My 1971 DT250 runs great new rings, burned out exhaust, new upper rod end bearing, running 32:1 temporarily until pump is rebuilt.  

after a minute at 55 mph it seems to ping.   Any ideas ?  Have not checked timing because I don't know how to use the gauge on a angled spark plug hole.  Any ideas ?

Did you verify jetting is still spec, any air leaks on the intake? Outside of these I'd think timing. It's not a bike known for pinging issues. Did you seal the hole where the oil pump normally feeds?

I was thinking about the fact you have an new top end. Are you fully through the break in of the new rings and following the recommendations? 

sounds pretty nice to me

Darin M.  I have yet to check the jetting and comparing with spec is a great idea.  I did close off the autolube entry to the cylinder.  I may try the timing first just to see if I can figure out how to measure the piston height against the timing.   I sure wish the spark plug hole was vertical.  

msavitt,
it is really strong and wants to run.  Just wish I could get the ping to disappear.

Adguy,
I sure haven't considered that.  Suppose I should but I don't think that would cause a ping.  I didn't replace the piston since it seemed tight and within tolerance.  May be piston slap but my recollection on piston slap is that you would hear it when releasing throttle at a standstill.  Could be I suppose....

Sorry to ruin ya day but i didn't hear pinging as such, but did hear ring dingin lean sounding in the video so could induce detonation on the road maybe. Actualy sounded a bit retarded at idle so check the points gap & see if barely opening, then set at 14--don't worry about timing just yet as will be in the ball park setting the gap--& then richen the pilot & needle, check main for size & partial blockage & go try it again. Sort of sounds low on comp at idle too plus lean. Hows comp feel on the kick start. should have a good bounce to it going by my DT1.
Also i hope the oil line to the cylinder is plugged or will draw air in & agravate the problem adding to lean.

During my woffling above i clean forgot the mag side crank seal possibility related to lean running. But they're a robust seal & if you saw what i'm running with no issues you'd not worry unless its fallen out or close to it.

RT325,
yes good compression with healthy kickback. Yes I plugged the autolube screw hole in cylinder.  I guess while I am in the flywheel side checking and setting the points I could look to see if there is any evidence of leaks to see if the crank seal is failing.  Or I could just change it while I am in there.  I will clean the carb and check main size and if sizes look normal I will richer the needle.  You are right there is no pinging while sitting.  I did t recognize the ring a ding lean noise but it has been a long while since I messed with these old bikes. Only under load at speed did I notice the ping.  Just riding around the trail in lower gears I can't hear a thing, only under load in top gear.  Thanks for the input.