Idle question C4

Right now I have my idle set at 1000 while in park/neutral. When I put it in gear it drops to 400ish. Is that too low? It feels like it wants to die but doesn’t. Any ideas or thoughts. I can raise the idle to say 1200 in park but that seems too high. I have not found any vacuum leaks. 66 with a 289 stock as far as I know.

Where is the vacuum advance hooked up at? If it is manifold vacuum, switch to ported vacuum.
Where is the base timing (vacuum advance unplugged) set to and at what RPM?

Excessive base timing makes an engine sensitive to loads. Put a load on it and the idle will drop.
Manifold vacuum advance is even worse. As you put a load to it (going into gear) vacuum drops. Drop in vacuum reduces timing. Less timing and the vacuum drops even more.

You should be able to idle in the upper 600s with minimal RPM drop when you put it into gear, low 600s. 1200 to 400 there is something very wrong. Timing set by ear or vacuum gauge is a big one. Use a timing light or it isn't set right.

Thanks Bowsher Vacuum advance is hooked to ported. Timing is 14 BTDC at idle. 1000 goes to 400 when put in gear, 1200 goes to 500 when put in gear.

What does it do when the idle is set to between 600 and 700 rpm? Probably drops to the same 400-500, but if it's still trying to die, or is dying, then obviously other things are still amiss.
But you should not have to compensate by raising the idle to an excessively high level like 1000 rpm. Too high for an auto in an old truck for sure.

Checked for vacuum leaks?

Good luck.

Paul

I would try backing the base timing off to about 8°. readjust the carb and see what happens.

If you really need more advance at speed, you have to get into the distributor and change the centrifical advance. You can't make up for it with base timing. Some of the emissions distributors have very wonky advance curves.

My C4 would do that also. I moved to ported advance and 10 degrees and it was marginally better. When I rebuilt my transmission I bought a re-manufactured torque converter and it was a better but not like a modern car. The converters are so tight they load up hard with almost no slip. I could set the idle at 800 and it would settle to 600 in drive. On a cold day with carb it would be very finicky changing from park to drive.

How many miles are on the timing chain?

Paul: I will set the idle to 700 today after work and see what happens

Bowsher: I will try backing the timing back to 8-10 and then readjust carb, I thought about that last night but it got too late. I don't know how many miles are on the timing chain.

Mark: Glad I am not the only one....well not really because it can be frustrating as I am sure you know.

I did notice oil leaking out the back of the intake manifold(even though the PO looks like he used a whole tube of RTV), could that be a source of a vacuum leak? Either way I need to pull the intake and get some new seals in place.

Is it worth messing with timing and idle before I change the intake gaskets or am I just chasing my tail till the gaskets are replaced and eliminating the leak?

If timing and carb tuning don't work, might be excessive timing chain stretch. Causing the cam to run retarded. Bad valve timing.

The factory valve cover decal on my '77 says idle at 600 RPM in gear. I've found that to work just right. It supplies enough air and coolant flow to keep it cool without loading the torque converter.
As Broncobowsher said, make sure the timing and idle mixture are set to spec first. This way the transition from neutral to gear causes the least change in RPM.
Also, any vacuum leak will result in higher idle RPM, but it can't hold the load of the torque converter when in gear.

mattyq17 said:

I did notice oil leaking out the back of the intake manifold(even though the PO looks like he used a whole tube of RTV), could that be a source of a vacuum leak?

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Normally, no. It would just be the source of an oil leak.
But if you have an internal leak, where one or more of the intake ports are leaking from the cranckase side, this can then be a leak at the end rail of the intake. But you would not see much oil coming out. Rather it would be trying to suck it in.
And to a certain extent, the PCV valve will be pulling some vacuum through any gaps, and can effect idle. But for now let's just lower the idle first, see if it runs, then check for vacuum leaks with one of the usual methods.

And speaking of the PCV valve... How is yours plumbed in?

mattyq17 said:

Is it worth messing with timing and idle before I change the intake gaskets or am I just chasing my tail till the gaskets are replaced and eliminating the leak?

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It's absolutely worth messing with first. It's not running correctly now, and the initial adjustments are easy and quick to accomplish. You're still in the experimental phase to even see what's going on, so do the quick and easy (and cheap!) stuff.
Yes, you're going to have to redo most of it after changing an intake, but again, it's quick and easy and might give you more information right away.

Things I would do would be to leave the timing where it is for now, and lower the idle speed just to see what it gets you.
Then lower the timing to the 8-10 degree range just to see what it gets you.

Obviously after you change timing you're going to have to raise the idle again, but that's the tuning-dance we all go through when changing things.
For that reason, of course you could actually change the timing first. I'm just thinking that changing the idle just needs a quick turn of the screws to see what it gets you. You could in fact do either one first. I'm just thinking simple first.

Of course too, whenever you change the idle speed you almost always need to tweak the idle/air mixture screws to achieve the smoothest and maybe even highest idle. Then mess with the speed screw again. Then mess with them all over again until you achieve the balance you're looking for.

After, or during whatever initial adjustments you make, by all means check for vacuum leaks all over the intake and carb area. I use carb cleaner on all except an engine with newer paint. Otherwise water or WD-40 or other more benign spray will work.
Spray any potential leak area, including any vacuum hoses down to the transmission, AND down at the transmission modulator valve.
I don't know this for a fact, but perhaps the others know whether or not a defective modulator could be causing any of this trouble?

