Help, I have a 1974 F250 4x4 that will start every morning and idle great with 20 vaccume. The problem is it will only do this for 4 or 5 minutes, then it acts like it's running out of fuel and dies. I have taken off the top of the carb numerous times and there is always plenty of fuel available in the bowl. The accelerator pump is delivering fuel with a nice spray. It only runs well for the first couple minutes. The engine warms up and the choke should not be needed, but when the choke opens it will die, and if you force it closed it will die
(After the truck warms I am not able to manipulate the choke to keep the truck running). Does anybody have any idea why it would only operate with good vaccume and smoothly when cold? I don't think it is electrical because when it begins to die it is not sudden it spits and spudders like it is starving for fuel. I have new fuel lines, new fuel pump, I rebuilt the carb (but I don't guess that necessarely means it means it is fixed) Any suggestions would be greatly appriciated. Love this forum. Pissed off in California.

 

Sounds like a vacuum leak somewhere. Either between the carb and intake or intake and heads.

 

If that were the case wouldn't it run rough all the time? I have sprayed 2 cans of Carb cleaner all over the intake, carb base, heads and everywhere else I can think of; the rpm's never rise. I also have taken a compression test, 150 on all cylinders but two and they were 140. Not to bad, the truck would be great if I never needed to drive it more than 5 minutes.

 

A few questions - when it is stumbling around and getting ready to die, are you able to feather the gas to keep it going? Also, what is your choke index (notch on the choke cap) set to?

 

Hi fmc400, yes I can manipulate the throttle to keep it running, but it is difficult (very rough) it's like it all of a sudden has a massive vaccume leak.
Like I mentioned before it idles great for 5 minutes, all the lines have been replaced. I can't hear or find a leak. The choke is one of the thermal ones, it is stamped lean 3 times on the cover, it is not indexed like the electric ones on the late 74's ( I think they switched choke's mid year ) But I kind of have taken the automatic part of the choke out of the equation, (I think) in that after I start the truck I am operating the choke and throttle by hand under the hood. (while watching a vaccume guage that is connected to the power brake assist. Idles 20 to 21 for 5 minutes then shutters like crazy. once this starts I can move the choke and throttle around to keep it running (if you want to call spitting and sputtering; running) I have checked the coil for correct resistance 1.6 primary 7,900 secondary (I think that is within specs) but is it possible the distributor could be heating up or the wire from the coil to the distributor failing? It sure acts like fuel but I am running out of reasons the thing will idle so good for only 5 minutes. Your help is very much appriciated. Still frustrated in CA.

 

What I meant by the choke index is the lean\rich setting - on the rim of the choke cap is a small notch. There are 7 notches stamped into the metal choke housing on the carburetor, where the black choke cap grabs on to. The notch on the black cap should line up with one of those 7 notches - if you look at the notches and count from the one closest to the back of the truck and go toward the front, the order is 3 Rich, 2 Rich, 1 Rich, 0, 1 Lean, 2 Lean, 3 Lean. On the valve cover sticker, it will tell you what notch to line it up with - it will say something like "1 Rich." Line the notch up with the correct mark and tighten it back down. Manipulating the choke with it hot won't really do anything.

To clarify the way the choke has "lean" stamped on it three times - original Motorcraft chokes have the word "rich" on it, with an arrow pointing counter-clockwise, and the word "lean" on it with an arrow pointing clockwise. That is basically to indicate what I just explained in the paragraph above. Now, yours has lean stamped three times - that just means if you rotate it in the direction of the text (L-E-A-N), it will go in the lean direction. It's the same thing as the old way, just a different (and not as helpful) way of doing it.

It sounds like the problem is more-so related to your idle mixture being too lean or the curb idle speed to low. Or, if your choke is opening too quickly, then refer to the procedure in the first paragraph.

 

Sounds like either your mixture screws are in way too far or there is a blockage in a fuel circuit in the carb.

