Tractor Repair: 9n back firing, volt coil, 9n tractor

Question
I bought my tractor about 6 months ago, my 9n tractor idles great and in low speed runs nicely up and down the road. But as soon as I ramp up the rpm's,or  place it in a higher gear, or put a load on it it starts back firing a lot, the back firing is very loud, then I lose power and wants to die. I thought it might be the distributor but I found that it has been converted over to a 12volt system and there are no points nor condenser. So I am not sure how to check if the shaft is wobbled out.I have cleaned the carburetor,but the problem continues. The repair manuals I have looked at are of little help because of the converted electrical system. What things should I be looking at? What further things could I be doing to repair the tractor? Thank you in advance for any help  or advice you ca give me.

Answer
Even though it was converted to a 12 volts system, it still has points and condenser. The change over the system the kit came with a harness, alternator and a 12 volt coil. Everything else is the same. Check the points and condenser and then timing. I have also heard that the bushing for the distributor wears out effecting the timing.