How can I tell if my power valve is bad?
 

I've got the Motorcraft 2150 on my 351M and rebuilt the thing carefully. Some posts I've read in my searches indicate that my power valve might be bad...or (I'm doubting myself here) maybe assembled incorrectly? My truck will start right up and warm up well. High idle at 1300 and low idle at about 700. When I drive it, it stalls every time I stop unless the engine has run for about 1/2 hour or more then it seems to go away. I notice that it bogs and coughs if I try to hard accelerate but that too goes away after a 1/2 hour warm up or drive. I also read that my air cleaner flapper valve may be the culprit. This I will check tomorrow but is there any check other than visual for the power valve?

More like a vacuum leak is present.

If the power valve functions without rips/tears and was assembled correctly, it is fine. It's not a sissy Holley power valve.

Thanks Jermafenser, I've got a weird light blue plastic "Christmas Tree" vacuum manifold dangling off the back of the engine that I didn't even know was there until I crawled up under the hood fooling with the choke mechanism. It looks shady and I think that will be the first thing to eliminate. I sure hope that's all it is. I'll let you know.

That's definitely a lean condition, which in your case sounds like a vacuum leak to me as well. This is the opposite of what would be happening if your power valve was bad. When this happens, the carburetor dumps unatomized fuel into the intake stream. The motor either floods and won't run, or will only run at open throttle and will dump black smoke out of the exhaust. It's rare for a Motorcraft carburetor to blow a power valve, but it can happen if you backfire out of the intake hard enough.

That would be your vacuum tree, the main source for all things that require vacuum (emissions, auto trans kickdown, etc etc). If it's not installed and sealed, that's one monstrous vacuum leak!

That's strange because I've thought I was running rich. My mixture screws are at about 1 turn out and the exhaust still burns my nose...even my neighbor commented that it smells rich. Maybe my mixture is off because I adjusted them with the vacuum leak.

If your truck doesn't have a cat, it will always smell rich. That's the purpose of the catalytic converter in the first place.

How did you adjust your idle mixtures? What's the vacuum reading at idle, not in gear?

easy way to check for a ripped power valve is to turn your idle screws in all the way, and if the engine dies your power valve is good, if it stays running, its torn.

if your lean, or you have a vac leak, the engine will run rough and hot, and if you punch the pedal it will back fire.

if your rich, it will take a while for it to warm up, it may hesitate off idle, depending on how rich it will smoke. and will definatley smell rich. but your truck will always burn out your nose hairs

the fact you have to wait 1/2 hr to warm up, probly rich. but hard to tell without seeing the truck.

these v8s heat up real quick when they dont have enough gas

I found several things wrong so far. First of which was the vacuum tree which has a metal fitting screwed into the back of the manifold was not screwed in all the way. In fact, it was swinging loose as a goose back there. I removed the oil pressure switch and put teflon tape on the fitting and screwed it in until bottoming out. All the vacuum caps were good and I reinstalled the oil pressure switch.

Secondly, the 'E' clip that holds my choke pulloff arm to the choke linkage had fallen off and so the choke pulloff was non-functional. I should have known as when the engine first started it would slowly increase RPM until it sounded like it would explode.

Didn't get a chance to run it around the block as I have a new baby to look after and she's not old enough to hang her arm out the window while we drive.

My other issue is this....I had to use a pair of water pump pliers to remove the oil pressure switch....I assume a stubby or offset wrench is needed? Otherwise, is there any other trick to use? I think it's leaking and I cannot tighten it enough by hand.

The oil pressure switch nut is under the big bell. You will have to feel for it then put a wrench on it.

Choke linkage: With it not working properly, it tells you it's been drawing air from the loose vacuum tree. It works hard to draw air, increasing RPMs. Otherwise, the truck would just die from lack of air.

As far as I can tell the vacuum leaks are all sealed and the choke pulloff is now working great. The truck warmed up and was driveable much quicker without any bogging or hesitations. Now it just stalls on hard braking. It will continue to idle at ~600 under normal stopping but cuts out on hard stops. Does this sound like I still have a vacuum leak as I suspect?

Did you check/replace vacuum hoses? 30 years old rubber hoses are beyond used.

Are the carb/spacer/EGR gaskets good?

How dirty is the carb? Eh, rebuild it anyway and tune it.

Power Brake line is original and vacuum advance is original Carb is newly rebuilt with new gaskets and spacer. I'll eliminate the two hoses to see what improves. A friend also suggested that sticky vacuum advance mechanisms inside an original distributor may mimic a vacuum leak...but wouldn't that only be on acceleration?

Replace them. I bet they're hard as a rock now. Pinch the brake booster vacuum hose off and see if it improves any. Disconnect any vacuum operated items and plug them up and see if the leak stops. Then reconnect one by one and you will find the source.

In that case, check the gaskets for leaks with an unlit propane torch.

Some trucks needed full manifold advance even at idle, some needed a ported vacuum advance. Is your truck's emissions intact or no?

Good advice. I'll try as soon as the rain stops.

Not sure what that means. What's the difference? My truck has a vacuum line from the carb body to the vac. advance diaphragm on the distributor.

No, my emissions stuff is gone.