Hey all,
I'm working on a 1974 DT125A and am trying to puzzle through a couple things.  I have chased down some wiring issues and repaired them enough so that the starter button works and it looks like I've got good spark.  I knew the carb was going to need some love, but tried to get it to start with gas.  No luck.  Then I pulled the carb and tried starter fluid.  I roared to life at high RPM for about a second and died.  I took that as a good sign that it was going to be a runner so I cleaned the carb and slapped it back on,  After repeated attempts the only times it sounded like it was going to go was as I was letting go of the starter button.  For example, I might hold the button for 5-10 seconds (sometimes longer) and it would crank vigorously, but then as I came off the button it would do one of three things 1) nothing - just stop (most common).  2) BANG! -  ignite fuel overrun in the exhaust (second most common outcome), or 3) run for a few revolutions (happened a couple times).

After some head scratching, I thought, maybe timing is out, but before I was going to check timing I hooked up my little test light to the points.  One lead to the bottom and one to the rocker arm.   When the points open, the light dims, but doesn't go out, When they make contact the light brightens.  I am not an electrical guy and maybe I'm doing something wrong, but shouldn't the light go out when points are open.  When I disconnect the orange wire from the dynamo, the light behaves like I expect.

Maybe I am doing something wrong because of the way I have it set up?  No battery in bike.   I have a lead that goes from the fixed side of points to a 12v battery neg, a wire from battery positive to light, a wire from light to points rocker.   As mentioned, with the orange wire disconnected, the light goes off with points open so I thought maybe a short in orange wire, but orange wire goes to the coil.  If I disconnect coil, and reconnect at dyna, light still works.

Anything thoughts.

As I'm writing this, I am wondering if I should take my 12v battery out of my set up and hook a battery up to the bike.  I don't know if that would make any difference, but I'm going to give it a shot.

Many thanks.    

So, I ran that test and I think I figured something out. With a 12v battery hooked up to the bike and ignition on the light is on when the points are open as the points close the light turns off. This confused me initially, but (if I am understanding it correctly) the current "flows" through the bulb when the points are open and when the shut, the current runs through the points (taking the shorter/ least resistant path) so the light shuts off.

I think that leaves me back where I started. Now I have to try and check timing and see if that is why it isn't starting. If timing is ok, I have to go back to the head scratching.

Cheers

6 Bike Garage

I don't know this model bike at all, over here in Aus I didn't think the DT125 had the dyna start, a friend has one or perhaps a predicessor by a couple of years.
But what you describe seems to fit my experiences with leaking crank seals or similar which suck gearbox oil into the combustion chamber and fowl the plug in a matter of seconds. So I'm thinking have you checked the plug is clean?

Here's a video I made several years ago timing my 1971 CT1. By the way, I don't remember what the noise in the background is, but it's not the engine running 

youtube.com/shorts/Gu8YPB2Rvzw?si=WY6fULMiiSFeFX7X

 Can you get it started using the kick starter? Maybe the electric start is drawing too much juice away from the spark?. Are you using a resistor spark plug? I think in your year they had a non resistor plug, like they did in 1971. The original spark plug cap has a resistor built in, so if you're using a resistor spark plug the spark could be weak. Are you getting a ton of smoke? That would be an indication that you have a wet side seal leaking.

I just fired up my 1975 DT125B after a complete restoration. Before I put fuel to it, I had the spark plug out and with connection to the top of the plug and the threaded end on the fins of the head. I cranked the starter and it was throwing bright blue sparks. I would start there. Old saying, if you think it’s the carburetor, check the ignition first.