No power or bad gas mileage.

1) Plugged exhaust.
2) Clogged gas tank vent, or fuel venting system.
3) Ignition timing retarded
4) Clogged air filter
5) Choke not opening
6) Secondary not opening.
7) Wrong main jets or rods
8) Dragging brakes.
9) Low tire pressure.
10) Automatic transmission malfunction.
11) Wrong or malfunctioning thermostat in cooling system.
12) Blocked or leaking exhaust heat passage in intake manifold.
13) Defective accessory (power steering pump, air conditioning compressor, etc.) causing drag on engine.
14) Wheels out of alignment.
15) Poor driving habits.
16) Float level much too high, or float partly sunk.
17) Ignition problems or needs a tune up.

1) Check exhaust. Look for plugged catalytic converter, bad muffler baffle, kinked or crimped pipe, dirt or other foreign matter in pipe.
2) Remove gas cap & see if performance improves. If so clean or replace the gas cap. Check the charcoal cannister, hoses to it & any check valves. Check the electric vent valve on the carb if there is one, & make sure it is getting power at the right time.
3) Check timing at idle and also for full advance when revved up. Look for bad or loose hose, hose hooked up to wrong pipe on distributor, bad thermal switch, leaking vacuum advance can on the distributor, a worn breaker plate, worn distributor shaft, sticky weights, point gap closed up, etc.
4) Replace air filter.
5) Fix choke or heat source problem.
6) Check the lockout: secondaries won
7) Check them. Replace if necessary.
8) Fix brake problem.
9) Increase tire pressure at least to factory recommendations.
10) Diagnose & fix automatic transmission.
11) Replace thermostat.
12) Remove manifold & fix it.
13) Diagnose & repair or replace the accessory unit.
14) Have wheels professionally aligned.
15) Reduce speed, quick acceleration, screeching around corners etc.
16) Set to factory recommendations, & check float weight, replace if necessary.
17) Do complete tune up & physically inspect the cap, rotor, wires, coil, plugs, points, etc. in addition to checking on the scope.