Original engine, 123K miles manual transmission. Used to require cranking about 30-45s and then would start, now just cranks but won't start. If I take off the air cleaner and spray a 2second burst of carb cleaner into carburetor, starts in about 5 seconds and will continue running. Also, engine has started to die/slip when gassing too much into a higher gear, and 4th gear is very slow to respond. Any help would be appreciated so I'm not paying a mechanic to run down too many rabbit holes.