And for an air horn with more visible “lines” please see image one below.
The casting lines were designed to bite into the gaskets forming a nice seal. And they do that...old gaskets show it, but if you'll check old gaskets with a mike or dial gauge, you'll find the screw hole locations of those gaskets will be thinner than the area between the 4 screw holes. This shows the gaskets are compressing at the screw hole locations and not compressing as much in the middle. Heat and this stress is causing the castings to warp. Only having 4 screws to hold these together adds to the problem. When those gaskets age, dry out and shrink, the areas in the center are going to be the areas that promote vacuum loss and gas seeping first. And later we’ll talk about how to straighten the air horns and float bowls.
How about using 2 gaskets to fix this? I’m not a fan of that technique. Please look at image two below to see why. These 2 gaskets came out of the carburetor that was most significantly warped. They had been stacked on top of each other and this carburetor worked just as poorly as you would expect. One of them either slid out r was sucked out into the interior bore and you can also see there was a nice gap where air was being drawn out of the float bowl every time this vehicle got past the idle/off-idle point.
These old and shrunken gaskets and the warpage all were working together to cause both a loss of vacuum and fuel leakage. Did new gaskets help? Only minimally. So let's consider vacuum loss is going to affect 2 things:
1--The idle circuit. It will make it run leaner.
2--The power piston/power system. Loss of vacuum there is much more sinister as it can cause the power piston to operate erratically or to fail to work at all. If the power piston disables the power valve, the whole thing fails and you have more fuel available to both the idle and main metering circuit all the time. Consider: The main jet in this carburetor is .058" and the restrictor is .0394”. If we calculate how much extra fuel will come in around the power piston and the hole, we come up with an area of the power valve of .0123”. If we subtract the area of the rod from that (.0058”) we get an extra amount of .0165.” That added to the main jet of .058” equals .0745” and that will be the effective size of your main metering jet now. So you’ve increased the fuel to your engine by almost 30%. You’re running really rich now, so you should celebrate. Buy stock in Chevron.
The Power Piston
Sadly all but one of the carburetors I had to study suffered sincerely stuck power pistons. When the power piston gets stuck, it always sticks in the down position. How does this happen? When you shut off your engine, the power piston loses vacuum and the spring immediately pushes it down. Gas has been sloshing around in there and will have wicked up into the gap. As the gas in your bowl evaporates (and it always will), it leaves a sticky crud on the piston which makes it hang in its bore. Once this starts happening, I can't see anything to reverse it aside from taking it apart and cleaning it. My suggestion is when you rebuild one of these work very carefully and try every single thing you know to get that piston out of there with no damage to it or the bore and then polish both it and its bore with MAAS or Flitz or something like SemiChrome polish and then grease it with cornhead grease mixed with graphite. That might give you more time before the thing sticks again, but it will stick again. It is a wonky design and I can think of a couple of different ways to achieve the same thing externally that would (a) not put a power piston down inside a float bowl and (b) not get stuck so predictably. But in fairness they didn't have the junky gas we have today and that's an issue for these.
Okay, back to some other problems...
Your idle pick-up tube may be missing. It was missing in one of the Rochesters I got. Please look at image number three below.
You can fix this with a piece of brass tubing and if you want your motor to idle you will need this. Please see image number four below.
There is a vacuum lip that can be knocked off in clumsy rebuilding. If this leaks (and it sits right next to a thin gasket at the air horn bore) it will make your idle mixture too lean. Take a look at image number five below.
