My ‘74 360 was 100%, or so I thought after venturing out on longer and longer rides without issue.

Today after replacing brake shoes - based on horror stories about separating linings and riders - I took a hot lap around my town on my detonation 360 and at around 55 mph in 4th / 5,500 rpms it just died. Pushed this pig in bike pants about 1/2 mile until neighbors kid in raptor took me last mile.

Took plug out and found it to be nicely sprayed with what appears to aluminum deposits. I will pull apart tomorrow but suspect I burnt a hole in piston.

How could this be if oil pump, timing, carb set right? Will post what I find with the goal of learning what happened to prevent a reoccurrence.

Ho HO Ho merry Christmas!

Ugh!

If you can pressure/vacuum test before taking apart it would be a good idea.

My condolences. That is ugly. With a little luck a fresh bore and new top end will be all it needs.

As far as remediating the root cause, I think you know what I would recommend...

Tdan - Stuff her ?

My best guess would be running too hot of a spark plug? That is assuming everything else is working as it should.

Comiserations & can't realy guess at the cause except keep them plenty rich on needle & main. Leaking mag crank seal is unlikely as are robust unless its popped out on an angle just. Keep the timing well back, like 12thou. I feel for ya though & been there before. Typing on my useless phone & cant see the heading thinking yours is an RT before '74, if after it'll be a cdi mag of course.

The '74 360 has a reputation of burning a hole through the top of the piston. It was mentioned in old magazine articles as well as recent experiences like yours. The parts diagram lists the standard main jet for the '74 as being a #140 with optional up to #190. The '73 is a #230 for standard. I run a #240 in my RT3 regular riders. And I avoid long sustained runs at high speed. Yamaha may have leaned out the '74 for emissions reductions back in the day.

Indeed. You'll be in there, might as well.

Dang that sucks. The DT360A is a bike on my bucket list but with all I've read here abouts I'm not sure now. My DT400 has a little detonation going on and It's been 1200 miles since a COMPLETE engine build so while waiting for my sleeper pipe this summer I pulled the head to de-carbon. I was pleasantly surprised that the top of the piston and head look like new after some cleaning. Apparently they'll take some detonation without damage. The 360 must really knock. I here a temp gauge is not fool proof but I use the Trailtech TTO 14mm temp gauge and watch head temp while riding. Vary activities if temp starts getting high. Good luck with your rebuild. I'm sure you'll post pics of teardown?