 

FMC400, I guess the choke housing has been replaced, because mine is as smooth as can be. But as I mentioned before I have been operating the choke by hand to remove any chance that the choke is opening prematurely or closing late. LxMan, I have started the mixture screws at 1.5 turns out and progressively opened them a half of a turn until they are almost unscrewed, seems to work best at around 2.5 turns out from fully closed. (but still runs horrible) I did find a leak with carb cleaner, it appears to be at the carb base at the fiber spacer. I don't really like these spacers, I think a regular gasket would have a better chance of providing a good seal? Also this thing has a cast iron plate for the EGR/Daspot. The intake maniford has a port for exhaust gas to go through the EGR plate. chances are i'm wrong, but didn't the 74 engines have a dashpot, not an EGR? I thought the dashpot just prevented the throttle plates from closing to fast to prevent the truck from shutting down during deceleration, I don't think it routed exhauts gas back thru the carb. This plate allows for the exhaust gas to circulate back to the carb. Wondering if the intake and plate are from a newer engine? Every autoparts store around here sells a manifold to carb gasket that does not allow for this exhaust port in the intake manifold. I will update when I find the right gaskets and fix the vacuume leak. ( though I don't think the leak is bad enough to cause this thing to run this bad ) Thanks for all your replys. More to come.

 

Ah - here you go. The dashpot and EGR are separate systems. Here is the issue with EGR. Those spacer plates are a giant hassle because it's very hard to find a good spacer-to-manifold gasket. Most parts stores sell paper-thin gaskets; they will not do the job. The correct gasket is thick and woven with steel. This is important for two reasons - first, the gasket is subject to extreme heat, and second, EGR spacer plates tend to deform underneath over time, so a paper-thin gasket will not do the job. I always use Fel Pro gaskets.

I'm sure you know this, but just in case, make sure you have the correct spacer-to-carb gasket as well. This gasket is about 1/4" thick and has grommets where the carb studs go through. These are important for a good seal and they act as a heat sink. The carb rebuild kits always come with crap paper-thin gaskets that are junk and should be thrown away immediately. Every time I've rebuilt a carb, I have always had to buy not only the kit, but also a carb-to-spacer gasket, spacer-to-manifold gasket, and also the air cleaner ring gasket, all bought separately.

 

Thanks fmc400, I will use a good quality thick gasket to prevent possibility of leaks.
(even if I have to buy thick gasket material and make it myself) Hopefully this will solve the leak. I am still curious why the gaskets the autoparts stores don't have the hole for the exhaust gas to go to the EGR plate?? My truck is a 74 but the ones I have been shown don't allow for the hole. It is making me think the EGR plate and manifold are possibly from a newer truck? Am I right in thinking that there is no EGR on a 74?? I thought 75 was the first year? Thanks for the tip on the carb to egr plate gasket. ( I do have the gasket with the spacers for the bolt holes 1/4" thick)
Thanks again, get back to me if you know about the EGR plate on a 74.

 

I know that EGR started on Ford passenger cars about '71 or '72. I'm not quite sure about the exact year it started for trucks, but it's a possibility. You also mentioned you are from California, and if your truck is originally from that state then I wouldn't be surprised if it came with EGR (stricter factory emissions). Also, it is highly unlikely that someone would swap in EGR. Usually people go to great lengths to get rid of it. Does your EGR valve have a sticker on it with a ford part number? That would give a clue as to the date. Also what engine do you have?

 

Yes, I am from California and so is the Truck. It is a 1974 4x4 F250 with a 360. I am the third owner. First owner had it for 30 years. 2nd owner was their nephew and he drove it for 3 months something quit working and he parked it in a barn for 3 years, then sold it to me. It is two tone and there in not a ding on this truck, even the paint still looks great (114,000 miles). Beautiful truck, lots of work cleaning up and changing hoses, bushings, rubber parts, bead blasting and pinting, etc... but well worth it.
I believe it is all original and you are probably right, (even if most states didn't have an EGR that year, that doesn't mean California didn't) No sticker on EGR, looks like there was one at one time but someone pulled it off, or age got to it. Thanks for the reality check, I agree, I don't think anyone would put any effort into trying to add a EGR to a vehicle that didn't originally have it. I will put this thing back togeather this weekend and update the results.
Thanks.