I have one 90´ktm 500 with SEM and another one with Motoplat. Motoplat always worked better for me. Bike always fires at first kick, even when cold. It is easier to fire the bike with Motoplat than with SEM, as the rpm spark threshold is lower (around 300 rpm) for Motoplat.
What Motoplat do tou have?, Look at the number on the flywheel. The standard MX ignition Motoplat is 9600 176-1. It has no light, only two wires. Spark threshold is around 300rpm. Used on: 390-495 1979-84 , 350-550 water cooled, 250 79-85 (86). Also used on 83-85 250 mx
Timing should be 2.1 – 2.2 mm BTDC. You will need a micrometer to properly set it.
Other Motoplats are:
MXC Ignition: 0616026
With light, three wires 6V 35W, for head light only.
This one was available for the big bores from 1982 and up, Spark threshold around 400rpm.
This was also the recomended ignition for the 500/85-86 for the ones who needed a head light. Same is true for all other big bores to replace the five wire 9600 162-1 With the five wire 9600 162-1 it was too difficult to start the 500...
It was used for: 420-495 82-84, 350-550 water cooled, 250 82-83 (can be used up to 86)
Standard Enduro Ignition: 9600 162-1
With light, five wires 6V 35/5/21Watt. Spark threshold around 650rpm.
Some people say this one was the bad one. Bike does not rev with this one. Probably due to a bad ignition curve?
This one was used on ALL KTM up to 1981.
125-400 75-78, 125 79-80, 250 79-86, 390-495 79-84, all water cooled 350-550
Only 125/81 got a new one, see below.
Enduro Ignition, New for KTM 125 1981 and up: 0616016
With light, five wires 6V 35/5/21Watt.
Seems they have changed the ignition curve here for the high rpm of the 125!
Some people say it helps the big bore too, but I doubt it. If it would, then KTM would have dropped the 9600 162-1, but they used it for the big bores until Motoplat died....
Used on the 125 only from 81-83, but will fit and run on big bores too!
New ignition for KTM 125-84: 0616028
With light, five wires 6V 3575/21Watt.
In 1984 they changed the ignition of the 125 again to 0616028. This ignition will not fit any other KTM! (I'm not sure about this, but I think at least they used a smaller mounting plate for this ignition.)
This one should fit all 125cc at least up to 1997...
Good luck.