I started my 68 DT1 after sitting a few years. i drove it around everything seamed to work. I parked it. It was on the stand idling fine.  I brought the revs up, nowhere redline. Then all of a sudden, the motor was redlining. I turned off the key, the thing kept going.  I had the carb boot off, so I choked off the air. I know under certain mechanical   conditions a diesel motor will run away. What causes a two stroke to run away? The throttle slide was not stuck open. Anyone had that happen? Stan

Air leak

Must say i've never experienced that but have you had the carb off & on tightening the flange gets slightly pulled at the ears--bolt holes, & then the slide sticks as pulls the body out of shape. Doesn't take much.. Does it operate now without running & the throttle go fully open & closed cleanly. Maybe any carbon got woken up & glowing kept it igniting, but still need air or throttle open to get runaway revs. Or crank seal out which would give more air on the left but more air & oil on the right.

Was the petcock on or off

The flywheel needs to be removed and the left crankshaft seal must be at least inspected. Normally, rare but in some cases that seal sucks air. I would contact ENDURONUT and see if he still has OEM NOS crankshaft seals. It’s going to be $50.00 to $60.00 if you’re lucky enough to find one. You need to inspect the outer lip carefully. Carburetor, head gasket, base gasket  air leaks do occur. It’s a 68’ treat her good…

a

I’ve experienced this freak out on two bikes now, it’s amazing I didn’t throw a rod at like 12k rpm with ignition turned off!!  

And it doesn’t take a big leak or a bad seal.  I had a bike go ape ^%^^ crazy from a minor manifold leak. I would pressure test before replacing stuff. i had another over 50 yrs that had brittle seals so If it has original seals they are probably hardened but could be leaking in intake or base somewhere else.  Once you fix then you can get it to run and idle really nice.  

I'll stir the pot a bit more!!. It must be getting fuel to rev. Can't rev without fuel--& air. Poster said hand across the carb stopped it, so can't be getting air from a seal or anywhere in useful quantities or closing the carb off with hand & if slide is 'down' then pilot is the only one left to suck from--plus what it can draw past a worn slide needle & jet. Apologies--too much time on my hands lol. But keen to find the 'real' answer when sorted. Oh yeah, there must be something very hot too to ignite--i forgot plug cap was off.

RT - makes sense to a degree.  I had run away with ignition off from very small leaks that apparently were big enough to lean things out to the point that it got hot enough to run like a diesel with ignition off.

if not a leak then a stuck WOT slide?  even if this were the case it would need a red hot something to run with the ignition off

my 50 yr old DT1E had crank seal on left that was as brittle as an eggshell

half the fun IMO is figuring this all out and getting things to the point where you can take a good long ride without having to call for help!

HNY

The "runaway" tendency with two strokes can happen if something hot in combustion chamber glows and engine gets into preignition...

More common though is a tendency for a two stroke under certain conditions to enter into more of a "chemical reaction" as I understand it...  A Japanese Company studied this tendency back in the 1970's and some efforts were made to develop low emissions, lean burning, high efficiency two strokes that would be started with "spark" and once warm would no longer need spark.  Initial development was for steady rpm engines like generators.

Initially named something like "ATAC", Honda secretly experimented with the concept and called their version "AR" for "activated radical"  They came up with high tech two stroke race bikes...  EXP1 and EXP2 I think were the names...  They raced them off road in Paris-Dakar and Baja. 

It's technically not the same combustion process as a compression-ignition "diesel"...  it's described more as "controlled detonation" and there's something about the high amount of exhaust gas in the combustion chamber of a two stroke under certain conditions and a chemical reaction that makes them run without spark. 

In the late1990's Honda built a carbureted production bike that utilized this concept...  the CRM250AR.  I happen to own one of the few in the US...  our UK friends might be familiar with them as an independent importer sold them there for years.  It actually "cruises" in AR mode where it's running very lean, spark plug is not sparking, and efficiency is very high.  You can kinda tell by the sound when it's in "AR" mode but Honda did a great job and performance is seamless...  it's just kind of a torquey two stroke. 

Yamaha may have killed it off when they came out with the very successful WR450R four stroke...  it probably was cheaper to go that route than continue to develop high tech low emissions two strokes. 

Anyway, engine has to be lean and obviously doesn't need spark so when it happens (and slide is not stuck open) flipping the choke on might kill it.  Hand over carb will also richen it and kill it.  Petcock off and lay bike on side will have it die when it runs out of fuel.  Old stand by is kill it with clutch in gear but that's my last resort.

Scary when it happens...  my 1979 DT175 was really bad.  Turned out to be a bad ignition side crank seal that held pressure but not vacuum would put it in "runaway" mode when warmed up and throttle blipped.

Here's an excerpt from a Dirt Bike article: 
"Activated radical combustion takes place in a two-stroke whenever a partially burned charge comes in contact with a fresh charge at certain throttle openings and engine speeds. Many dirt bikers have seen examples of Activated Radical Combustion. It’s what takes place in most engines that mysteriously run on, even when their ignitions are killed. What is happening is not “dieseling.” It’s a complex reaction caused by the highly reactive intermediate molecules (active radicals) left over by a previous ignition, not the direct result of heat or pressure."

dirtbikemagazine.com/when-honda-tried-to-save-the-2-stroke/
 

My 800SX Jet Ski did this to me one time when I was blowing the water out of it in the parking lot after a ride. I started it and blip the throttle a couple of times to blow the water from the exhaust system and the engine went wide open with the throttle closed and the tether pulled. The only way to stop it was to pull the hood off and cover the carb intakes with my hands. I thought the crank would come thru the side of the cases. I sure it was wide open for 20 to 30 seconds.  That was last year.  This year I'm cruising down the lake pinned and a big thump and the motor died.  Turns out it snapped a rod in half.  It never seized, it just broke a rod. This is a stock JS800 motor.  I believe the wide open incident on the trailer weakened the rod.  Anyhow destroyed the complete motor. Had to replace everything between the carbs and the exhaust manifold except the head. I believe it was all caused by the dieseling effect and on a jet ski there is no kicking it into gear and letting the clutch stop it.