A Couple of (Possibly Related) Fuel Problems

[basketcase0302]

"390xlt"]Well, I might have spoken too soon. I went for a test run yesterday, and it ran pretty bad at speed, but idle was fine. Again, I figured it needed a little fine tuning, but I wasn't too worried about it.
Fast forward to today: it fired right up and idled great for a minute or so. Right as I was about to put a vacuum gauge on it, it stumbled a couple of times and died. Went to start it back up and it was doing the same thing as yesterday, not wanting to start, then if it does start, it won't stay running unless I'm giving it a lot of throttle.
Since I can't get it to idle, I can't test for vacuum leaks. I did use up a can of carb cleaner testing it before I took it apart and didn't find any leaks. I doubt if there are any now, but there's always a chance that something didn't get sealed up when I put everything back together.

PS The tank seems fine so far. Mounting holes for the sending unit and the cab lined up exactly.

That still sounds like air leaks to me and/or the accelerator pump on the carburetor. A second set of hands to keep the truck running while you spray the carb area for leaks would be nice...

Joseph, the O-rings are bad...about egging out of round leaving gaps where they're not sealing.

[390xlt]

I won't be able to do anything about it until next weekend, so I guess I'll start recruiting a helper.
Could the choke pull off be causing me problems? It goes up and down like it's supposed to, but I did notice a couple of holes in it while I had it apart. Since I've got manual choke and don't use it very much anyhow, I didn't sweat it, but it is vacuum operated and does have holes.

[basketcase0302]

"390xlt"]Well, I might have spoken too soon. I went for a test run yesterday, and it ran pretty bad at speed, but idle was fine. Again, I figured it needed a little fine tuning, but I wasn't too worried about it.
Fast forward to today: it fired right up and idled great for a minute or so. Right as I was about to put a vacuum gauge on it, it stumbled a couple of times and died. Went to start it back up and it was doing the same thing as yesterday, not wanting to start, then if it does start, it won't stay running unless I'm giving it a lot of throttle.
Since I can't get it to idle, I can't test for vacuum leaks. I did use up a can of carb cleaner testing it before I took it apart and didn't find any leaks. I doubt if there are any now, but there's always a chance that something didn't get sealed up when I put everything back together.

PS The tank seems fine so far. Mounting holes for the sending unit and the cab lined up exactly.

Can't stress enough about being able to spray the carb while it's running with a vacuum gauge on it as your basically flow testing the carb-while checking for leaks. One common issue with all carbs on our older trucks is the throttle shaft bore on the aluminum body of the carb itself wearing out and leaking, (sucking air) between the bore and the brass throttle plate shaft. When or if the bore is leaking it's time for a new carb. One way to tell is it will usually be discolored, (the aluminum bore) where the throttle plate shaft exits the bore (where the linkage hooks up) usually a darker discoloring indicating it has been leaking for awhile. Good thing that I love about the Motorcraft carb on our old trucks is that it is still available for a decent price if you need to replace it.
http://www.autozone.com/fuel-delivery/c ... &year=1970

[390xlt]

So this thing is really starting to drive me crazy. Now I can't get it to run at all. Every once in a while, it will fire for a few revolutions, but it won't stay running, even when I give it gas. The throttle shaft had a tiny bit of play in it, but I can't imagine it's causing such a huge vacuum leak that it won't even start. Except for the choke pull off that I mentioned before, all the rubber is new. There's plenty of stuff I can check once it's running, but I've got to get to that point first. Any more ideas?
Did I maybe miss a vacuum port? The distributor and pcv are both connected, and I've got the large port on the back and the small port on the front both capped.
Mixture screw adjustment doesn't seem to be making any difference, so it certainly seems like a vacuum issue, but how can I diagnose it without the engine running?

[basketcase0302]

If that is the Motorcraft 2100 it is a pretty simple carb to rebuild, (as long as nothing on the carb was too far out of spec to start with). I believe you have all the ports sealed if you have the large PCV on the rear and there should be two smaller ones, (one on the front of the base / and a second one half way up on the side). Plug all of them to get it running right-then hook up your vacuum advance after you've done your tuning.
Having a handheld remote starter here is a lifesaver as you can crank the engine over while spraying for leaks and if you find one you'll see/hear it in an instant, (even if it won't start). The base gasket is one problematic area but usually not enough to cause the engine to not start-just run roughly when it starts leaking.

