From my experience your problem is most likely a incorrect plug gap,,,try setting your plug @ 020 i am sure you will find your problem will stop.
I like to try too aliminate the easier stuff first,before we get in to the complicated stuff,i hope you do not mind. The back fireing and irregular idel problem could also be caused by water or dirt in your fuel and carb,this would also cause the hard to start problem,,could it be possible that your carb is gummed up?are you useing fresh fuel? how long did this machine sit before you tryed to get it too go? when did it run properly last? you are my eyes and hands in this project,i appreciate the information that you send back too me. thank you
thank you for your imput could you please do one more check for me,would you remove the plugs and tell me if they are white/brown or black? I am still thinking you are haveing a fuel problem,the bowl should be full,,,a white plug would indicate lean burning causeing your symptoms.. If your motor keeps turning and does not start it will slow down your starter working against your built up compression. 80% of engine trouble is carburator so lets check the plugs and go from there. If she was running fine up to recently i doubt your timeing suddenly changed....please let me know
thank you,,black is a indication of rich fuel mixture,,there is a main jet on your carb,bottom/front,,try rotating clock wise a little at a time to see if it will help smooth out the idel,,i still think you are experienceing carb problems,,have you run it out of gas recently? if you find that rotating it clock wise makes no differance try it counter clock wise just a quarter turn. If you find it makes no differance either way your carb needs to be torn down and cleaned out. Please let me know the results
now we are on the right track......if you adjusted the jet and it made no differance your carb has dirt or water in it. being carefull not to break gaskets remove the carb from the engine,watch how your linkages are hooked up,,,after removal undo the bolt at the bottom of the carb,be carefull!! there is a float there so hold it up side down when removeing the bowl.Look carefully and you will see that the float controlls a needle valve inside,there should be a swivel arm and spring atatached to the float,you will see a slot screw head,remove the screw being carefull not to loose the spring!!! the float,arm and spring will come off,your needle valve is in side try to remove it and be sure to watch how it comes out/pointin. remove the other jets turning them in first to count how far out they are set. i use compressed air to blast the port holes to be sure everything is clean.Replace your jets back to original setting,when putting the needle valve back in start your screw a bit first slide the rocker pin and spring under the screw making sure it is connected to the needle valve and float. watch for contaminates through out this process put your bowl back on and place your main jet at 1. Put your carb back on the engine. Start the engine and fine tune with the main jet.
I hope this has been a acceptable answer,i am here if you need further assistance