Good Morning Everyone, 
I've been trying to sort out why I can't get the kill switch or the key to shut the engine off on my 74 DT250A , pretty sure I have it wired correctly black with white stripe from kill switch wire plugged into same from main harness and solid black wire from kill switch plugged into black triple from main harness , the only thing I can think of is , I took the top half of my original key switch and installed it to the bottom half of the key switch from a 75 dt400 that I got with the speedo & tach I got from ebay that didn't have a key and my original one does . I was wondering if when I reassembled the key switch if I may have gotten the orientation of the innards somehow wrong, and before I open the key switch back up again , if someone might have run into this issue before and may be able to help.
thank you 
John

If you are comfortable with circuit testing, then you can do this by disconnecting the wires that come from the main/ignition/key switch and test each connection in line with what Yamaha call the "legend" shown in the wiring diagram for the bike.

Post again if I haven't explained myself very well! 

Thanks GR8UNCLEAL, not sure I quite understand, so I would unplug the plug from ignition switch and take one probe from multi meter( one at a time ) and stick it in the plug ?Take the other probe from the multi meter and place it somewhere else? I'm not to savvy with electrical . The key switch works fine as far as controlling the horn , turn signals and tail lights
Thanks for your response.
John

That is correct, John.

Alongside the wiring diagram are normally little summaries of which coloured wires connect with which, in each switch position, in each switch.

I use a testing light when I do mine, but a voltmeter would do the job just as well. Connect one probe to one terminal of the coloured wires for whichever circuit your testing, the other end of which should go to one terminal of a battery (doesn't matter which). Connect the other probe to the other coloured wire in the circuit, and the other end to the other battery terminal. The test light will come on when the circuit is complete.

It can be a bit of a faff if the terminals are encased within a block connector.

Any switch or even a complete harness can be tested in this way.

Edit - I've just checked and there is a service manual for this model in the Tech Library. The wiring diagram is at the very end, just before the DT360A wiring diagram. Only the ignition switch legend is shown there (manuals for later models often show these for all switches), but this is all you need for what you're looking to test.

Thank you Gr8uncleal , I'm going to do some testing now .
assuming I do have the wires connected right , i should see no light on the test light when I probe the black/white wire or the black in the key switch plug and touching the other probe on the battery positive or negative? This might tell me I have a problem in the key switch?
hopefully I'm understanding this right . It could be possible I have a junk right hand control it is a new aftermarket 2 wire, i have the original 1 wire right hand control , am I correct to assume that if I touch one probe from the test light to the 1 wire switch and the other probe to the battery the test light should come on and should shut off when I switch the kill to off and come back on when I switch it on? Sorry if I seem stupid when it comes to this.
thank you

Another thing i was wondering is , does the headlight have to be connected to complete the circuit for the ignition switch and the kill switch?
thanks again
john

Light and kill switch are 2 separate systems…
Kill switch is only ignition.
Sneezles61

Thanks Sneezles61, i think I'm starting to understand 
 

On your key switch there should be a black/white wire and a black wire.  With key off, the black/white and black should be connected to each other.  With key on, the black/white and black will not be connected to each other.  You can test this with an ohmmeter or continuity tester.

Black/white connects to the little wire going into the ignition coil. (Please note that the ignition coil wire I'm talking about is NOT the larger diameter wire that goes to the spark plug)  With an ohmmeter of continuity tester you can test if the black/white.that comes out of the harness at the key switch is connected to the little wire into the ignition coil (it should be black/white too)

The black wire coming out of the key switch must go to a good ground.  Again, you can use an ohmmeter or continuity tester to make sure the black wires you are connecting the black from the key to is grounded. 

Diagram below shows the wire from the points to the ignition coil highlighted in red.  The stuff in the blue circle is all under the flywheel. 

You can see the wire from the kill (main) switch tees into the little wire going into the coil (Black/white)

You can see the other wire out of the key switch goes to ground. (Black)

When key is "off" the contacts in the switch are closed which grounds the little black/white wire going into the ignition coil. 

When the key is "on" the contacts are open and the spark happens because the wire into the ignition coil is no longer grounded. 

Hopefully this helps you find and fix the issue.

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