surging idle, has to be something simple

[78ak]

I’ve got a 1978 chevy 350 with auto trans – I bought it a few days ago, and guy said its been in the family since 1980, and his grandpa did a “complete overhaul” in 1986, and he got it in 2007, hasn’t been driven more than a few hundred miles since then, and not at all in the last 4 years, but it was in a heated shop. Drove it home, battery shot, died 4 times, stopped and a got new one. Made it home, and saw volt meter only charging around 9 – changed alternator – now pulling 14 or so, working good. Issue is that it used to die when putting into gear. Done a couple small things, now it doesn't die (most of the time), but it runs rough. When driving down the road and coming to stop, idles choppy and way too high.

So far, all I’ve done is put in a fresh tank of gas, changed the plugs, checked all the vacuum lines for leaks (including the brake booster which I thought it would be and the line on the back of the transmission), new vacuum advance, new sending unit for the temp gauge (it was bad), new fuel filter in the carb, turned the distributor about a quarter inch counter-clockwise, and new PCV valve. It idles good. Choke works well, and it kicks down to probably 600-700 RPM after warm-up, sounds good, minimal to no missing. Before I did anything to it, it would die instantly when put into reverse or drive. Now it will sputter a bit but not die every time. This is all after warm-up. If I put into gear while the choke is on, it shifts into reverse and drive perfectly, no hard shift, but it surges from low to high idle. When let off the brake, it will drive at high idle RPM, way too high. I’ve done a little research and found everything from torque converter to fuel pump to ignition control module, to timing, etc. Before I do anything more, thought I’d throw the question out. I know there's a chevy guy out there who knows exactly what it is. Thanks for reading and responding if you have any ideas.

[78ak]

Forgot to mention, driving down the road it shifts perfectly, runs strong, doesn't hesitate when pushing on gas. Problem is only at idle.

[geezer#99]

Is it all stock air cleaner, heat riser etc.
You need to take some pics of the motor!
You need to get a timing light and check the timing!

[78ak]

I'll post some pics of the motor tonight. Whole truck is stock.

[Bears63]

You did some messing with the distributor so the very next thing you need to do is hit it with a timing light and then go from there. Make sure that timing is properly set, it can make a huge difference.

[rich weyand]

Originally Posted by Bears63

You did some messing with the distributor so the very next thing you need to do is hit it with a timing light and then go from there. Make sure that timing is properly set, it can make a huge difference.

Yeah, that too. Read this. More than you ever wanted to know.
http://67-72chevytrucks.com/vboard/s...d.php?t=646219

[rich weyand]

Just poor tune adjustments, mostly, I think.

1) Put the vacuum advance on manifold vacuum.

2) Set full warm idle to 700-750. It should idle at 550 in Drive.

3) Separate adjustment for high idle. Dial it back. If you hook a warm engine on the top step of the fast idle cam, it ought to idle at 1200.

4) Oil the choke linkage. (it's one of those spring doodads in the manifold, right?) Make sure it isn't binding up.

[78ak]

thanks. I'll give it a try.

[Axle]

If it has an electric choke, make sure it has a good contact. My "new" 81 had a corroded choke terminal, worked one time, not another. Cleaned it up and alls been good with that.

Consdering it has sat so long, you'll also want to put a few miles on it to get things limber again, choke linkages, fluids moving, etc. The 81 is now starting to loosen up a bit and behave and sound smoother after a couple weeks of driving it.

Alex.

[franken]

Fuel filter, or the sock in the tank?

[78ak]

hoping it's not the sock in the tank, and I changed the carb fuel filter already, and I did throw a pint of sea foam in it just to see if it was a quick fix - oddly enought it ran a little better after that. It's literally been driven 40 miles in 4 years - so that stears me toward what Axle says...maybe I just need to oil up the carb choke connections and drive the crap out of it for a few days and see what happens.

[Dead Parrot]

The seafoam is a good idea. Whatever gas that was in the tank when you got it could have been 4+ years old. Plus whatever was in the carb was long gone, leaving behind dead gas goo in the small idle passages. As you replace the old gas in the tank with new gas, it will probably run better. I would change both the transmission fluid and rear end oil soon. It is likely that the last time either was changed was during grandpa's complete overhaul if then. If it still has one, check the spare. Wouldn't be outside the realm of possibility that it is still the 36 year old original. Also check the rubber suction line from the gas tank to the metal gas line. If that is still original, it is end of life.

[rich weyand]

Which choke is it? If it is the electric choke, you need to adjust it properly, too.

[78ak]

I know I need to check timing, but I don't own a light, and it's on my list. But, I decided to try and rule out some simple things tonight. Figured it'd be a good even once I fix the problem. Which... I didn't.

Tonight I changed wires, cap, rotor, coil, and ignition module.

I don't know. Seems like it's starving for fuel. Carb has seen better days. Can't figure out why it's only at idle when it runs like garbage.

[77 350 Shorty Wide]

Good Morning, I didn't read all the posts thoroughly but one thing I didn't see...
I certainly would check for vacuum leaks.

Have a glorious day and God Bless, Terry

[rich weyand]

Originally Posted by 78ak

I don't know. Seems like it's starving for fuel. Carb has seen better days. Can't figure out why it's only at idle when it runs like garbage.

Did you check to see if the vacuum advance is connected to ported or manifold vacuum? Being connected to ported vacuum can make the idle suck, even though everything else is OK.

[78ak]

Morning. I did check all vacuum lines by sight, then sprayed carb cleaner all over - no leaks.

Also, yes, vacuum advance is connected to manifold vac.

[rich weyand]

OK, good. You need to get a timing light, and set the base timing, say to 12* BTDC for now. Then plug in the vacuum advance and check it again and make sure it is pulling in. If the vacuum can is broken, you will get a crummy idle. Also, if it is an aftermarket can, and it is set wrong, the timing could be hunting. You should be able to see the timing hunting with the timing light.

[Axle]

That carb looks rather good as is.
All it may need is the idle passages sprayed out with a can of carb or throttle body cleaner. And yes, that means pulling the airhorn and getting a carb kit if the gasket shreds upon disassembly.

Alex.

[78ak]

bad news, pulled spark plug wire for each cylinder, 3 of them made no change in the idle, 5 did. I'm going to run a compression test but I know it's going to say I've got problems. that's an expensive fix in Alaska

[rich weyand]

If you join Amazon Prime, you can get a new GM 350/260hp crate engine, with free shipping to Alaska! No kidding. Another guy got one shipped free to Fairbanks this year.

[motornut]

can't see if any bushings left in the vac advance area.
the parts wear oblong holes in the weights,rust and seize up like mine
float/carb could be gummed up.-> seafoam!!
my 79 has a green close to that

[78ak]

Hey thanks, I'll take a look at the bushings - I did put a tiny drop of oil where the bushings would be if they were there, I noticed when I had the cap off that the weights didn't fully move back into place after flexing the spring. I'll check it out tonight. -- also, I've got a can of sea foam going through the tank right now. --- any thoughts on running a can of it through the oil? I've never used seafoam until this but I've heard good things about it.

[77 350 Shorty Wide]

Good Morning, I reread some of the posts, things you have done already, I did not see where or if you had changed the plug wires, I might have missed it. That would be my next thing to check before the compression test.

God Bless, Terry

[78ak]

for 77 350 shorty --- I've changed plugs, wires, cap, rotor, coil, ign module, vacuum advance, fuel filter in the carb, and its timed... bought a compression gauge last night, worried what I'm going to find ---- ever heard of heads that have been sitting for years come alive again? or is it a 'once they're gone they're gone'