715 carb

there are several possibilities. First, how much is the idle screw turned in after it first starts lifting the butterflies ? If it takes too much adjustment of the idle screw to keep an idle, then you've uncovered too much of the transfer slot, and the mixture screws will have little effect.

Next, I would check the floats, besides being set too high, one of them might have a leak, filling it up with gas. While you have the floats out, take out the needle and seat. you might get lucky & find some garbage that is keeping the needle from seating & causing the gas level to be too high.

Next on my list would be the power valve. Replace it. They are cheap, & it might have failed. While you've there, make sure all the jets are tight. A loose jet will make a carb run rich.

Did you try running without the air-cleaner ? A clogged air-cleaner will give you rich symptoms. Last on my list would be the fuel pump, unless it been changed lately that is, (an old pump isn't likely to be putting out too much pressure).

Z. Ray

 

Added to all excellent suggestions Z. Ray would be to check the
secondary butterflies adjustment. It's done from the bottom of the carb
through a very tiny screw. If the secondaries are slighly open it will
draw gas and cause a rich mixture and no reaction on the idle screws.
Texas Swede

 

Typical Holley problem, you need to clean out the passages and orifices, and replace the bowl and block gaskets.

 

if you do end up doing this, be sure to get the blue (non-stick) replacement gaskets. They can be reused several times.

Z.

 

Have the carburetor rebuilt, the problem is most likely the gaskets between the separator plate the main body, they would go bad a lot in the late 80's early 70's and everyone just bought a replacement Holley from a speed shop. Very common problem but with the age of the carburetor, just bite the bullet and have it rebuilt by a professional. I had a carb redone by Pony Carburetor in Arizona very happy with the results, wasn’t cheap, but it was done correct. If this the orginal carb to tht car, it would be worth it and all of your problems would be over, also check for bad intake gaskets

 

hope you can get the issue resolved without a costly and possibly unneeded professional overhaul.

Z. Ray

 

Before you do anything, get an INCH lb. torque wrench and check how tight the bowl screws are. (The 4 screws that hold the fuel bowls on the carb) The spec is 40 to 55 inch pounds, both primary and secondary. Since the metering body is sealed to the main body by a gasket, and there are passages with vacuum close to passages that flow fuel, it is very easy for a leak to occur. Checking the bowl screws is the first step. The only reason I list the torque specs is so you can get a feel as to how tight they need to be without stripping the main body. (I have seen this happen Holley used to market a heli-coil kit to fix it). All of the fuel going into the engine at an idle, with the exception of a very small amount flowing through the secondary metering body should flow through those idle screws. Closing them should kill the engine.
Good Luck