Posted by oscar on Monday, March 17, 2003 at 2:21AM :
In Reply to: Rough Running flathead posted by Patrick on Sunday, March 16, 2003 at 10:30PM :
Sounds like it is running (barely) very very rich. A stuck closed choke, leaky float, stuck/leaking carb needle valve, over pressure fuel pump, bad gas, or water in gas.
Grab a flashlight - check that the choke opens completely.
Use a timing light to check for solid regular spark to each cylinder. No cross fires or cylinders with multiple or weak sparks.
Check the points gap and timing. (Standard tune stuff). Check for incorrectly routed spark plug wires (maybe someone has played a prank on you).
The black smoke indicates a fuel flooding problem causing the plugs to foul & misfire.
Old fuel + sitting leads to a sticky/gummy buildup in the carb float needle. The needle valve either sticks open or closed. Open and the engine floods - rough running, black smoke, backfires, very poor mileage. Stuck closed and the engine refuses to run unless you pour a bit of gas down the carb throat.
I doubt it is an electrical (ignition) problem.
Most likely the carb has a bit of dirt / rust or a residue causing the carb needle valve to stick open.
Clean the carb & change the fuel filter.
You may want to drain out the gas and replace it with fresh gas if it is older than about 4 months.
A flathead runs just like any other gas engine. The low compression flatheads make them a bit more tolerant of poor quality gas and tuning settings but they are just gas engines.
oscar