Hello all, i recently bought a 1976 f350 super camper special with a 390. when i first got it, it needed a new fuel pump. I installed a new one and started the truck for the first time in about a year. It ran fine for a few weeks but soon began chugging and making black smoke. Thinking it was a fuel problem, i rebuilt the carb (believed to be a holly 4160) and reinstalled it. The problem still persisted. I got a barely used carb from a friend and installed that to see if it made a difference, but once again it was pouring gas into the engine. Since its done the same with both carbs, i believe it doesn't have to do with them, but instead something else on the engine. I thought it might be the fuel pump createing too much pressure, but im not sure if that idea holds water, so to speak. There is also a slight vacuum leak somewhere in the lines, but I'm not sure where. Again, that may or may not be the issue. any ideas? thank you

 

If you see it pouring into the carb i would replace the needle valve and seat. You mentioned its a holley, if its running and it slowly starts bogging down it could be a blown power valve. If you ever get it running and you can screw the idle mixture screws in all the way and engine continues to run then the power valve is blown.

 

If you're using an aftermarket carb, install a fuel pressure regulator. The stock mechanical pump outputs around 8psi and Holleys and Edelbrocks don't like much more than 6. I have mine set at 5.5psi for my Holley Truck Avenger.

 

If you have a Holley carb (not sure about a 4160) see if you have a small screw on the left hand side of the fuel bowl (facing the carb, passenger side of engine). This is the fuel level sight plug. Remove this plug and the fuel level should be at the bottom of the plug holewith the engine running at idle. Fuel should barely drip out of this hole when properly adjusted according to the Holley repair manual. The adjustment is at the top of the fuel bowl. It is a large (a little smaller than dime size) with a hex nut underneath it. Using a flat blade screwdriver loosen the screw and turn the hex nut with a wrench (5/8 I think) to the left to lower the fuel level. You are actually screwing the needle and seat down into the fuel bowl. As stated in the other post, check the needle and seat. The viton tip on the needle valve could be deteriated or have have varnish or trash under it keeping it open. Another problem could be the float itself. Metalic floats develope pin holes in them and fill up with fuel and sink keeping the needle open, plastic floats get saturated too sometimes, but its rare. If the float is suspected, remove the float and place it in a container of fuel and see if it floats.

 

Thanks everyone, im gonna try again to get her to fire tomorrow. I think the floats and needles are all set because the carbs that i tried were both recently rebuilt, but ill go through and check all this again includeing the power valve. Thats one thing i didnt really get into too much. Thanks for the advice