D15 160 gas

My d15 has been a constant ignition problem child. It ate points in a few months so I had it rewired and pit in a positive ground pertronix kit. It blew two of those I thought it was a bad key switch so I replaced that with the same key switch which is on my d17 and works great there. After blowing the second pertronix kit this summer I switched it back to points as they are cheaper.   Well tonight it has started to diesel when you shut it off. I have been using it off and on all day for the last week cleaning out my barn so it has run pretty steady all week. Shutting it down only when I want a break. I checked the timing when I put the points on it is timed with the f 25 mark dead center in the window at full throttle.   I am stumped as to what could be wrong. I have replace the cap rotor coil point condenser wires and the plugs are new last fall. Tr wires with a new key switch. It has a one wire alt which has been rebuilt for positive ground.   
I was wondering why it would diesel after it has been allowed to idle for a few minutes and the key is shutoff. It takes several seconds maybe 15 to actually stop not at all its normal behavior.   This 160 is a much more fussy motor then my 226 in my d17 that is for sure.

A-C #44 spark plugs would also help.

     Hey Dan,,,,, most generally, dieseling can be caused by too high an idle, timing, or the most frequent one, carbon deposits in head creating hotspots that trigger detonation of the unburned fuel still in system after key shut off.. I've not ever used it but read a lot of good things bout Sea Foam working on the carbon problem. Since the butterfly in carb is never completely closed, and the engine is still "turning over" due to dieseling, it will continue to suck and run on residual fuel in system.
 Since you have already addressed the timing and idle RPM's, it may be the engine has developed some carbon build-up. One way to address the dieseling is to shut off while in gear,,,,,makin sure of your brakes,,,,,,
   The 1 wire alternators are notorious for not producing at low RPMs ,,,you don't reckon that was the issue with burning out  the Petronics before,,??
   Changing the wiring is not that big a deal if you decide to change to the conventional alternator when the 1 wire has to be replaced,,,, hope this helps you.

Dan - PM sent!

Joe you are probably onto something with the carbon I had a skip and the #4 cylinder is definitely had carbon on the plug. I also just had a nice chat with Orangeman about it and will be trying some higher octane fuel.

Dan - Good to hear - the 160 when set up properly is a fine engine with an excellent torque rise!