Wico xh on Kohler welder

[oldiron 1952]

Ok this one has me stumped. It is a Wico xh 2485B sn 2-204898 on A old Miller welder with w K662 Kohler engine. The mag has new pionts,condenser,coil the hot coil 5700 it has super spark when cold let it run for about twenty min and it will stop with little to no spark. I have tried different condensers taking the kill wire off putting the old coil back in nothing makes any difference. It does have good magnetism and looks clean inside. The old welder is not worth buying a new mag for.Thanks in advance am at wits end with this one. Ps if any one has a used mag they would like to part with reasonable please let me know. Thanks

[CommandoTom]

I'm not an expert here and I'm sure someone else will pipe in if I'm way off but it is my understanding that most of the old mag's have a capacitor build in to the rotor under the windings. When they age (get old) heat becomes the enemy and as the mag warms up during use the capacitor function "breaks down". As far as I know the only cure is to have the rotor rewound, replacing the capacitor in the process.

[oldiron 1952]

Update: out of frustration I put the mag on my lathe and spun it fast enough to get out of the trip speed with the cap and rotor off I hooked a jumper wire up to the coil out put terminal and let it run for an hour great spark so then I put a heater up to it to get it as warm as the engine would still great spark then I put the rotor down on the metal part of lathe bed and held my jumper on the rotors contact and a small spark jumped to it so I put some heat to it and the spark got stronger Thur the insulated part of rotor. What do you guys think cracked rotor? Going to order one Thur Mainely Magnetos in Main great guy to deal with. Yea maybe.