Hi everyone,
1,500 km after a complete restoration and now a hole in the piston; bugger!!
Doing 80 - 90- km / hr on the open road, mild weather (22 C) at the time.

I have ridden it harder (100 -110 km/hr) in the past, in hotter conditions.

It has pinged occasionally in the past but was jetted on a dyno.
I have a Rex electronic ignition and 12V kit on hand (for a DT360A Special project); considering fitting it.

Had been hard to start prior to the breakdown.

This has never happened with my DT360A; is this common with the 400?
Any troubleshooting suggestions (other than a new piston!) much appreciated.
Cheers,
Richard

Any suggestions?
 

Most likely jetting.  Cruising at higher speeds means high revs at low throttle.  Main jet is out of the equation, this is a matter of pilot jet and needle clip position.  It can be tempting to lean out these low throttle circuits to crisp up the low rpm operation but it can be dangerously lean at high revs.  Much care must be taken to strike the right balance and it would be difficult to accurately replicate this low load at speed condition on the dyno.

Other causes include improper ignition timing and air leaks.  Probably is jetting though.  Many thousands of crank seals have been replaced attempting to cure a jetting problem.

Hope the flywheel hasn't damaged the key & changed ignition timing very slightly, but unlikely as would happen shortly after first start after assembly. Keep it rich everywhere & forget about fuel economy. Plus a cold plug NGK9 etc. Tis a bugger though.

DT Dick - very sorry to hear about your piston meltdown, that sucks
I did same on my DT360A a couple of years ago, big hole with piston ring fragments blowing out muffler
Root cause was likely having a #140 main jet vs. specified #180 or #190, combined with a B7ES plug and likely some air leaks too....my mistake was assuming that old bike still had original and proper jets and not taking the time to check this or the plug.  Piston cost me a left nut.  Ended up having to rebore due to a big gouge by a ring, so I redid the entire engine properly, including pressure testing it.  Luckily crank was still good, which I attribute to Lucas semi-synthetic injector oil which I swear by.

After many DT400 restores I have found that they are almost impossible to jet for Highway use and off road use and have both work good. If jetted for Highway or high speed where you are using a steady throttle position you better be rich or you will have a melted piston.  If you jet for off road and take it easy on the highway you can survive going a little leaner. Not a jetting expert  just  my experience after 15 restores.  I find i can lug these engines at lower RPM and they last forever, but if you spin them to high for too long they retain heat and bad things happen.  Some kind of fuel injection system would be the ticket.  One thing I would like to try is a LECTRON Carb but I can't bring myself to spend the thousand bucks on an experiment.
Doug

First thing I thought of as the cause was the statement "jetted on a dyno".  But don't know enough about what the goals were and what was changed from stock. 

Timing was my next thought...  prudent to check with timing light.  Air leak could be issue as well.

P.S.  @run103...  On the Lectron, I bought my last one for my DT400D not many years ago and had them custom machine to fit the stock boots...  total with shipping was only $420.  Hard to believe they are $1000 now?  I never got around to trying it because I haven't ridden the 400 since before I got it.  I have Lectrons on 250 Husky and TS400 and they work great. 

It would be great to have a clean running DT400 at all rpm and load  ranges.  The 250's are pretty good but the big bores need a different train of thought.  My belief is they create and retain too much heat, and the incoming fuel charge cannot cool it down for all loads.  Had the same issue with my yz465's and yz490  in the good old days.
doug

Modern fuel just isnt the same.
Retard timing to 2.4mm BTDC, that will directly result in less heat in the motor
You shouldnt notice much if any power loss.

akara1, not arguing with you just reporting what I've just experienced with my DT400C I had it at just over 2.5mm BTDC and it was hard to start, I got it dialed in to 2.9 now and it starts first kick.  I do have the Vape 12v conversion.  Again, just my experience but appreciate your input!

I think I posted here before that I like 0.100" BTDC for timing on 360/400.  About 2.5 mm.  Rarely will kick back and runs great in my experience.