Another tip. Check and see if the main body is warped, it's a common problem with Holley's, and your kit will only last about a month before it starts running bad again.

When you get it all torn down, you will end up with the large square center part of the carb left. On the front and the rear you will have flat faces with a bunch of holes showing. If you have one of the vacuum secondary carbs that have no jets in the rear, you do not have to check the rear face.

But check the front face. Get a good flat file, and lightly run over the front face with all the holes. You will probably find the 4 threaded holes in the corners of the face are high, while the center is low, the file will not be touching the center. You can see right away this would cause sealing problems with the gasket. When you first put it all together it will seal for awhile, but as the gasket shrinks, it will start leaking through all those little holes, causing weird stumbling and running problems.

The best way to fix this is to mount the carb in a milling machine and fly-cut the surface flat. I have filed them also, but you have to be very careful, and keep the file flat, and keep changing directions from which you are filing. You also could possibly glue some sandpaper to a flat piece of metal and try that. As soon as the file or whatever you are using starts cleaning up the center part that is low, you can stop. Then take a can of carb cleaner with the little red tube stuck on it, and spray all the little holes clean of the filings.

 

I agree with the others about not rejetting. There is something wrong, and I suspect it is the power valve, but you need to fix whatever is wrong and get it back to where it should be before changing anything like the jets. And, the power valve protector is a good bet because just one backfire can blow a power valve. It should have been installed when they built the carb IMHO.

I also agree with Dave on checking for warpage. Been there, done that. Another approach to taking the warp out is to use sandpaper on a piece of plate glass. If a piece of glass is laid on something approaching flat it'll give you a reasonably flat surface to work from.

And, I agree that you should be in the 12-14 MPG range when everything is correct - depending on your driving style and the rear axle ratio. I had an '82 351W/C6 that got 12.4 MPG on the highway at 62 MPH and it was running 3.50 gears. A difference between it and yours is the carb - it had a 2150 2bbl, but the 4160 4bbl actually should do as well if not better if you stay out of the secondaries.

And, btw, if you do use styrofoam make sure the carb has no gas in it or on the parts when you put them on the board. And, keep the carb or brake cleaner far, far away as those chemicals will devour styrofoam in very short order.

 

When I was a kid I "tried" to use a styrofoam cup to clean a small carburetor.

 

Dave, what causes that wear? Shrink/swell in the metal while those points are tightened down?

 

Carbs are made from pot metal. Very soft. Tightening the screws too much distorts the metal around the screws, and tightening the screws more to seal a leak makes it worse.

I like to use 1/4" sheet glass with wet/dry 600/1000 grit paper glued to it for truing. Same way I sharpen my plane blades.

My 351W/C6 gets 12 city and 14 highway if I keep it 65 and under.

And the power valve protectors are a good idea.