No one even mentioned the word compression and it makes me wonder about you people If the engine turns over without a plug in, it's because there is little resistance to turning over in a well oiled engine. With the plugs in the pistons now must generate compression of the air fuel mixture and this takes much more force. The power supply may be adequate to turn the engine over without plugs, but it may not be adequate to turn the engine over with plugs in. MAKE SURE you jump start CORRECTLY, with very good contact on terminals(you must have the right voltage which takes any contact, but right amperage as well which takes sufficient contact to pass that much currently in a given time which is why battery terminals are the size they are and not tiny wire connections) The engine should easily enough turn over with a breaker bar and socket on the crank pully if there is an easy way to verify this(only rotate it forward) if it takes a lot of resistance the engine is gummed up or starved for oil(or sludged) and good luck at this point.

If you are saying that the engine won't physically turn over- so that the screen of the engine turns part of a turn and stops, then you likely have a compression release issue. This could be caused by a defective camshaft, loose rocker stud etc which there is lots of info on the forum to search for. So, if the engine cranks over well only with the spark plug out- that is the likely cause- provided that fuel hasn't bypassed the carb and hydraulically locked the engine. If there is a presents of fuel- know that an open spark can make a bad day worse and cause a fire. If in fact your engine does crank well and you don't have spark, disconnect the harness at the engine or the small wire at the coil to be sure. If still no spark, then you have a funky coil.

I don't change them unless I have problems not explained by something obvious. Admittedly, my skills may not be up to some others here. I got my initial experience from the VW Idiot's Book with our 59 VW Bug, 65 Squareback and 68 Bug. I rebuilt the '59 engine. I had an old Ward's 14" chainsaw that I ran for 25 years. I guess that I may not have had its tuning set to best advantage because I tended to get fouled plugs with some regularity, but I just had several and cleaned them up in batches and reused them.

I took a plug and bent the ground electrode pretty much straight upward. The spark still jumps the gap, but not going all the way to the tip of the electrode. Seems like it is jumping erratically to the left and closest part of the ground electrode. Thoughts?