Garden tractor won't start.?

Changed Head gasket on my 1970's Allis Chalmers 712h, 12 hp riding mower. Turns over but won't start. But when I pull the plug boot off and jump the spark with a screwdriver in the boot and shaft of screwdriver 1/8th inch away from spark plug (spark plug still in cylinder head) the mower starts and runs. Put the boot back on the plug and it won't start. Stole a plug wire from my other mower, also a AC, and still won't start unless I pull the boot off and do the same as before. Bought a NEW plug wire and same thing. Won't start unless i pull the boot and hold screwdriver 1/8" away from sparkplug. Anyone know whats going on here. I'm baffled.

Garden tractor won't start.?
By pulling the plug wire and holding it 1/4" away, it is actually intensifying the spark by delaying the coils discharge so the amperage builds up a bit more and it also changes the timing a bit. Your problem depending on the configuration is going to be either a coil, rusty flywheel, points or timing setting.


I assume you did replace the spark plug.
Reply:It could be something as silly as a cracked spark plug. Most spark plugs are resistors of sorts, and a cracked one can lose some resistance and not spark. By creating a gap at the top of the plug, you actually "force" a spark at the tip...





Sounds silly, don't it? That may not be what is causing it, but it might help to replace the plug.
Reply:this is just a thought , have you changed the plug ? if you have to archthe plug there is a break down of voltage to the plug ,does this unit have a set of points? if so give them a close look
Reply:I believe the spark is tracking down the side of the plug insulator to ground . The plug gap may need to be reduced so as to have less resistence to ground.I would recommend dielectric grease placed on plug insulator to restrict spark tracking to ground on the outside of the plug.
Reply:Try a new plug, I had this happen in the past. Plugs fireing from side instead of on the center electrode.



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