Hi Everyone, 

No spark here. Here's the tests I've run:

• Source Coil resistance 
• Condenser 
• Points Continuity
• Continuity between wires
• Voltage from Black/White wire coming from stator unplugged from wire harness. (Reading is all over the place even at consistent RPMs, could be my tester is wonky though.)
• Voltage from black/white wire going to ignition coil, key turned to on, kill switch off, (don't know what this reading should be but there is current.

No voltage coming from spark plug cap so I tested resistance values and they seem weird. It's an after market coil, but it's brand new. Primary Coil Resistance - .3Secondary Coil resistance- 1.91k

Seems like I should be getting some type of at spark plug cap regardless of these not aligning with factory IC resistance values, right? Is this a grounding problem perhaps? Does anyone know the voltage values I should be getting from source coil (assuming 6V) and the b/w wire?

You say readings all over the place even at consistent rpm which leads me to think it must be running. Good old mag never fail so give points a scratch & she'll be good. Could bypass the plug caps by removing it & wriggling the lead onto the bare plug thread just to try. & run mag wire straight to coil bypassing the loom.

Yeah I tried a few more tests so I think the ignition coil must be bad:

• Points closed are reading at 0
• Old and new plug straight to IC without plug cap = No spark
• Move IC to known good ground point, still no spark. 
• Voltage readings from plug wire seem way too low to produce a spark but it would be helpful to know exactly what it should be. I'll take another look back through he service manual and see if it's in there. 

All of these tests were run outside of the wiring harness so I know it's not another ground or kill switch issue.

Best of luck & often a simple answer. Funny thing is nowdays i'd have more luck recovering spark in a cdi system than a good old mag setup as back in the day--which was my day--i only ever recal repacing 'one' points mag source coil & that was on a honda 50 in the late "60's early "70's.

Thanks, yeah you'd think a simple system would provide a simple answer, but that doesn't mean it's an obvious answer! 
Anyway, I adjusted the points and checked all the grounds. I tested the voltage again and I'm getting consistently higher readings now, but still no spark! 
I'm stumped! 

Almost every "test" you've done is pretty much useless in my experience. You've found that out already. 

With the black (or black/white) stator wire to points disconnected, check the resistance from that wire to a solid ground with points open and points closed. 

Should be zero...  not almost zero...  zero resistance with points closed.  Same as when meter probes are touched to each other. 

Should be between 1 and 2 ohms with points open.  Analog meter with a Rx1 scale works best for this test. Many digital meters jump all over and won't give you a good reading. 

If that passes...  then what happened to the original ignition coil?  If you have it, try it.  At least six members on here got spark in the last year after putting their original ignition coil back on in place of the shiny new Chinese one they just bought.  Coils are like the Chinese carbs...  some work great, others don't work at all.  Emgo makes a coil that work good for less that $15 the last time I bought one...  with inflation might be more than that now. 

 

0.0 with points closed, 1.4 with them open. But yeah I think you're right. These after-market coils are garbage. I did see that that Emgo one, I'll give that a try. This bike came to me in the form of several boxes full of parts. Never did find an ignition coil in any of the boxes!

I'm just looking at my old yamaha workshop bible of lots of models & i'm doubting what i'm reading as DT3 "73 which i'd think would be ball park similar to a "74 DT250--i know the mag changed, but--being a similar points mag it says 0.508 for the source coil but if i go to DT2 it says 1.53 so blowed if i know what's right or wrong any more. If its not grounded it should be ok at what you said which i think was1.4ohms. In fact i guess if you 'could' unearthed it at the mount screw then check to each end of 'that' coil you could find out if grounded somewhere in its windings to the coil pole. Just also looking at DT250-400 in the tech library & its full of bull s$%t in resistances yet meant to be genuine books copied from. I saw something like 1.36 for a 400 source which is cdi of cours but think they put the decimal place in the wrong place. 136 more likely. I dunno, i think you've got to be a little knowledgable about electrics or you could easily get taken on a ride--& have ta walk home.

When I first joined this forum I found this great how-to on ignition testing:

yamaha-enduros.com/index.php/forums/how-...a-no-spark-condition

After testing all other components, I ordered a new EMGO Ignition Coil  that @Mark T suggested and bingo! Big blue spark now even at low RPMs. Buyer beware of the cheap Chinese ones. The extra $10 for the EMGO did the trick. Here's the part number: EMGO 24-72400
I found it on eBay for $25 with 2-day shipping.