firewall
4 - 8
3 - 7
2 - 6
1 - 5
front
NUMBER 7
mine did the same it turned out to be the injector
its on the cylinder head. just like any exhaust manifold.
Passenger side 3rd cylinder from the front.
right front
www.motorcraft.com shows ( 10 degrees BTDC ) for the 1991 F-150 with the 4.9 L ( 300 cubic inch ) straight 6 cylinder engine - confirm with engine compartment sticker
Ford F-150 ( V8 ) firewall 4 - 8 3 - 7 2 - 6 1 - 5 front of vehicle
drivers side of engine , back cylinder ( nearest firewall )
firewall 3 - 6 2 - 5 1 - 4 front ( for the 4.2 L - V6 - in a 1997 Ford F-150 )
3 - 6 2 - 5 1 - 4 front
firewall 4 - 8 3 - 7 2 - 6 1 - 5 front of Ford F-150 ( V8 )
You could have a dead cylinder in the engine, or you could have a warped rotor, out of balance tires, etc.