diagnosing this fuel/spark issue

[BiggSixx1970]

Seems like the past couple weeks I've been on here a lot lately! My 1970 c20 is being very temperamental this summer.

I finally whipped my issues with my cooling system after flushing it out and replacing everything from the coolant to a water pump to an upgraded radiator.

Some time along that way, my engine picked up a little hiccup that I think has something to do with fuel but idk.

It runs fine, if not just a tad rougher now, but you have to keep your foot on the gas pedal. Accelerating it seems to run good while climbing, letting off it pops a little more than normal (due to a leaky manifold gasket, this hasn't presented itsself as an issue, pretty much every 250 I6 has an exhaust leak) and when I put in the clutch, or shift into neutral, it dies.

It got a major tune up last summer: Fuel pump, carb rebuild kit, new distributor, cap, plug wires, plugs, fuel filter, along with other misc minor misc hoses/gaskets, air filter, coil

This spring I gave it another with: Plugs, distributor cap, new radiator, water pump, cooling system flush, and another air filter

Everything in my ignition system should be up to snuff, correct? and my carb rebuild/coil/plug wires/distributor/fuel pump are only a year old.

Whenever I let off the gas in neutral, or push in the clutch it dies.

I did notice after driving it around after I replaced my cooling system, that it smelled kind of rich. Could this be a carb air/fuel mixture problem? It's a rochester monojet, there are just 2 screws i think. Messing with that carb is something I know nothing about. I have always been told if it's running, and running good (which it has up until recently) to leave them alone

Thanks for your input! You all are a big help

[domeier]

I'd guess you have a vacuum leak. Try plugging off your vacuum ports one by one to isolate where. Distributor vacuum advance diaphragm might be shot.

Two screws on the Rochester are idle speed adjust and idle mixture control. You can do a rough adjust on the mixture by turning clockwise until it stops, then 1.5 turns ccw, but the truck needs to be idling in order to set it correctly - if you haven't touched the mixture adjust and it ran right in the past - leave it be. Otherwise you'll just add to your mystery.

[BiggSixx1970]

That's exactly what I'm afraid of, adding another problem to the mix. I have never touched the idle speed and idle mixture control. The vacuum advance on my distributor is new, it came with the new distributor

Now I don't know much, but I thought that if the vacuum advance was acting up it would idle correctly, but run rougher or cut out as the rpm's increase?

I will start by trying to track down a vacuum leak, but my engine has almost NO vacuum lines. Theres no power steering, no power brakes, no ac. In fact I think the only vacuum line it has is to my distributor for the advance, and my oil pressure gauge.

If memory serves me correct, when I replaced my distributor last summer, to set my idle timing I set it without my vacuum advance connected, meaning the engine should idle without the advance, just cut out as the rpms increase, right?

[Rufton]

Once you set idle speed and mixture one time you will find it very easy. You can adjust w/ vacuum gauge and tachometer or just adjust by ear. It's a teach a man to fish kinda thing

To make tinkering low risk put a paint/sharpie mark at 12:00 position and keep track of changes. I'd mark 12:00 and turn CW till hit seat; write the results. Then you can get back to original clocking if desired.

[chevy88]

Hook up a vacuum gauge it can tell you alot, have a look at this website for instructions http://www.earlycuda.org/tech/vacuum2.htm, it a cheap and easy diagnosis.

[domeier]

Originally Posted by BiggSixx1970

...but I thought that if the vacuum advance was acting up it would idle correctly, but run rougher or cut out as the rpm's increase?... If memory serves me correct, when I replaced my distributor last summer, to set my idle timing I set it without my vacuum advance connected, meaning the engine should idle without the advance, just cut out as the rpms increase, right?

Vacuum leak will cause it to idle rough or not idle at all. When you adjust ignition timing or idle mixture, the vacuum advance diaphragm needs to be disconnected *and* plugged off at the manifold/carb.

Think about it this way: The engine needs a particular mixture ratio of fuel and air to run, so if you have a leak in your vac system, you're sucking in additional air and making the mixture too lean. It'll be farty, poppy, rough or no idle.

Or... maybe your distributor hold-down clamp loosened up and allowed the ignition timing to change?

[BiggSixx1970]

I may not know exactly what i'm talking about. This is my first truck, first carburated vehicle, and first delve into oldshcool technology. But it seems to me like it may be getting too much fuel, when it dies you can kind of smell it, i think it smells like fuel but it doesnt just smell like straight gas, i'm a rookie so I guess it smells kind of rich I think. Like when it's at higher RPMs it has something to do with the extra fuel but it has too much to run at idle? I dont know, just my rookie speculation, trying to give you guys as much info as possible

[Rufton]

Plugs from lean (white) to rich (fuel wet and/or black carbon):