I posted this in the engine thread a few hours ago (no response yet) but I think it would also fit in this thread because the engine is from a '74 F250

Hi everyone. I have been using this forum for the past few weeks to try to get my 1965 F250 with a 1974 390 FE running. Most of the problems the seller told me about were electrical. Since I got the truck, I've replaced the ignition switch, starter relay, installed a pertronix ii ignitor, flamethrower coil and bypassed the pink resistor wire. I also did the basic stuff like new plugs, oil, and air cleaner. It was also missing the alternator and regulator so I bought and installed those. I also rebuilt the carburetor.

Anyway, as soon as I turned the key, it fired right up but died almost immediately. I have the timing set as good as I can get it by myself. (It won't idle long enough for me to use the light). I thought my gas might have been bad so I replaced it with fresh 87 octane fed from a 5 gallon can directly to the fuel pump. No change. The only way to keep it running is to close the choke completely and keep it around 1500-1700 rpm (I'm guessing). It runs so smooth at that point that you could balance a coin on it.

(By the way, I used the dowel method to measure the stroke as well as measuring 1 and 4 at TDC. Both measurements came out just over 3 3/4" confirming the 390 I believe.)

I've checked firing order, compression, float level, and I've tried looking for vacuum leaks with carb spray but it's hard to do while I'm working the throttle at the same time. So far, everything has checked out fine.

One thing that really has me stumped it how rich it smells. I thought because it only runs with the choke closed that meant it was lean? I've got the idle mixture screws out 1 1/2 turns on both sides. It just smells like burned gas and the plugs are almost completely matte black. Is that because I have to give it so much gas for it to run? I have to pump it a few times before I start it with the key.

At this point, I'm thinking it has to do with the carburetor. When I bought the truck, I noticed the air cleaner was from a 302 but now I see that it has the 1.08 venturi and 49 size jets. Could that be my problem?

Are the symptoms more indicative of a vacuum leak? The egr and distributor advance are the only 2 vacuum lines I am using right now. All of the other ports are capped off. I've tried running it with the egr and vac advance hooked up and capped off with no change. The pcv seems to be working(it's hooked to a vacuum tree on the intake. I have a vacuum gauge hooked up to the un-used pcv port on the egr plate. It reads steady between 20 and 23 when I have it "idling" at 1500-1700 rpm.

I'd appreciate any help you can give me. I am 22 and this is my first carburated vehicle. I do have prior experience working on cars but they have all been fuel injected and computer controlled so bare with me 😅

(Sorry for the long post)

 

Lots of possibilities.

Have you checked the voltage getting to the coil ? I'm not familiar with your ignition system but what you have going sounds like a bad pickup coil in the distributor on many systems.

Your timing shouldn't be off far enough to make it die if it runs at all but you could move it around a little and see if it runs better until you can check everything out.

what is the carb ?

 

Back your idle screws out about 4 turns and see if it will idle(albeit idle high). It sounds a lot like a vacuum leak somewhere to me, but could be lots of things and not wanting to run doesn’t help discern. Check the carb stubs and make sure it’s tight. Check all vacuum lines and caps for cracks and leaks. The choke system is supposed to richen the mixture out by choking off air. I assume this is a 2100 carb? They and the 2150 love to leak air around the throttle plate shaft after years and years of use. My truck would only idle about 4 1/2 turns out because it was sucking so much air from the throttle shaft.

 

Thanks for the replies!

It has a 2150 from a 302 apparently. 4 things led me to that conclusion. It has a 1.08 venturi, size 49 jets, a 302 air cleaner, and I believe 1975 was the first year for the 2150 on trucks (I might be wrong about that one).

I have the carburetor tightened down as hard as I feel it can go without stripping it.

The distributor was still a points and condenser style in '74 and when I got the truck, it didn't have spark to the plugs. I installed the pertronix ignitor ii (which uses the original distributor) and their matching 40,000 coil. Both of which are getting power. I've tested the spark on a few different cylinders outside of the engine. I have the plugs gapped at .040 and there is a nice healthy spark.

I have advanced the timing to the point that the starter was turning slower with only a very slight improvement in running quality. (Vacuum advance disconnected). I backed it off a bit from that point.

I have had the idle mixture screws as far out as 3 1/4 turns with no noticeable change. I know some adjustments need to be made there, but with the "idle" so high, it makes it difficult to diagnose because I believe they become less and less a factor the more you open the throttle

I just can't figure why it's displaying rich symptoms (smell and matte black plugs) but won't run without being fully or almost fully choked. That's why the screws are at 1 1/2 turns out. (Still smells very rich)

My rebuild kit was pretty bare and didn't come with any throttle shaft gaskets or seals.

I'm sorry if I'm not listing enough symptoms. Is there anything else I should look for? It just runs so smooth at a very high idle or if I hold my hand on the throttle.

I've got a tachometer coming Tuesday so I can give you the rpms for sure then. It "idles" with the choke almost totally shut at 1500-1700 rpm (I'm guessing) but smells very rich or if I hold the throttle open and open the choke, it will run with a slight stumble around 2000 rpm or so. I don't want to screw the idle speed screw in that far, so I just hold it.

Thanks again

 

It all screams "vacuum leak". Are you using a carb spacer? One for the PCV connection? Check the mating surfaces and make sure you have a good seal everywhere there is a gasket.

I used to have to use two gaskets to fill in the "imperfections" in the manifold/spacer/carb area. (well that would be four, but it's been a while....)

 

Thanks. I will check again with some carb spray.

The way its set up is a very thin gasket between the intake and egr plate, then the egr plate, then a thick gasket (maybe 1/4" +/-), then the carburetor itself. The way the egr plate sticks out, it's almost impossible to get the carburetor spray under it to see it it increases the idle. If there was a leak, I would think it would be there as the carb spray didn't seem to increase the idle when I sprayed it around the other areas of the carb. All the plugs on the vacuum trees are new (installed myself).

 

Power brake vacuum lines and booster itself checked?

 

Would the undersized carb cause any of these symptoms or would it only show signs of being undersized above a certain rpm (say 3000-4000)?

 

If you don't have spark or only partial spark in the hole at idle you end up with undetonated fuel. Also spark at the wrong time leaves unburned fuel and incomplete combustion.
If the choke is the only thing keeping it running at idle then fuel pressure as the result of a worn fuel pump or an improper float adjustment can be suspect.

 

It doesn't have power anything. I have every port plugged as best as I can tell.

I have even plugged the vacuum lines that run to the choke because I converted it to a manual choke

 

That carb size should work fine. It will limit you at higher rpm, but usually a carb like that would give you an excellent idle and very good low rpm response.

 

I spent about an hour setting the float the other day and I think I have it spot on.

How would I know if it isn't sparking while inside the combustion chamber. I hadn't thought of that because of how well it runs at a higher rpm

 

Thanks! I guess I can cross that off my checklist for now

 

I have a clear fuel filter going into the carburetor which stays mostly full and at a steady level the entire time the truck is running. I have a fuel line going from an external gas can to the input of the fuel pump. Does that still sound like it could be a problem?

 

Would a vacuum gauge hooked to the intake show a leak while the truck is running? When I have it "idling" at 1500-1700 rpm, the gauge is steady around 20 - 23 depending on how high the rpm is and how long it's been running. Would the gauge be jumpy or erratic if there were a leak somewhere? Thanks