4cyl. has points and condenser. cold engine starts good, when warm will run at highway speed for 15-20 min. then slows down and will even backfire if you try to stay at speed but if you slow down to 25-30mph it runs good. it won’t run at highway speed again until engine is cool. that’s not cool.
1974 volvo
Sounds almost like it may be running out of fuel. Maybe the fuel pump is weak and not holding pressure. Or it may be that the choke is sticking. Check that first.
If you are certain it is spark related, you can start with a good tune up and replace the ignition coil as well.
Carburetor Choke Problems
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