I was getting ready to run to the Post Office and I pulled my bike out in the driveway.  The bike started just fine on choke and then I knocked off the choke to let her warm up.

While she was idling, I busied myself with putting on my jacket and helmet, that is, until the engine suddenly quit.

I went over to start it again and when I switched the choke on, it fired once or twice, but then no more.  I switched the petc0ck over to PRI and tried again.  Nope.  Then I opened the bowl drain.  Yep, I've got gas

At this point I remembered and used my headlight switch and cranked it a few more times.  While doing so, things sounded normal except that on a couple cycles I noticed the choke knob sort of bounce out and in.  Hmmm.

At this point, I decided to feel the exhaust and put my hand over the tailpipe.  While spinning her over again, I felt a very strong suction in a steady cycle with the engine rotation.  Hmmm.

I pushed her back into the garage, and will get into her after awhile.

So, now while I'm taking the truck to the Post Office, what does everyone think has happened?  Any guesses out there?

s'pose the exhaust valves are staying open for one reason or another - you didn't port that head a little too far did ya?

Good thoughts all...humorous too  I'm also sure that you read my other post where I rode a good part of yesterday without any problems.

See, when this happened, I thought that I'd do like so many people do, except that for a change I'd throw in a couple initial tests that I did while trying to start it.  After all, I know that you've NEVER HEARD OF...a post like "Help, Bike Died..."

Anyway, I just pulled the plug (iridium) and it was nasty black and even wet.  I cleaned it and checked for a spark.  Yep, got plenty of spark....and I didn't even shock myself

Then I connected the compression tester.  Supposed to be checked when warmed up, but if it doesn't run that won't work.  I also decided that I wasn't going to pour any oil in the cylinder because I really wanted to see just how low the compression might be.  Book says 142 - 200 PSI is normal when warmed up.  The minimum is 114.  I got 140 PSI, 2 times, with it stone cold.  Yep, it's got "Squeeze".

Then I put the plug back in (through the plug door) and she started right away.  I turned it off again.

Why was the bike idling fine, then die and have such a dirty plug?

Going back down to the garage...

Stinker messed with your carb settings when you weren't looking.

LANCER wrote on 12/31/69 at 16:00:10:

Yeah.  I'm trying to turn it into a "dieseling 2 stroke" just like when my old 400 would go ballistic on a miss shift

My guess is the valve lash is too tight. The engine runs till it gets warm then the valves move a little bit and stay open just enough to loose compression out the carb. It cools back down when you are checking it and then everything goes back to where it will start again. Of course it could be a bad valve seat or binding valve springs but the lash is the easiest to check. Kind of humorous in a twisted sort of way . Hope it is a easy fix.

Did you ensure the fellpian tubes are clean?

Maybe low speed jet fell out