I had my 1951 Plymouth Cranbrook out this weekend (1st drive of the year) and out on the highway it wouldn't do over 45 mph and really slowed down going up a hill. Gas pedal was to the floor and seemed like the engine was missing. Going down hill it would sound better but when under load it would start to miss. Took it back home after about 15 minutes of driving- didn't want to be left stranded, shut it off to open the garage door, then it started real hard. No problem 2 years ago but last year I had clutch/transmission problems so I didn't drive it that much and only around town. Clutch/transmission problems fixed at a shop that works on older cars but I can't think of a connection between working on the clutch/transmission and a high speed miss other than the problem happened after it was fixed. Plugs/points/coil/condenser are about 7 years old but have less than 1500 miles on them. Every fall Sta-Bil is put in the gas and tank is topped off.
Ideas on what to check?
Answer First do you have a fuel filter on the car? If so there could be some restriction in the filter like dirt build up. I would pull the plugs and get a look at them. With driving so few miles the plugs could be fowled a bit. A defect in the ignition coil could also cause the same problem. Also check the exhaust system for restriction. It has not been unknown for critters to crawl in the tail pipe and build nests plugging the exhaust, and also check the air cleaner.
All automotive including antique and collectible. However if the car has been modified I can only answer in general terms and maybe get you pointed in the right direction.
Automotive tech instructor.
Syndicated auto columnist 1970's though the early 1990's.
Syndicated auto radio talk show, Ask Brad About cars, CBS Radio 70's through 90's
TV Show "Last Chance Garage" 1980's PBS-TV syndicated.
Auto instructor for the following companies:
University Of Conn
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Sugar River Technical Center
Grew up in a family garage in Needham Mass and turned wrenches from the age of 14.
Publications Manchester Union Leader, Nashua Telegraph, Motor Service Magazine, Yankee Magazine, Popular Mechanics (Saturday Mechanic early 80's), Los Angeles Times, New York Times, and lots more.
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Basically Franklin Technical Institute in Boston, Northeastern University, Fitchburg State Teachers College, Tufts University, and a lot of factory schools along the way.
Awards and Honors Moto Award winner. And much more.