1975 dt 400 B lost spark

Just changed the rectifier, seemed battery was dropping voltage. Rode bike about 10 miles & went to start & no spark. Checked all connections & they're good. Have voltage out of stator,but not sure past that. Any ideas ?

I have a 75 DT250 and I had same problem! I changed all the electrical parts (battery, points, condenser, ignition coil,....etc) what I found was at the steering column the wiring harness was worn and found some broken wires! A simple soldering job and some heat shrink and I was good to go!

Try going to YamahaEnduro.com, there you will find LOTS! of good information and plenty of knowledgeable people that rebuild these old bikes and that can help! Ask or look for forum member DEET, he makes new harnesses if that's the problem!

Thanks for the info,checked switches today & they're good. I'll check those wires & then head from the coil back

After checking wires,all looks good. Wondering what voltage should be after the Cdi box,I think that may be my issue. It reads around 30v out of the stator. What should the voltage be after the Cdi box & after the coil???

Im not able to figure out how to post a picture on here. I have a wiring diagram for a 75 DT400B but can seem to upload it. Do you have a copy of the wiring diagram? If so look at the ignition coil, if there is a ground or open in the coil you will loose spark. If you getting 30 volts out of the CDI I think it would be good. Im just not sure what voltage you will get.
There should be a black wire with white stripe coming from the main switch and the kill switch. This wire will ground the CDI and shut off any spark to the ignition coil.; check this wire and make sure it is not grounded. If its not grounded, disconnect this wire from the CDI and see if you get a spark. If you dont then I would think its a bad ignition coil. Can you check resistance of the ignition coil? If you have a owners manual for your bike it may have the values of the ignition coil in it.
Looking at the diagram Im still thinking you may have a bad ignition coil or its grounded some how. From another forum, there are a lot of stories of the ignition coil going bad.

Thanks for all the info!!! Yeah I have a manual,but doesn't give voltage,just ohms on some things & pics of meter images. I tried another coil & got the same response.ive bypassed keyed ignition as it looks in wiring diagram & get no spark. BUT, when I switch the key back & forth as I kick there is a quick nice blue spark, but not consistent. It sparks about 1/2 way through the turn. Wierd right....so now I'm thinking its the Cdi box .

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