Looses speed going up hills. Will not keep 55mph. Put new air filter on. Cleaned carbs with seafoam. Still does not keep speeds. Sounds fine but will not hold speed going up hills and it does not take off very fast. My four cylender mini fan takes off a lot faster.

Coil on plug ignition system , coil misfiring ? What engine does it have ?

Sounds like lack of fuel. Try backing the air fuel mixture screw back 2 turns and see if it helps. Once it starts, see if you can keep it running by spraying carb cleaner into the carb. If you can keep it running long enough, place the palm of your hand over the carb inlet to choke the carb causing a sudden suction in the carb. Some times this will dislodge foreign material and help it run.

Hey Fred, I AM SOOOOO JEALOUS!!! My teenage years Saturday nights were spent in a 64 Fairlane...(4 speed - 390 - WOW!!)..
Any way.. your carb needs some TLC, a valve clearance setting-get a qualified pro to do this!!), new spark plugs (wires maybe) .. a new air filter.... mainly the carb needs a QUALIFIED PROFESSIONAL to clean and set it once you have put in the plugs/air filter.. This will eliminate possible rough idle issues and give the mechanic peace of mind when he is adjusting the carb.
Good luck...

Sounds like its a fueling issue, or there lack of.. When your hand is over it you're restricting the air flow and the engine is receiving the right amount of air to fuel.. But with the filter on, its drawing too much air for the amount of fuel its getting.. Check the fuel lines, fuel filter, and carb settings if it has any tuning screws. If its a float type, make sure the needle valve isn't sticking.

Remove the tank and flush it out with denatured alcohol. Shake it up really good than drain tank and let it dry. Next remove the fuel shut off valve out of the tank and clean screen with air pressure. Also remove entire fuel line up to banjo fitting on carb. Blow the line out with air pressure than spray a good amount of carb cleaner thru line and blow out line again. If carb is new id take it off and remove fuel bowl and see what is in bottom, dirt, water or rust. Depending on how severe it is you may have to rebuild carb using carb dip tank and blowing out every hole you can find. It sounds like the fuel screens in the fuel shut off and fuel line are plugged up.

I’m happy to help further over the phone at https://www.6ya.com/expert/david_c087e4b7d13c0163

may hav worn ignition parts that are gittn hot--they gain resistence-test them with a ohm meter--clymer manual has procedure

sounds like tranny problem/ and a bit of carb trouble i suggest clean the carbs/clean the tank....put some seafoam in the oil drive about a mile then change the oil don't dive any farther than that with seafoam in the oil or the clutch might burn out (seafoam is at any auto store 8-10 bucks does awesome things)

Sounds like the carb has not been cleaned properly, gas overflow like that could be caused from a worn seat/needle. Did you put after market exhaust on it?

This is called hunting, your fuel-air ratio is not right. Your carb. jets need cleaned

Sounds like a fuel problem, possible stuck needle valve. Did you check for vacuum leaks? How about the fuel itself, did you treat it before storage to prevent gelling and varnish ? If not your entire fuel system may be restricted. Purchase a can of Seafoam...pour in 1/2 of it to a tank of gas, use the other 1/2 on the next tank. Change fuel filter afterwards because any foreign materials will be in the filter from the cleaning process. Third tank normal fill.
Good luck...i hope this helps.