Good luck.

Paul

I agree Paul, I am going to adjust the idle down, see what happens, then adjust timing, re-adjust idle, idle/fuel screws(edelbrock 1406 since I haven't mentioned it yet).

The PCV is going to the front big port of the carb.

I checked yesterday for vacuum leaks with carb cleaner(engine paint is not a concern) and didn't notice any change in idle. I didn't check down on the transmission modulator though so I will try that today.

So far this is all fun for me, I enjoy working on it and learning new things....up to a point

Question regarding idle mixture screws. I have read both and understand both but want to know which is better for my use,(weekend cruiser, drive to work when the weather is nice, forest service roads, no hard core wheeling). Stock 289

1. Turn clockwise for a leaner set up on mixture screws(economy).

2. Turn counterclockwise for a richer set up on mixture screws.

I am thinking # 1 for my purposes.

mattyq17 said:

Question regarding idle mixture screws. I have read both and understand both but want to know which is better for my use,(weekend cruiser, drive to work when the weather is nice, forest service roads, no hard core wheeling). Stock 289

1. Turn clockwise for a leaner set up on mixture screws(economy).

2. Turn counterclockwise for a richer set up on mixture screws.

I am thinking # 1 for my purposes.

Click to expand...

1½. You want a happy spot between lean and rich. Too lean and it runs rough, random lean misfire. As you richen them up it will smooth out. Keep going rich and it will usually still run happy then start idling down. If you have a wide band O2 sensor in the exhaust it is really easy to dial in. Doing it off feel and sound alone is a bit of an art. I generally set them to where they run good and go toward the lean side of the happy smooth idle. Both sides should be nearly the same setting.

Big then is be gentle. if you bottom the needle out in the seat don't force it. That puts a little ridge on the needle and it will never flow smooth again. You should be able to dial these in by finger. Reach and heat often force a small screwdriver, but only needs the force of a couple of fingers. And do it fully warmed up, as in out driving 20 minutes warmed up. Often this will mean a rough adjustment when you first start it and again after going for a drive.

mattyq17 said:

...I enjoy working on it and learning new things....up to a point

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I hear that!

Paul

And to add to what broncobowsher said, the two screws do not have to be set the same as each other. The numbers you see in print are only "guidelines" and may not have any bearing on your own personal reality.
So twiddle, tweak and tune until it runs the best for you.

If you do it cold, just to get it working, that's fine. But unless you do it fully warmed up like he was saying, you're just going to be doing it again when it does warm up, because there can be quite a difference between hot and cold running depending on the engine.

Paul

Oh, and how you set the idle has little to next-to-nothing to do with overall economy or power. It's just the idle you're adjusting.

Only main jetting has any great effect on overall running and fuel efficiency.
In your carb, that's both the main jet sizing AND spring and needle sizing working together.

Paul

I just went through this dance myself and it was a vacuum leak. Did you hook up a vacuum gauge and see what it reads at idle? Do that first and see what ya got. Stock 302 should get around 20 or so. Mine started at 11 and I kept finding little things until the big leak was found. My emissions Holley List 6919 had the wrong curb idle mixture needles in it! I had the typical fuel mixture needles when it they were supposed to be the slightly thicker, blunter air metering needles. With the vacuum leak so bad, the C4 didn't shift right and the vacuum advance wasn't right either....it was a mess. Once I fixed the vacuum leaks, it was now time to set idle, set the idle mixture, time the engine and adjust the C4 vacuum modulator.

By the way, the other smaller leaks were carb base gaskets, PCV hose, new PCV valve and grommet. Also rebuilt the carb.

It was a wonderful day when it ran like a champ when all was done!

Mikey said:

I just went through this dance myself and it was a vacuum leak. Did you hook up a vacuum gauge and see what it reads at idle? Do that first and see what ya got. Stock 302 should get around 20 or so. Mine started at 11 and I kept finding little things until the big leak was found. My emissions Holley List 6919 had the wrong curb idle mixture needles in it! I had the typical fuel mixture needles when it they were supposed to be the slightly thicker, blunter air metering needles. With the vacuum leak so bad, the C4 didn't shift right and the vacuum advance wasn't right either....it was a mess. Once I fixed the vacuum leaks, it was now time to set idle, set the idle mixture, time the engine and adjust the C4 vacuum modulator.

By the way, the other smaller leaks were carb base gaskets, PCV hose, new PCV valve and grommet. Also rebuilt the carb.

It was a wonderful day when it ran like a champ when all was done!

Click to expand...

Mikey, Yes I will be picking up a vacuum gauge this week, between then and now I will youtube how to use one, lol.

So I think I have success. Warmed it up and took it for a 20 min drive. Dropped the idle to 800 and it dies as soon as I put it in gear. I used edelbrocks procedure to adjust the idle mixture screws but couldn’t get the idle to come down past 1000 by adjusting the idle screw. Sat there looking at it for a minute and realized I needed to adjust the high idle screw to give me room to lower the idle. Got out the timing light and hooked it up and my tach is 150 rpms off compared to the tach on the timing light. 800 rpms was actually 600 %)%). Set it to 800 rpms off the timing light and put it in gear..... idles at 600 and has no sound of wanting to die. I will see how she does and report back if I have any more problems. Thank you all for the help