77 F250 351M Just bought....Serious running problems

[Vanner1993]

Ok, well its actually a 77 351M but I figured it would be the same setup in a 78/79 bronco. So here's the specs, 77 351M, has a 4 barrel edelbrock intake, 80,000 original miles, carb is a holley factory emissions 4 barrel (has that spring rod on top of the front metering block).
Anyway, bought the truck misfiring, checked the plugs they were all SERIOUSLY fouled. Changed them, ran good, well better then misfiring. The truck runs so freakin rich my eyes tear and it smokes like an old idi diesel. If I stomp on it, it rolls coal..haha. Will not fire up unless I have foot on the floor, and sometimes backfires through carb and just barely runs and runs like crap. Now when it runs decent, the idle will change from like 1200, then 1500 and I lower it to 1000 rpms. Now I don't wanna make this long as I need some help with this. So hear you go..
-Hard warm starting, just cranks unless I have foot on floor.
-Cold start is just bad but better then warm starts
-I cant run vacuum advance, if I do no matter where timing is, the engine stumbles all rpms and runs poor backfires through carb and exhaust.
-metering screws all the way in and unresponsive
So at this point
I have no vac advance hooked up, and have idle at 1200 rpms, runs so dam rich, metering screws bottomed out, and has a dead spot under throttle.

Any help? I will get back to your responses quick!

 

[adsm08]

Stuck or damaged float, stuck choke, damaged jets....


Just rebuild the carb.

 

[Dune Runner]

My first thought was stuck or sunk floats. Tear the carb apart.

 

[Dune Runner]

Oh and do an oil change too. It's gotta be full of fuel.

 

[Will]

Sounds like a ruptured power valve, and I'm pretty sure that's what you have. But we're talking mummy-speak here.

I was a hotrod guy a long time ago. I knew everything about troubleshooting these things. I could get it back if I felt like it, but I don't. I still have boxes of jets and springs. I have the special tools. But once I learned about electronic fuel injection and believed in it, I stopped thinking about carb stuff. I also stopped thinking about distributor advance curves (I still have those springs as well).

A 351M is probably the worst and most boring engine ever built. It's a monument to Ford's lowest and most pathetic stooping in the history of the company. It's a 5.8 liter Briggs and Stratton. I don't really understand Ford's thinking at this point. They started with the 351C, which made sense. I had one, a '71 4V engine with huge valves and ports and in a low-geared car ran like a burning bison. Then, they had the 2V motor, which still had big valves, but not as big as the 4V. But as big as Chevy motors that would carry a 4-barrel. But Ford didn't carry a 4V on those so the 350 4V motors were faster. Ford was very stingy with 4V carbs and only put them on special motors. Ford also refused to standardize a motor until the 80's 5.0 so they were way behind.

When Ford did come out with a good motor it was a port fuel injected motor that dominated for years--or decades. The 5.0 Mustang is the best thing Ford ever thought of. No GM product ever competed with it. The Buick GN and such were always pitted against it in magazines, but those vehicles were limited productions.

The 351M, to get back to the subject, was a horrible thing. Ford was trying to end 351C production in favor of 351W productions, but couldn't make volume. So, in their wisdom, they put 351W cranks into 400 blocks. The 351W and 400 share the 3" main where-as the 351C has a 2.72" main, I believe. Bore spacing is the same. The problem with the 351M is very real. They used the 400 rods, but had to make a strange piston with a really tall compression height to make it work. The 400 block has a much taller deck height compared to a Cleveland engine. The piston is very tall and rocks in the bore and can't be used at very high piston speeds.

The good news is, a 460 has the same bellhousing pattern as a 351M. A 460 is a great engine. Where the 351M was an unfortunate afterthought, the 460 was brilliant and is still a viable pulling motor despite its age.

 

[85_Ranger4x4]

When Ford did come out with a good motor it was a port fuel injected motor that dominated for years--or decades. The 5.0 Mustang is the best thing Ford ever thought of. No GM product ever competed with it. The Buick GN and such were always pitted against it in magazines, but those vehicles were limited productions.

I think the fact that they basically made a V8 go-cart with a foxbody had a lot to do with it too. The Camaro was quite a bit bigger, the same theory helped keep the SN-95's within gunshot of the Camaro into the '00's. The Camaro's only clear hope was when they would grab engines from the Corvette parts bin.

LS>Mod4.6>SBC5.7>SBF5.0>SBC5.0 was the pecking order when I was in high school in the early '00's.

 

[Vanner1993]

Ok, I installed a brand new holley 600 before. Starts right up, idle screws responded like they should have, and ran smooth. BUT As soon as I rev it, its smokes BLACK like a diesel under load. And when vacuum advance is hooked to ported vacuum, it misses through all rpms besides idle (since the ported vacuum). So I have no advance hooked up right now. I fixed the egr thing on the passenger side of the carb, it actually had an exhaust leak coming from the square plate. Timing is about 8-10 btdc so its more then good. I just dont get this rich issue.... I will work on it again tomorrow.

 

[Will]

Know what a power valve is?

With a carburetor, you have to rely on several layers of magic for the engine to run smoothly. The first layer is the idle circuit. A gas motor needs a specific fuel/air mixture to idle strongly.

Then you need to accelerate. There are several circuits in the carb that help with this. The first is the accelerator pump. When you step on the pedal, the pump squirts raw fuel, powered by your foot, through orifices that spray over the venturis. It's as crude as Fred Flintstone, but is your initial crutch against stalling.

Next is the power valve. We're talking Holley here, which isn't the best, but is the most common. Anyway, when you stomp the pedal and open the throttle plates, there is a big pressure rise in the manifold as you let all that atmospheric into the manifold. Holley knows it takes time for the air to speed up through the venturies as the engine gains rpms. In the meantime, they decided to have another fueling circuit that was sensitive to that big pressure difference from throttles closed to throttles opened.

At low power requirements, the throttles are closed because your foot isn't down on the pedal. When you need power and stomp the pedal, there is a valve connected to the intake plenum that feels the pressure rise from all that atmosphere you just let in, and it opens a valve that lets a lot of fuel straight into the venturi.

Problem is, if there is a tear in the diaphragm of that valve, it will always think you are wanting power and always be letting in lots of fuel. When it's working correctly, you stomp the pedal and the accelerator pump covers the initial bit of fuel so it doesn't hesitate; the power valve covers the next moment while the engine sorts itself and begins to draw air; the main jets take over and make the real power.

A ruptured power valve diaphragm is a very common cause of what you are suffering.