When you say engine turns very slow, am I to assume you mean when cranking engine with starter?
Still 6 volt?
Have you checked the switch on the starter?
Does the ground strap connect to the frame to rear of battery or direct to the starter?
Is there a ground strap between the frame and the starter?
The cables size should be either "0" (zero/aught) or even "00" (double zero/aught), but no less than "1". The lower the number the larger the size. Most 12 volt battery cables are "2" or "4",
to small for 6 volt systems.
Is the battery at least 540 cold cranking amps? 600 would be better.
Converting to 12 volts is not to expensive and seems to work much better. I have changed all my trucks to 12V negative ground.
You would need to change the battery, the coil and add a ballast-resistor to ignition circuit. Starter will still function at 12V, just do not over crank (20 seconds)else it will melt the solder connection inside the starter.
Amp meter will still work OK, but gas gauge and temp gauge if electric would need to be changed, as well as all lights and turn signal blinker. A voltage reducer for heater and radio if so equipped. Are wipers electric or vaccum?
I have done the conversion several times.
Good Luck and Happy Trucking.
