There was a factory mod on the wiring at the fuel injection computer. Unplug harness and cut into the wiring, find the purple wire and shave insulation. Find solid brown wire[ground], cut into it and use a spare piece of wire to join the solid brown to the purple. This will raise rpm's by about 75.

Another explanation:
1. look for X26 connector (rectangular box with 12 pole connections) under the lower fuse box on the engine compartment on the drivers side

2. identify the pure violet (or purple) wire, unplugged the connector box, open the male connector side and remove the
violet wire-male connector from the connector box.

3. Plug back the connector box and ground the male connector violet wire to the ground.

the female connector is the hot wire from the ignition switch.

the male connector is line to the ECU and this wire is to be grounded to increase the idle by 50 to 100 rpm. if this wire is
unplugged and not grounded, you will note a high and fluctuating idle.

if you want to be sure if you got the correct wire, test for voltage and should have no voltage with ignition on.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor