported vacuum runs the ignition late at idle. Since a richer mixture also burns faster, setting the idle rich helps mask the incorrect timing by reducing the time it takes for the flame front to go from the spark plug to the piston face. Or, another way to put it, when people adjust the idle mixture for best idle performance on ported vacuum, they get to a much richer spot.

So you need to lean out the idle mixture screws for best idle performance when you switch to manifold vacuum. Your mileage will go up, your engine temps will go down (because you won't be dumping a still-burning mixture out through the exhaust runners in the heads), and you will lose any hesitation coming off-idle due to the need for the vacuum can to adjust to the proper advance.

 

[rich weyand]

On the fast idle sticking, take a look at the fast idle cam plate on the driver's side of the carb and make sure it isn't wobbly or catching on anything. You could be missing a spacer or have the wrong shoulder screw on the pivot.

 

[bwilhite1]

Read this thread so you understand a bit more about timing and mixture:
http://www.gmsquarebody.com/forum/showthread.php?t=14508

Basically, ported vacuum runs the ignition late at idle. Since a richer mixture also burns faster, setting the idle rich helps mask the incorrect timing by reducing the time it takes for the flame front to go from the spark plug to the piston face. Or, another way to put it, when people adjust the idle mixture for best idle performance on ported vacuum, they get to a much richer spot.

So you need to lean out the idle mixture screws for best idle performance when you switch to manifold vacuum. Your mileage will go up, your engine temps will go down (because you won't be dumping a still-burning mixture out through the exhaust runners in the heads), and you will lose any hesitation coming off-idle due to the need for the vacuum can to adjust to the proper advance.

Great thread. Thanks Rich. Any idea what would cause an increase in smoke?

 

[rich weyand]

Dumping the still-burning charge into the manifold causes secondary combustion in the manifold. That was the whole point of the emissions setup, and the AIR pump was to add air to this fire to burn up emissions before they went out the pipe.

This secondary burning process due to ported vacuum may have been burning up the smoke, while under manifold vacuum there is no secondary burning process. That is, you may have had whatever is causing the smoke for a while, but it was partially masked by being on ported vacuum.

In any case, manifold vacuum is how all GM engine vacuum advances were hooked up from the beginning of vacuum advance in the late 1930s through 1967.

Lean those idle mixture screws out for best performance. You want highest RPM, then turn in for 20rpm drop, called "best lean idle". You want to be a bit leaner than the highest-RPM point to keep from fouling plugs.

Here is the carb instructions. See page 11.
http://edelbrock.com/automotive/misc/tech-center/install/1000/1404_manual.pdf

 

[Rusty Nail]

Hydrocarbons go through the roof using manifold vacuum.

It is vitally important that your vacuum advance canister is correctly matched to your application using manifold vacuum.

This has very recently become a HUGE source of contention within this MSB for no good reason.

I use ported vacuum in all of my vehicles because it sucks and I think it's wrong.
I do NOT use Edelbrock carburetors, because I think they are great.

My hotrod is faster than yours because ported vacuum is bad.

Quite frankly, people that ask about this, have no business running manifold vacuum and will likely do it wrong ending up with less than desirable results. That said, my official stance is that you should run ported vacuum because your truck is not a race car.
If it was, you wouldn't have asked.

See what I did there...

Wasn't broke but you fixed it anyway, how's that workin out for ya?

 

[Georgeb]

Hydrocarbons go through the roof using manifold vacuum.

It is vitally important that your vacuum advance canister is correctly matched to your application using manifold vacuum.

This has very recently become a HUGE source of contention within this MSB for no good reason.

I use ported vacuum in all of my vehicles because it sucks and I think it's wrong.
I do NOT use Edelbrock carburetors, because I think they are great.

My hotrod is faster than yours because ported vacuum is bad.

Quite frankly, people that ask about this, have no business running manifold vacuum and will likely do it wrong ending up with less than desirable results. That said, my official stance is that you should run ported vacuum because your truck is not a race car.
If it was, you wouldn't have asked.

See what I did there...

Wasn't broke but you fixed it anyway, how's that workin out for ya?

I resectfully disagree with you. I made the switch to manifold vacuum on my '78 350 with excellent results. If there is no concern about emissions the hydrocarbons are of little worry. That truck never ran so good after I reset the base timing and tuned the carb in again. I was running a stock Q-jet. I have also had good luck with eddy carbs. The key is tuning it correctly.
I guess run your stuff the way you want to.

 

[rich weyand]

When I learned to tune a car, back when the ink was still wet on my driver's license, all vacuum advances of the major mfrs were run on manifold vacuum, because, pollution aside, that's the best fit to the combustion chemistry. It had been that way for nearly 30 years. My '61 Chrysler, my '62 Plymouths (both of them), my '62 Buick, and my '66 Chevelle all ran on manifold vacuum. There wasn't even a carburetor port for ported vacuum.

Ported vacuum is one wall of the emissions structure that emerged in 1968. Ported vacuum, a different vacuum advance canister, a different base timing setting, different carb tune, an AIR pump, and a catalytic converter were the other walls, the floor, and the ceiling. Running ported vacuum without the rest of the components doesn't make any sense. It's like having one or two walls of a shed in your back yard. You are suffering the disadvantages of ported vacuum -- reduced gas mileage, hotter engine temperatures, and off-idle hesitation -- without the benefit of the reduced emissions that resulted from the entire system being in place.

If you are going to go the full route with the emissions package, then by all means use ported vacuum. The emissions package won't work right without it. But if you are running without the rest of the emissions package, then hooking up the vac advance the way God and GM intended it is the way to go.

It is correct that you need the correct, pre-emissions vacuum advance can, and I make that point in the thread I linked above. The AR-23 or equivalent is the best fit for these engines, and was a standard can back in the day.

All that said, hook it up and run it the way you want. For me, I just tune 'em the way I learned to, back in the 1960's.

 

[donnieray]

I always love the threads on manifold verses ported vacuum and I think I've read them all. I run manifold vacuum on both of my 350's, the one I built myself and the one that was bought from a guy who had it ported. I also use Edelbrock carbs and intakes on both. Manifold vac is the only way mine will run as they should. From idle to wot they both run great. Can someone shed a little light on the instruction manual that came with a crate 350 from GM that a friend of mine owns. The instructions state that NO vacuum advance is to be used. If I hadn't read it myself I wouldn't even have believed him.

 

[rich weyand]

Yeah, even the General gets it wrong sometimes.

 

[Rusty Nail]

Yeah, even the General gets it wrong sometimes.

Blasphemy.

 

[donnieray]

Yeah, even the General gets it wrong sometimes.

Best answer I ever got for that question!