Hi Renato,
Sorry for the delay in replying to your question to me on the Holley vs. Carter. The answer is, just my experience and opinion. I'm no mechanical or hydraulic engineer. The Holleys seemed to deteriorate more with age, soaking in carb cleaner, wear, and rebuilds. After re-reading the whole chain of your post, I have a couple of 'geezer' thoughts.
1) All the other folks have had good suggestions, most all of which are cheap fixes or checks well worth doing on any older car; a tranny check (I didn't see if you had had a tranny shop open it up and check the clutches, pressures, etc), brake adjustment and check, valve adjustment (if not hydraulics), etc. are either fairly cheap at a shop (you have already done many or all), or easy to do in your carport without bringing too much attention to you; as long as projects are (or appear to be) wrapped up each day, as opposed to having a car on blocks and parts and pieces strewn around with oil running down the gutter (the kind of thoughtless behavior that resulted in HOA 'no car work' rules), I bet the HOA will be silent.
2) With an automatic tranny, depending on your differential ratio, and a tired engine with a 1920 Holley, 13 or 14 mpg combined city/highway may not be that bad. My all-time favorite slant 6, my 74 Swinger with auto, A/C, and PS-PB never got better than about 17/18 hwy and 14-15 combined, even when it was fresh from a one-year bumper to bumper resto. But I loved that car, just seemed to 'fit.' After 250k and significant work again being needed, I let it go to a young couple who needed some cheap reliable transport, and moved on; it was my last slant 'daily driver.'
3) Now that I understand your personal situation a bit through the chain, i.e. no garage and an HOA, I'm backing away from my suggestion to mess with a Carter instead of a Holley, UNLESS you want to check the vintage bone yards for a Carter BBS that has linkage that will bolt up to your car without having to fab new parts, and have it rebuilt as an experiment. Your car should have the throttle rod that comes out of the firewall and goes directly into a rubber grommet on the carb. But the 80 engine would have been set up for a cable throttle. So if you found a good old Carter, it would likely have been set up for the rod. If not, you may not want to mess with a cable-back-to-rod conversion.
4) Also, your 80 engine should have hydraulic lifters, which negates the valve adjust issue, unless what you have is an 80 block that was rebuilt with all-mechanical valve lifters.
5) I'll wrap up with a philisophical question. What do you really want? Your mechanic seems to have nursed the mileage up to within the low end of reasonable, and identified that you have some eventual internal engine issues. If you want max mileage, you could look for a late 80s or 90s Nissan Sentra or Toyota Tercel. They were tough and dependable and my wife had a Sentra with a manual that got 40 mpg on the highway. Oh, yeah, it wouldn't be a cool old Valiant!
So, you may want to think about it this way. Best case, with your car with an auto tranny and everything tip-top, maybe you could get 18/20 mpg hwy and 15 or 16 combined. So that would be a what, a 20% or so better mileage? How much $ and pain are you willing to spend to get there?
So look at how many miles you drive, and what that would be in a year, at 3.75/gallon. If you are commuting a couple hours every day, well, I'd agree it may pay to spend the outlay to improve the mileage. On the other hand, if you don't drive that much on a daily basis, and you like the car, and it generally runs well, and (I assume) you have a fairly low initial investment, is it really worth stressing and spending large dollars to try to squeeze a couple more mpg out of it?
If you are interested in learning how cars and engines tick, and getting greasy, that's cool. If it was me (it ain't), I'd probabaly not be able to resist continued tinkering (and I'll never, never again live in an HOA (although when they send me to the old geezer home, I guess I'm done). Sounds like you need to find a garage where you can learn and 'play.' And maybe another car so when you have to tear into things, you aren't trapped into working all night to try to get it back together, or cab fare for a week.
Advice: as long as it's running well, you are enjoying it, and not spending hundreds of dollars a month on gas, you may not need to stress on it. I don't mean give up, just no stress; try things as you can and see what works! Based on your description (and assuming your wrench, who seems to have made progress and is likely not an idiot), when the engine finally starts its death spiral (who knows when that'll be?! But you'll know...), it will be plenty of time to make choices about what to do with an engine rebuild or swap.
The whole idea of an old car is to have fun and have folks look at it and say, 'Wow, what a cool old car!' If you need stress, get a copy of the 2012 IRS regulations and start reading them...
Have some fun with the old gal. Cheers! :thumblef:
