Pioneer 1200

Well, if it backfired, you must have had a spark, right?
 
Take a look at the RPM limiter. I presume it shorts out the ignition when the limit is reached. Try disconnecting it.
 
I'm sorry. I'm not familiar with your unit, so I can't offer well thought-out suggestions.
 
Here's a tip for diagnosis. When you replace something, for instance, the spark plug you mentioned, and it does not change things, put back the old one. The idea in diagnosis is to remove variables, that is, to narrow down the possible causes. By introducing a new part that doesn't change things, you are instead adding variables, for now there's the possibility that the new part fails. Once you've figured out what the real problem is, you can go ahead and change the plug if you like.