I have been puzzled for a while. I have no spark to my spark plugs when I have the key turned. It will sit there and crank but no spark. And every one in a while when I let off the key turning it from start to run position I will get a single spark. I have changed the ballast resistor and put a different battery in it. But no luck. Any help?

 

It is a 71 dodge demon with a 340. It has an msd ignition system in it. If that helps any.

 

check this
http://www.aa1car.com/library/no_spark.htm

 

Put the key in the run position and put a nail (or anything metal) across the starter relay (I may be using the wrong terminology). It has two nuts sitting about an inch from each other. It should crank and start like it was a key.
FF

 

Sounds like your key switch is bad, or the wiring from it thru the firewall. The IGN1 wire gets power only in "run" position, not while cranking. It feeds upstream of the ballast resistor. The IGN2 wire alone gets power while cranking, and only then. If feeds in downstream of the ballast, directly to coil+. See if you have >10V on coil+ (rel. to BATT-) while cranking. I'll bet you don't. If your key switch is in the column (later cars), it is harder to change. You didn't bother to describe your car. See www.forabodiesonly.com for a post w/ photos on changing a column switch.

 

71 Demons have the ignition on the column. My 69 Coronets have the ignition on the dash. Forabodiesonly and forbbodiesonly are great sites for classic cars. I am 1969CoronetR/T on forbbodiesonly.