Hello everyone, I am new to this site and have recently purchased a 1970 F250 with the 390 motor. It is running a bit rich and I want to lean it out tad. My question is this, the two adjustment screws coming off the carb to the front that are accessible with a screwdriver under the air cleaner are for air or fuel adjustment? I am not sure and depending on which they adjust will make a difference in how I proceed, Thanks in advance..........

 

Redneck,
Those two screws are for the idle mixture only. They are adjusted at hot idle speed for best or smoothest idle. Once the carb is off idle these circuits shut down and the mains take over. Check out the power valve located in the square cover pointing straight down between the two screws you mentioned. You'll have to take the carb off to get to it. The cavity in the square cover is a vacume chamber and if you find it full of Gasoline then the power valve is blown and needs replaced. That is a common occurance with that carb.

good luck
Ken

 

Thanks for the info Ken, I'm going to take a look.......

 

Ken,


I pulled off the carb and there was a small amount of gas down in that chamber, does this mean the power valve is blown or will there be a significant amount of gas down there. Also if it is blown how much work to replace and also where is the fuel/air mixture adjustment located on the carb. Thanks again for the help............

 

Redneck,
The only way gas can get into that chamber is through the power valve diaphram. I think a 15/16 wrench will unscrew it and it is availible from your local auto parts separate from a carb kit. Even the carb kits aren't too much money.

should be pretty simple
(famous last words)
Ken

 

Your fuel mixture is set by the brass jets in the bottom of the fuel bowl (where the floats are.) They are not adjustable - but you can replace them with smaller ones if you are running too rich.

Like the others have said, the screws in front only adjust the idle mixture.

 

Thanks for the responses guys, but I went with plan B, put a new Edelbrock on it. The old carb looked pretty well done in after a little further inspection. Figured the best and easiest way was to just go new. Thanks again......

 

"Some" liquid in the power valve cover does not always mean the power valve has ruptured. Like redneck said his had a small amount in his and was not full of a liquid.

If you have a high float level, heavy float, defective needle and seat or too high fuel pressure which raises the fuel level in the float bowl can cause fuel to siphon into the intake manifold after the engine has been shut off. When the liquid fuel enters the hot intake it vaporizes and spread everywhere under the throttle plates. When the engine cools and the vapor condenses, it puddles and one of the places it will puddle is inside the power valve cover.

I have tested many power valves over the years that the covers were full of a liquid and the power valve was good. FoMoCo published a TSB about this back in either 1978 or '79.

The two mixture screws will affect air/fuel mixtures up to about 2500 rpm, lessas rpm's increase. You do a lot more driving below 2500 rpm than you realize, especially around town and they can have a big affect on gas mileage.

You need a tach to adjust the mixture properly (or an infrared exhaust analyzer). You turn them in or out to reach the highest possible rpm. You then slowly turn them in until there is a drop in rpm. If you adjust both screws in from the peak rpm until there is a 25-rpm drop you should be set to go. This will apply to almost every carburetor you install on your engine.

When making these final adjustments idle speed is important and so is clean engine oil. Fuel contaminated engine oil being drawn into the engine via the pcv valve will affect these adjustments.