I have been through some of this before, and it seems like a typical situation faced by most old Volvo owners.

I also had carbs that were barely worn, spraying stuff on them didn't really show a leak but I was never able to tune the correctly, either they ran too rich or the plugs showed that they were too lean, and the front cylinders were never in the same shape as the rear cylinder.

If you are running a cast iron combined manifold, you might want to check for cracks, as cracks are common on these pieces.

A final thing you may want to consider when looking at your budget, tools, and skills, is an investment in an entry level wide band oxygen sensor. They can be had for about 200 dollars and they provide a whole lot of really good data. Of course you can get your carbs rebuilt for that much, but after that you still need to tune them. The amount of information provided by the O2 sensor can save a lot of time and money spent trouble shooting a fuel system. The initial expense is also less when you consider that you can move the sensor from car to car and get all of your cars tuned perfectly.

The best part about getting a wide band oxygen sensor is that you will soon realize how impossible it is to get a mechanical fuel metering system to perform exactly how you want it to under all circumstances and you will end up selling your carbs on ebay and got to a programmable fuel injection system that can be tuned with your handy wide band sensor to run just how you want it. With my wide band sensor I regularly run at AF ratios above 17 while cruising, with plenty of power to spare when I put the pedal down.

With this setup I can get really good mileage if I drive nice. But I have found a difference of well over 5 mpg depending on how hard I drive my car.

On a final note, if I am not mindful when parking after filling up, I can lose about a gallon of gas right out my filler neck. That really kills the gas mileage and eats holes in the asphalt.