Easiest thing that will help around $100 is swap to an electric fan with a thermostat so it isn't running all the time. Considering the emissions equipment on it has probably been nonfunctional for 20 years I would remove it. I would swap the old cat for a junkyard converter off an obd2 car that has the honeycomb guts in good shape and it would probably pass emissions even without all that crap on it making it run bad.
If you can dig around the junkyards and find a smaller crank pulley so you can underdrive the belt driven stuff that would be great as well. If it has a quadrajet on it you can tighten up the spring tension on the air valve on the secondaries and make it open slower so it doesn't run quite as rich at wide open. I don't think I would bother changing metering rods because properly tuning it is a pain and it won't really gain you much unless you run it pretty lean.
There are a lot of cars made around that same time that have a rear end that is basically a bolt in swap and could be done in an afternoon if you have experience messing with them. Look in junkyards and find a better gear ratio than what is currently in the car. Also pull the speedometer gear out of the junk car's trans when you get the diff so the speedo will stay accurate on the car.
I think that car should be higher mileage than what it is getting, my 80 camaro with a 350 got around 18-22mpg with normal driving so I think that car just has an issue somewhere that nobody has noticed. Maybe the carbon canister has lost its carbon and it is letting gas escape and getting dumped on the ground? Lots of old stuff on that car that could need to be gone over to make sure it is right.
Now for my $0.02. 12 MPG is PERFECT. I had a same body style caprice (actually an '88 9C1 with the police 350ci) and I routinely got 8MPG in highschool, I think close to 10 once, but never over 10.
The aero on these cars is the same as a brick. Camaros are more strealined and the newer ('92+) Caprice/GM wagons also had an awesome streamlined body and could get 16-20 pretty good.
But for that age and body, 12MPG sounds perfect. Your plans to seafoam and look over the obvious sound spot on. I'd shy away from cutting out cylinders or the secondaries. For how little it's driven it won't matter much and the fondest memories I have from high school/college are the times when the secondaries were open :-). Especially as a party car it wants the secondaries.
The 'best' in my opinion beyond driver skill and going slower, would be major engine work, increase compression, increase advance, etc, because these things will add power and fun to the party wagon AND boost FE a bit. For now just increase the spark advance a bit but I would not go all the way to 'just before pinging' as suggested. Because with a car that old longevity is more important and I've killed engines with too much advance, I'd back off a bit more than than, maybe go 1/2 to 3/4 between current position and pinging.
Electric fan is a good idea, now sure what kind of increases you'll see, I've yet to do it to my PU. It sounds good in theory but I've seen mixed experiences.
A lower gear final drive is a big one and as was said you can find one very easily in a junkyard and a swapping isn't too bad, but it will still be a hassle and probably isn't too worth it and again makes it less fun when the secondaries open.
The only other thing I'd really do is increase tire pressure to 50psi or so and make sure they're wearing even, etc.
The last biggie would be to repack the wheel bearings and make sure all wheels spin freely, but this can be a hassle and pricey for not much gain. On the other hand I had a bearing seize on the mentioned 9C1 and can tell you that replacing the whole hub (that magically exploded) is a lot more expensive that just doing the bearings.
Well, my friend got her car back, a while back and just the other day was able to get a list of things that were done to it, it looks like it has been gone over pretty well! carb was rebuilt, tranny fluid changed, spark plugs, wires, cap, all changed, new cat and muffler, all new belts, I think it even had a valve job.
and it only has 72,000 miles... it's been driven a bit more seeing as how it's winter and we have alternent side parking where you have to move to the other side of the street every day (waste!), the sugestion of a differnt vehicle was met with a firm "No", of course they think my Citicar is goofy looking and are the only friend who said that they DID NOT want a ride when I have my electric car working... seems like a good filter for picking friends... drives a gas guzzler, dosn't like electric cars... hmmm.
Best $100 to spend on this car? Depends on how it is driven. If it is for short trips with only the driver, then buy a bicycle. Use the bicycle for short trips, single person. Take the money saved on gasoline for these trips and buy a moderately sized used motorcycle. If someone travels by themselves 95% of the time, that is a good, economical, pleasant and fun way to travel as long as the weather is good, or one really likes punishing themselves.
Take the money saved on gasoline and buy a service manual for the big station wagon. Learn to maintain it so it can give the best performance and FE it can.
There are certain things a big station wagon can do really well. And if it is paid for, and only has 80,000 miles, so much the better.
__________________ "We are forces of chaos and anarchy. Everything they say we are we are, and we are very proud of ourselves!" -- Jefferson Airplane
Dick Naugle says: 1. Prepare food fresh. 2. Serve customers fast. 3. Keep place clean.