I've got a '78 F250, born with a 460, but I found her with a 351M.

I took the vehicle to a harsh, dusty dry lake bed, for an extended period. And now the truck doesn't have any power, and it seems to be missing, etc. I figured that the fine dust has clogged some holes in the carb. I've never done a rebuild before, but thought I'd give it a try.

Well, I've found a few completely rotted hoses, so I'm happy to get those replaced. And I got the carb off the engine, and I'm preparing to begin the cleaning. But, I've noticed a few problems already. Perhaps you can help me solve them.

1. The air horn assembly has two holes in it, on the flat round part, so the air is getting sucked up into the carb from the bottom. They clearly aren't supposed to be there. Should I get a new air horn assy, or just try to JB Weld the holes closed. They are a little less than 1/4" in diameter, and I have no clue how it happened. Any advice?

2. The other problem is on the tube that runs from the choke, through a plate on the header and back out to the rear of the carb. The tube rotted out and completely cracked off at the plate. I'm not sure of the tube/plate name, but it looks like it is there to bring the heat to the thermoreactive elements in the carb. Is this a part that I can get at the parts store? What is it's name? Any tips? Is it really necessary? I can fashion a small cover out of Aluminum to block it off, if I can get away with that.

I've got the carb off of the engine, and I'm going to start disassembling things. Wish me luck...

Another thing. Now that I've got the carb off, how should I check the EGR valve? I've heard of them giving people trouble in general. It's probably better for me to check/fix it now that I've got good access. Any tips?

Thanks,
Dan

 

1. Is it possible to get a quick picture of the holes? Are they perfectly round? I can't think of too many ways a hole would get in an air horn unless someone purposely drilled one. Are you sure it's not a screw hole? There are also a couple holes back there that are actually supposed to be there, so more details could help.

2. This sounds like a choke stove - only you said header, are you sure it's not a plate on the intake manifold? If this is the part I'm thinking of, then you can get kits from parts stores to replace this (at least from my experience). If you run out of luck with that option, hopefully a junkyard could help you out.

3. As for the EGR valve. When you do a carb rebuild, it's important to not just tend to the carb, but the EGR spacer plate and valve. When you remove the carburetor, it can disturb the seal above and below the plate, so you need to replace the spacer-to-manifold gasket with the thick steel-woven type. I always replace the EGR valve gasket at this time too (this gasket uses the same material). As for the valve, you don't have to necessarily do this with the carburetor off, but apply a vacuum source to the port at the back and see if the rod moves in or not. An EGR valve is a simple linear vacuum motor. If it moves, it's good, if it doesn't, it's bad.

 

Thanks for the great information, fmc400.

Unfortunately, my camera isn't with me at the moment. The holes look like they were drilled out (smaller than a 1/4"), but the pot metal of the air horn assy had cracked and gave way, resulting in a couple of messy holes. They do not have any connections to anything, and everything seems hooked up, as far as I can tell. They go straight through the plate to underneath it. While the truck was running I could hear an extremely loud hissing vacuum leak from behind/underneath the carb, but I never found the source. These holes could be the culprits. I can't honestly imagine the purpose of those holes, except to bypass the air filter (not good on that dusty lake bed!!). Are there time tested techniques to fixing things like this? Perhaps bolts with gaskets? Or just JB Weld? I guess if I use the bolts I can just open them up easily if I do find their purpose.

You're right I was talking about the intake manifold. I'll call around for choke stove parts. I hope I don't have to drive out to the junkyard, but if I have to...

Okay, I'll replace the gaskets underneath the EGR spacer, as well as check it's performance.

 

What other options do I have? It seems that a new choke stove will be difficult to procure. Can I just put a plate on the hole, and run the vacuum lines to the same places? What exactly does the choke stove do? The parts counter guy told me it was only for warm-up periods. I'm fine with a slightly rougher idle until my vehicle warms up if I can avoid fabricating this piece myself.

 

It cycles hot air through the choke at all times. This warms the choke up. Even after the choke is open all the way, it still is cycling hot air. You absolutely have to have a choke; some other options include an electric choke conversion, or hopefully someone can chime in about advice for building a new one out of some line. Without the choke, you probably won't be able to start the truck, let alone drive it to work in the morning.

 

So, help me understand how this works...

When the engine is cool, the choke is closed, allowing the truck to start. After the truck warms up, the hot air from the choke stove makes the choke open up. (I hope I don't have that backwards!)

So, if I get rid of the choke stove altogether, the choke will never properly open up, right?

This sounds like I'm going to need to do something to warm up the choke. I'm pretty handy with tubing and whatnot. Perhaps I can make a tube that wraps around a different part of the engine (exhaust?) and let THAT be like the choke stove? I've got some 1/4" copper tube lying around. What do you think?

The only part that bothers me about this is the plumbing. If I'm recalling correctly the choke stove's two ends were connected to the choke and into the bottom of the air horn assy, on the rear. The air horn sucks air in, so it seems like the hot air is headed towards the air horn, and away from the choke. Am I going crazy, here?