Ignition off: can you see a nice steady stream of fuel looking straight down the carb when you operate the throttle linkage from the idle stop to the full throttle position? You should see two steady streams coming out of the nozzles when you do that-if not something went wrong in the rebuild, (which is what sounds like to me). One other thing to check is that the fuel tank is venting, (via the gas cap) as if it doesn't vent your fuel pump will be fighting to pull fuel up to the engine and never do it). Take the gas cap off / cover it with something to prevent it from being a fire hazard / then verify your picking up fuel by the pump using the method above looking into the carb, (or a fuel / vacuum gauge hooked into a tee fitting after the pump to verify pressure). A lot...of folks will sell new fuel caps for the bump era trucks that don't vent. Can't tell you how many locking types like this I'd bought only to start having mechanical issues.
One other thing to look at that I found when building my truck is that if you have power steering on the truck the fuel line actually routes behind the power steering pump, (from the factory). Mine was crimped/crushed down to 1/4 of the 3/8" diameter tubing creating fuel starvation from the factory!

Unless there is something other than carburetion wrong here...

[390xlt]

All the vacuum ports are sealed off. Moving the throttle does give a nice spray of fuel in the carb. I've got a new gas cap, but I tried the whole thing with it removed and got the same results. My fuel line doesn't run close enough to the power steering pump to cause any problems, but it does run between the frame and the steering box. It's got a little bit of play on both sides, but since I've recently replaced the box, I pulled it off just to be sure nothing was crushed; it wasn't.
I'm certainly willing to entertain the idea that the problem isn't with the carburetor, but since the original problem seemed so obviously fuel-related, I want to eliminate that first, and it looks like we have.
Where do I go from here?
What really baffles me is the fact that it has started up and run just fine a couple of times, but only for a minute or two each time. And I did manage to get it to the gas station and back -- once.

[basketcase0302]

If you have a steady stream of fuel coming from the carb nozzles-you should be able to keep the engine running long enough to check for a vacuum leak.
Did the carb rebuild kit come with a new accelerator pump?
How many miles are on this 390? Have you ever ran a compression check on the engine?
Condition of the points and how old is the coil-is the coil grounded to the intake manifold well?

[390xlt]

It did come with a new accelerator pump. The old one didn't seem to have any holes in it, but I did replace it.
The odometer shows just under 27,000 miles, and I've got to assume that's rolled over at least once or twice. I'm not sure if the engine's ever been rebuilt. I have not run a compression check, but when it runs, it runs great.
The cap, rotor, points, condenser, and coil all look fairly recent, but looks can be deceiving, especially when everything else on the engine is so covered in grease and grime. The coil is not grounded to the manifold at all.

Since it's not a huge undertaking, I'm going to take the carb apart again this weekend and double-check that everything is clean and correctly assembled. If I'm still having issues after that, I think I'm going to start trying to find an electrical problem. Does that seem like the reasonable course of action, or should I be going at it from a different angle?

[basketcase0302]

The coil has to be grounded or it won't keep the engine running, (the bracket that mounts it to the intake manifold is the ground point-then the ground strap from the engine to the battery finishes the ground connection). Really hard to tell from your description but I believe you've got another electrical gremlin, (I remembered you having one here before):


If you can get it running long enough at night, (in the dark) with as much outside lighting around you turned off-is a good time to see electrical gremlins come out. You'll see arching and grounding where you'd never see them in the day time. If you pull the carb back apart double check the float level as it could be starving for fuel.

