Fellas,
Here I am so close to the end of a major overhaul, and I'm (literally) sputtering out. I believe that all I need is some minor fine-tuning adjustment to get this truck running, but I just can't seem to get it. Here are the details:

1970 F-250 Camper - 360 V-8

Heads removed, reconditioned; valve job. New fuel pump, new alternator wiring, new plugs, plug wires, distributor cap. I rebuilt the carb using a kit.

This truck has a manual choke.

Right now, I can start the truck when cold with the choke closed. It idles VERY high (over 1500 rpm) and I can't seem to get it to slow down.

Other data:
Timing: 6 degrees BTDC with vac. advance disconnected and plugged.
Dwell: 32 degrees (after I set the points. Before doing so, it was down around 12 degrees)
Idle mix screws on carb: 2 1/2 turns out.

At this speed, the truck does not produce any black smoke or anything unusual. It seems to run smoothly.

The PROBLEM: After warming up for 15-20 minutes, I still cannot open up the choke at all without it dying. I cannot get the idle down to normal.

There is mild hesitation when I try to rev the engine, but then it revs OK.

After it's hot, if I try opening the choke the slightest bit, it will either die immediately (as though I've turned it off), or sometimes it REVS, then dies, then REVS, then dies, then finally is off completely.


I would really appreciate any guidance here. Are there more tests I should do? Should I start from the beginning? Could this carb be bad somehow?

Another note: before I did the head work, the truck idled OK and the choke operated normally (that is, after about 5-10 minutes of warmup I could open it completely). The reason for all this work was an exhaust gasket leak, leading to broken bolts, etc. etc. The only other previous symptom was stalling at high speed, which I read might have been due to the points (dwell of 12 degrees being a bit below spec)


Thanks a bunch, I'll look forward to hearing from you.

- james

 

I'm a novice tuner myself. But I'd be looking for a vacuum leak. Plug everything at the carb/manifold and see how it acts.

I'd also bump the initial timing advance up to about 10-12 BTDC. back off if you hear pinging at higher RPMs.

 

Sound like a vacuum leak to me too. I would however leave the timing alone until the leak is located (too lean +too much timing=damage).

 

A vacuum leak... good idea. It reminds me of the last clue I forgot to post:

Two or three times during the tuning process, I noticed that an ounce or two of fuel had leaked onto the intake manifold. I could see that it had dripped down the carb mounting plate, but could not tell exactly where it had come from. I've tightened the carb mounting bolts down pretty good, but if the seal between the carb and intake manifold is bad, that could be the source of a vacuum leak, and I suppose the source of fuel spillage if the engine didn't fire or was cold.

Tonight or tomorrow, I'll try a different carb mounting gasket and see if that helps. I didn't use any gasket-maker silicone in that area because of fuel exposure.

I guess the other possibility is that the carb itself is leaking fuel, and maybe vacuum. How can I determine that?

Thanks guys!

-j

 

It could be in the carb itself, especially in the throttle shaft area. Leaky shafts can give a host of problems that are extremely hard to diagnose.
Greg

 

A leaky throttle would explain where that gasoline came from.

Perhaps I'll convert to fuel injected! hehe

Thanks agian, I'll post tomorrow if I find anything. I realized in searching today that my spark plugs are slightly the wrong gap, so I'll do that tonight as well.

 

Your idle speed is directly related to the choke. With the choke on, the fast idle cam should be engaged so I wouldn't worry too much about that

Although I would tend to agree that a vacuum leak is the most likely culprit, you might also look into a leaking or improperly adjusted float. Your fuel dripping into the manifold supports this to some extent as well. If the fuel level is too high, it could be causing a flood condition at idle, but with a higher engine speed the engine is drawing enough fuel to prevent this.

 

Just to be certain the terminology is correct

When the book says Vacuum advance disconnected and plugged.

They mean :*the Hose running to the advance* is supposed to be plugged...(Golf Tee's work great) ........

not plug the vacuum advance tube.

I just had to make sure

 

You can use carb cleaner to find a vaccum leak. SPray it down your lines. When the engine bogs you've found your leak

 

i just had this exact idle problem, when i took the carb off because i thought it was bad i noticed my throttle plates were open almost 1/2 way with no acceleration.


check the position of your throttle plates, and if there open more then a fraction of an inch it's going to idle fast!
adjust them to specs!!

1970 f-100 390 mtrcrft 2100d
dual flowmasters 2 3/4 inch pipes out the bumper
runs like a raped ape!

 

OK, I've determined the following:

1. The throttle plates are properly adjusted to be nearly closed at idle.
2. The float is at the proper adjustment.
3. All my new spark plugs were carbon-fouled... probably because of my initial efforts in setting the timing, mixture, etc. I cleaned them and I'll see if it happens again.

Now here is the interesting part: when I opened the top of the carb to check the bowl float, the bowl was EMPTY! Shouldn't the bowl still have fuel in it, since I had the truck running just the day before? This leads me to believe that there IS a fuel leak somewhere.

Another disturbing fact: as I mentioned before, a couple of times I saw that a pool of fuel had spilled onto the manifold. Two times out of three, that fuel has been BLACK with dirt, or oil, or some sort of junk. Is there a way to drain my fuel tank and see if the fuel in there is black? Or should I just siphon some out, is that easiest?

Tonight my homework is to manually fill the bowl in the carb with fuel, and watch to see if it comes out somewhere.

Thanks for any additional advice!

- james

 

Power valve in carb??? George

 

I dont' remeber what the terminolog for all of this but way back my auto shop teacher showed me hoe to bleed the bowls and set there level. He opened the screw on top of the fuel bowl and backed off the nut on that same brass screw ubtil fuel started coming out. All while the engine is running. When fuel started coming out of the screw and nut asembly he would back the nut back down to the point that the fuel was not coming out any more. Then re tighten the screw. He told me do to this any time you put on a new carb or one that has been sitting. I don't know if this will help but I do it every time I change a carb. I think it helps to remove the air pockets that may form in the fuel bowl and set the seatting level of the bowl.

Let me know if this sounds familiar to anyone?

 

What does your fuel filter look like? You shouldn't have anything in fuel bowl except gas. Oxygenated fuels and gas additives are hard on anything rubber (fuel lines, gaskets, etc). I have filters on mine on in-bed tank, and also on fuel line ahead of carb, just for insurance. If it was mine I would rebuild carb and check all vacuum lines for leaks. Dont forget the pwr booster vac line if you have one. I have found it easier to just change out old lines with new when acquiring used vehicles, doesn't cost much and they are usually borderline anyway.

 

You could take a one gallon metal can as use it as a test fuel tank. I assume you have the 2100 2bbl carb? You can take the top off and see what the fuel level is doing while the engine is running. Also, install a vacuum guage.