85 F150 Engine runs rough

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Old 01-08-02, 09:35 AM
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85 F150 Engine runs rough

1985 Ford F-150, 2WD, carbureted 4.9L 6-cyl, 4spd, overdrive, 160,000 mi. went from relatively smooth running to running very rough at all motor speeds virtually overnight.

Sitting at a light, the exhaust smokes (mainly black/grey) at idle. I have to rev the engine to prevent stalling. Pulling away, first a cloud of smoke and an absence of power. It tends to buck under load. Revving the motor in the driveway blows black smoke. The plugs (Bosch Platinums installed about 6 months ago) foul. Compression on a reading holding compression gauge ranges fairly evenly from 165 to 180 psi. across all six cylinders. I can't tell anything more without a helper.

Spark seems pretty solid, but the rythm seems somewhat uneven.

I tried to find a vacuum leak by pressurizing the vacuum manifold with low pressure compressed air and using a soapy solution on the vacuum lines. No leaks were detected. I've replaced some vacuum tubing and the pcv valve.

I did a timing adjustment to prevent stalling. Can't find the timing mark with a strobe.

I'm not sure where the carburetor adjusting screws are or that I should mess with them. Albeit, I think the mixture is too rich.

Where do I look next?

900t
 
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Old 01-08-02, 09:46 AM
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Have you checked the choke? Sounds a little like it could be stuck partly closed.
 
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Old 01-08-02, 09:49 AM
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Or, could be float problem; can't remember if that engine's carb has a history of sinking floats or not, Joe will let us know hopefully.
 
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Old 01-08-02, 01:04 PM
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you should make sure your choke opens up all the way when warm, but it was a common problem for the base plate to loosen up on the carb on the straight six fords and also the gasket on the midsection goes bad causing fuel to bleed over and run to rich so you will probably have to rebuild your carb or replace it.
 
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Old 01-08-02, 04:03 PM
Joe_F
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I agree with both of these guys.

Grey smoke is usually burning transmission fluid though. If it has a vacuum modulator, check that to make sure there is no fluid in the hose.

Barring that, it's probably timing chain time for one (with this mileage...memory escapes me if this has metal gears that mesh or a chain...but slack here will cause it to run badly).

These had Carter 1 barrel carburetors on the straight six. Leaking junk . The Motorcraft/Holleys on the 2 barrels weren't superlative either. Ford never got the carburetion right....they went to fuel injection early.

I'd check as they suggest and if the smoke is truly black, we're talking rich running. Likely carburetor time.
 
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