Finally got my wrist pin bearing and gasket set for the 1970 DT1 I bought a couple weeks ago. Got it all back together and fired up on the third kick. I was having some trouble with throttle return but it seemed to work okay just sluggish returning. Took it for a quick test ride and it ran great, tons of power and was idling fine. I was letting it idle and gave the throttle a twist and it stuck wide open. It doesn’t have a key switch and of course I was in such a hurry for the first ride I didn’t have time to hook up a kill switch so I had to reach in and pull the plug wire off. After I got it unplugged it kept running and the plug boot was at least 6” from the plug and no arcing. I ended up shutting off the petcock and had to run it out of gas before it would stop. Anyone else ever see this happen before because I haven’t and would have a hard time believing it if I didn’t see it for myself.

I had a similar thing happen. I ended up putting the bike in gear and letting out the clutch with the brake on. The combustion chamber had enough compression and was hot enough to burn the fuel. Sounds like the throttle stuck open.
So did you learn from this incident.

Classic air leak symptom. (lean)

Should have dropped it into first, popped the clutch and killed it. Scary, huh?Sometimes over tightening the throttle cable top will stick the slide. Doesn’t have to be super tight.

I’ve been playing with engines small to large for 40+ years and I experienced same freaky run away on 1st start and ride of my dt360 that sat for 40 yrs prior to resuscitation. I popped clutch. Cause was precisely per MarkT, main seal on magneto side was toasted to a crisp so I replaced all 5 and then I was able to tune carb. I knew I had a vacuum leak before this happened because newly rebuilt carb pilot air screw didn’t do crap.

This blew my mind too.

I had this happen on my sx800 jet ski. I was riding on a 90 degree day and I got the machine on the trailer when we were done and instead of blowing the water out on the loading ramp I pulled it to the parking lot and started it and blipped the throttle and it went to red line and stayed there even though the throttle was closed and the tether removed. I ended up removing the engine cover and pulling the plug wires off even though they were not sparking and then I had to cover both ends of the airbox to kill it. Got the heart beating i'll tell ya Luckily no apparent damage but I'm sure it took some life out of it.
doug

I have had the runaway but not as bad as you guys, turned out I miss adjusted to throttle cable so the slide was lifted off the seat. Was able to kill it with the kill switch. I learned a hard lesson with sticking throttle cables as well turned out that the throttle grip was rubbing on the end of the bar so sliding it out off the end of the bar a little bit took care of that problem. Of course I spent north of an hour and 1/2 lubing cables, taking slide off carb, checking routing, yada yada before figuring out the true reason the cable seem stuck.

Plus one on the routing of the throttle cable. I tried a slightly different routing on my CT-2 and had it run away when I turned the bar to the right. Back to the left released the bind in the throttle cable. Rode it the mile back home, went back to the original routing, no more runaway.

Also, check that you have not overtorqued the carb mounting screws/bolts at the manifold. You can warp the carb barrel slightly and cause the slide to bind in an open position if you have tightened those til your elbow popped twice instead of just once.

Thanks for all the advice on what the problems could be. If I was on the bike I might have tried getting it in gear and use the brakes to stop it but it had me in such a panic all I could think of was to get the plug wire off and that didn’t work. I’m going to get the throttle working right and wire up a kill switch then try it again but sounds like it could be crank seals. It was running nice and strong before it did this though, seems like it would have been obvious right away. I don’t know if it could be part of the problem but gas was coming out of the overflow so I need to look at that too. I’ll report back when I get it resolved.

Be careful. It may happen again and not be the throttle sticking.

Blipping the throttle on a warm engine and having a runaway like you did where killing the ignition won't stop the engine is a classic symptom of an air leak... usually a crank seal.