 

That's more or less the jist of it.

Actually this is the opposite. That line that goes to the rear of the carb is a vacuum line. The air-flow is in the opposite direction than what you described, so the hot air passes over the choke. The air horn doesn't really provide vacuum itself; it's just the top casting of the carburetor. It is the manifold underneath the carburetor that actually provides a vacuum source.

The choke is probably the most complicated mechanism on the carburetor, because there are many overlooked mechanisms to it, and each has its own tuning procedure. Aside from the lean\rich setting of the choke cap itself, there is also the choke pulloff clearance, the fast idle speed, and the fast idle index. All must work in harmony for proper engine warm-up.

As for fabbing a new stove, if this were my vehicle I would probably have a metalworking shop try to whip something up. Whatever line you need (I can't remember if copper will hold up to the heat or not) will probably need several bends, a flare fitting will need to go on the end, and the other end will probably need to be welded to the stove plate.

 

The easiest way around the rusted choke stove is to use an electrically heated choke spring. Here's one: http://tinyurl.com/ysepdp Theyre about $30, but you can snag a few from carbs in the junk yard. Check the heaters with an ohm meter. It's usually a direct swap, I've done a bunch of them.The wire for the heater comes from the "S" terminal of the alternator so that it only gets power when the truck is running. If you look at the harness on the firewall just above the right hand valve cover, there is usually a 2 wire connector dangling there. One wire may be used already (idle solenoid IIRC). The other wire is for the choke heater and comes from the alt "S" terminal.

 

Great, apparently I already have a electrically heated choke spring. It just wasn't wired up. I'll look for a dangling wire and check it for proper voltage when ignition is on. Then I can just put the choke stove back with it's broken tubes to block that hole in the intake.

Thank you, forum, for your help.

 

The wire won't have power with the ignition on, only when the alternator is spinning.
Look at the wire color on the "S" terminal of the alternator to help locate the right one. If you don't have the connector on the firewall, you can come straight off of the "S" terminal to the heater. This keeps the heater from heating if you only have the key on and the truck not running.

 

Thanks again, lsrx101. The wiring has been fooled with before, so I'm not going to trust it. But, I'll just probe around on the alternator till I find the terminal that is 0V with the engine off, and 14V with it on. Seems straightforward enough. First I just need to finish putting the carb together again. I've found enough problems that I'm sure the truck will work much better with them all fixed. I'm looking forward to that first firing up.

 

The stator terminal should only be 6V with the engine running. It is not full battery voltage. Also I don't recommend probing back there with the engine running.

 

I found a dangling wire that looks like it could go to the electric choke. I'll test that out first. Of course, I won't be able to do that till the carb is back on.

The rebuild instructions tell me to turn my choke 90 degrees "rich". Which way is that exactly? That is supposed to close the choke valve, right? So that there's less air, and hence a rich supply of fuel? The choke has "lean" written on it. But twice, and in both directions, so I've got no idea which way to turn it.

Thanks again for your help.

 

Here is a run-down of how the choke works. As you can see, the choke assembly includes a plate at the top of the carb. When it stands straight up, there is no restriction of the throat of the carb and air flows like normal. But when it begins to close, there is less air available for the carb. However, the manifold vacuum is still the same inside the throat of the carb, so more fuel is pulled per unit air. Engines naturally must run rich (more fuel per unit air than standard) to run when cold.

As you can see, you can loosen the three screws that hold the choke cap, and you can rotate it two directions. Rotating it toward the back of the truck will make it "rich." This takes the choke longer to open, so the enrichening effects occur longer. Rotating it toward the front of the truck will make it "lean," meaning it takes less time to open. The choke says "lean" on it because it is supposed to imply that rotating it in the direction of the text (sometimes there are arrows) will make it run lean. Equivalently, rotating it toward the back of the truck, like I mentioned, will make it rich. Some chokes even label both directions.

When you are ready to set the choke index, there is a specification you must follow. On the metal choke housing that holds the choke cap, you will find seven notches. Going from the back of the truck toward the front, the order of these notches is 3 Rich, 2 Rich, 1 Rich, Neutral, 1 Lean, 2 Lean, and 3 Lean. You will also notice that on the top of the choke cap on the perimeter, there is a small notch. You must line that notch up with one of the seven notches I just mentioned. The knotch you must use will be listed on your rebuild sheet, but it will also be on the valve cover sticker. You will see something like "1 Rich."

The reason that they are having you rotate the choke during the rebuild is because there are some settings that must be done when the choke is fully closed shut. Rotating the choke 90 degrees rich will ensure that the choke is fully closed because it's all the way over. Now of course, you won't run the engine with the choke 90 degrees rich like that.

 

The voltage at the "S" terminal is actually 12V AC. And that is where the wire to the choke heater is fed from on all Ford vehicles.