[farmallmta]

My son bought a '63 Fairlane that had a similar issue to what you're describing. After going through it all (carb, distributor, ignition switch, starter solenoid, several fuel pumps), here's what he found was the actual culprit: the fuel lines from the tank to the fuel pump and from the fuel pump to the carburetor were clogged with tank-reseal material applied by the previous owner. The reseal material was turning loose and working into the fuel line, along with the rust that was being turned loose, too. Clogs of this gunk were at the bends in the line where the fuel slows down. The clogs could not be seen, so weren't diagnosed. Gunky debris accumulated at the bends and eventually nearly completely closed up. Just enough porosity existed in the gunk blockages to allow fuel to come on through slowly. Occasionally a big enough hole in the clogs would open up to allow enough fuel to come on through and let it run normally and perfectly for a while, until it would gradually re-clog with more gunk from the tank. So the car had a very intermittent problem that was not consistent enough to be accurately diagnosed for quite a while. When a thin piece of wire was run down into the short metal line between the fuel pump to the carb, chunks of the gunky debris came out on the end of the wire. That's how the problem was finally discovered.

Some suggestions have been made here about resealing a tank. I have never been comfortable with doing that. That seems to me to be a snake oil solution to a major leaking problem. If the tank is rusty or has pinholes, spend the money to replace the tank and replace the metal lines. Put a clear plastic Wix brand filter ahead of the canister filter. Put another metal filter just ahead of the carburator. These screw into the bowl of the carb inlet.

For what it's worth, I have my carbs rebuilt by a retired mechanic who now specializes in cleaning and rebuilding carbs in a shop behind his home. Like many other touchy mechanical items, carb rebuilding sounds simpler than it really is. Lots of experience and lots of practice help a lot. No rebuild he's ever done for me has ever been problematic. He's well worth his very reasonable rates, usually $40-60 plus the kit. He's a really neat guy: he's well into his 80's and started his professional mechanic career in the 1940's as a crew chief on the first American all-jet bomber, the B-45, stationed at an airbase in England. He's also a drag racing innovator, holding several speed records achieved with twin 6 AMC engines, supplied to him by AMC.

Hope this helps.

[390xlt]

The wiring on this truck is questionable at best, and I'm no electrician, so I'm hoping I can sort it out one small problem at a time, but I guess time will tell how (and if) that works out.

As far as I know, the original tank was never sealed, and I'm sure the new one isn't. Of course, that doesn't mean that any matter of trash couldn't have gotten stuck in those lines over the course of 47 years. I'll blow them out before trying anything else.

[71Fe2O3]

You said that when you had it running, it idled but ran poorly at higher rpms. This can indicate a clogged fuel filter. Did you replace that after you got the new tank?

[390xlt]

It was the condenser. For $6 and a no more than a few minutes' work, I'm back on the road. It was fine when the engine was cold, but once it got anywhere near operating temperature, it would fail slowly enough to mimic fuel starvation. Replaced it, and the truck fired right up with barely a touch of the starter. And that's before I even had the mixture screws tuned.

I was so sure it was fuel related that I got tunnel vision, but once I started thinking about heat and electrical components, I had it sorted pretty quickly. Okay, maybe not pretty quickly, but relative to how much time I spent chasing a fuel problem, it didn't take too long.

On the plus side, I now know that everything from the gas tank to the carb is spotless. The tank probably would have lasted a little longer if I hadn't taken it out and had it cleaned, but the pinholes it had would have shown themselves eventually. While it turns out that a dirty carburetor wasn't my main problem, it was in pretty bad shape and definitely benefited from its cleaning. And I'm pretty sure I can now rebuild an Autolite 2100 while blindfolded.

Time to move on to exciting new projects like having a functional parking brake and decent lighting.

Thanks for everyone's help on this. I know it took forever to fix. A lot of that had to do with the fact that I had very limited time where I could actually put a wrench on the truck, but I'd probably still be standing there scratching my head if it weren't for the help I've gotten here.

[1972hiboy]

oh no. I should have remembered that one. I went through a horrible stretch of bad condensers. Keep one in the glove box, if that one last 5k miles , that will be a miracle. The new condensers are such horrible quality. I ended up finding an old one in my stash of parts and putting that on my truck because I had gone through several new ones. So glad you found the problem and yes it easy to get tunnel vision when your set on something.

[colnago]

... Keep one in the glove box, if that one last 5k miles , that will be a miracle. The new condensers are such horrible quality...

That's one of the reasons I went with Pertronix, then eventually switched to Duraspark. Still no guarantee, though ... Pick your poison, I guess.

Joseph