I have a 1975 DT125B that will turn the motor very slow with the electric start, I’ve got a good new battery, verified 12v going to pale green wire, have cleaned the commutator, have cut the Mica to be lower than the commutator, have cleaned the brushes and they are well above the wear line. I downloaded the service manual and gone over everything. The bike still will barely turn the motor over, if I pull the spark plug out it will turn it over faster because of the less resistance. I have continuity between the positive brushes and the body of the dynamo only when the key is on. I’m stumped here. Anybody have an expertise? Thanks in advance! I will include a picture of the wiring 

Welcome. This could get very interesting as we're into an almost identical post on the site here. Does it charge at the moment, if so then at least its half good [sorry stupid answer]. Keep a watch on the other post too below if you're not already doing so.
I guess the heavy wires are still 'heavy' & not been replaced by small wires. That goes for battery positive over to the start solenoid & battery earth as well for sure. Cheers from NZ downunder.
yamaha-enduros.com/index.php/forums/74-7...ng-woes-im-lost-help
On the face of it, it sounds like the armature or stator is faulty in the windings--but i've been wrong before.

Check your grounds. The ground wires are a major cause of electrical issues.

Hello Clegg,
 You may want to retest the voltage getting to the pale green wire while cranking it over. I’d expect to see a 12 volt battery delivering around 8-10 volts down there while cranking. A working battery drops a little in voltage , that’s why I’m wondering how yours can deliver a full 12v DC to starter. There are resistance losses before it gets there.
  An electric meter set to DC with one terminal hooked to positive on battery and the pale green terminal (the big fat wire) on dynamo while cranking will likely show one or two volts. This is the electric loss from traveling through wires, burnt starter solenoid contacts and 50 YO connectors. If it measures much higher - investigation into what’s wrong in the delivery system is in order. 
 

Ha, I came to the forums this morning to check if anyone is having this exact issue as my bike ('75 DT125) is as well. If I can come up with anything on my bike I'll be sure to post it here.
Good luck, please update us if you've found the issue.

I have the same bike and the same issue. Once I ride the bike, it will crank over fine. But my initial start after setting for days or weeks is never possible using the electric start. It kicks over so easily and starts on just a kick or two, so I simply accept the fact.

Any of my dodgy batteries in my vehicles show up just with a volt meter across the battery while starting.
The voltage drop is there to read.
Need a battery with high cold starting amps to spin those DT125's from cold.
Ideally kick start cold.
Also be sure any heavy wires haven't been replaced with a lighter gauge on anything like battery earth, or positive side to the solenoid & down to the stator starter terminal. Brushes seem to work [until they don't] but more length gives more spring pressure.
Commutator needs to be good & flat to get full brush contact but can be machined.