Public Service announcement.

Intended to educate, not denigrate anyone.  Everyone's input is valued.

There seems to be some mix up between pre ignition and detonation on this thread.  
Detonation rarely occurs on a partial throttle because the compression is not sufficient to cause it.

However, pre ignition can occur due to wrong plug, advanced timing or hot carbon deposits.  Again, partial throttle and low rpm  usually precludes hot deposits unless they are severe and the engine is in sore need of a decoke.  If really severe, the engine may carry on running after the ignition has been turned off.

I have spannered on and tuned road racing bikes in a previous life, right to the point of detonation on normal pump fuel, extremely fast bike, but ate pistons.  In my effort to effect a cure and prevent the team manager from going bankrupt,  I researched two stroke detonation at length and also had a good supply of pistons to examine.

So two stroke detonation is a spontaneous form of ignition entirely separate from other ignition sources.  It usually occurs at full throttle, high rpms.  It is usually confined to the squish ring area and is due to overly high localised extreme adiabatic  compression.  It breaks through the laminar gas insulation boundary of the piston crown and eats away at the metal in a process called cavitation.

Pre ignition or "ping" is caused by the rising piston hitting an already rising pressure wave due to the mixture starting to burn early and compressing it further.  This causes an intense and uncontrolled pressure rise in the chamber and causes spontaneous and instantaneous combustion of the remaining unburnt fuel.  Literally an explosion in the cylinder.

The damage is again caused by cavitation, but located where the source of the cause of ignition is and covering a wider area.

There is another condition where the fuel can spontaneously ignite due to a chemical reaction in the chamber, but when I was spannering in pre internet days, it was noted but little understood.  Certainly not by me anyways.

So it is important not to bandy words about interchangeably.  They are the same animal but with very different causes and symptoms.

As some have mentioned, weak mixtures can excaberate the condition and richening the mixture can prevent it by cooling things down, but are rarely the sole cause of the condition.

Edit.

BTW, the OPs plug looks fine to me, but definitive results can only be found using a brand new plug, fitted at operating temperature and run at one specific portion of the throttle range ie wide open throttle and kill the engine before closing it. An old plug such as the one shown only gives a general indication of an average condition.
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@RT325

I know it sounds unbelievable. The gearing is 17/44 so it's actually turning less RPM at 90 than the stocker is at 80. The kicker is the altitude. I'd have 20% more power at sea level. Tapped out at 7000 RPM it would be doing about 100, assuming the engine would put up with a top end pull like that. High revs, high heat, risky business.

@tinkicker

The difference between detonation, knocking, pinging and pre-ignition is when the spontaneous ignition occurs.  Pre-ignition occurs before the spark lights off the contents of the combustion chamber, detonation/knocking/pinging occurs after the spark.  Neither one is likely at low revs and partial load and I've never seen a two stroke exhibit either condition anywhere other than full throttle.  These old things are unique in that way.

For all I know you could very well be right but I find pre-ignition to be extremely unlikely.  It requires some kind of superheated ignition source that persists inside the combustion chamber during the blowdown and scavenging period.  At 3000 RPM and part throttle there is not much heat and plenty of time between combustion cycles to cool it off.

As far as I know the 74 360 was the worst of them when it comes to pinging at part throttle but if you wick that throttle open and run it hard, the pinging completely disappears.  Any hot spotting in the chamber from deposits or an incorrect heat range would only worsen at full load.

Luckily the compression is so low on these things that they can withstand a good amount of pinging without sustaining damage.  It should be addressed in any case though, if nothing else it is irritating.  At worst it'll cause rapid and serious engine damage, which is about what my 360 was trying to do in stock form at a steady 55 mph cruise.  If you didn't recognize that sound and you just kept on it you would be looking at a locked up motor in 30 seconds or less.

Tinkicker, thank you for sharing your experience.  My ideas/definitions/experiences are very slightly different...  probably splitting hairs. 

One thing I did want to mention is that the "runaway" two stroke many of us have experienced...  where ignition is off and engine keeps revving (very scary)...  is almost certainly caused by the "chemical reaction" phenomena you spoke about and not preignition.  It was the unique tendency for a two stroke to go into a "runaway" condition that prompted the research that ultimately led to Honda's CRM250AR...  which actually runs without spark during certain riding conditions!  It's a very efficient combustion cycle that Honda named "Activated Radical Combustion".  Emissions are cleaner and fuel economy is greatly improved.  Honda used this technology in the Paris-Dakar race race bikes and later in the production model which you may have seen in the UK. 

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Turbodan- I'm once again confused.  In an earlier post in this thread you said "I do hear detonation occurring sporadically in the video.  It's a unique sound, hard to describe but I think you already know.  Just like you would imagine a small, powerful explosion would sound confined inside the combustion chamber"

Then you just said "The difference between detonation, knocking, pinging and pre-ignition is when the spontaneous ignition occurs.  Pre-ignition occurs before the spark lights off the contents of the combustion chamber, detonation/knocking/pinging occurs after the spark. Neither one is likely at low revs and partial load and I've never seen a two stroke exhibit either condition anywhere other than full throttle.  These old things are unique in that way."

These seem to be conflicting statements...  for the record the only part of either statement I tend to agree with most is the last sentence in the second quote above...  which is the sentence that contradicts your earlier statement saying you heard detonation in the video?  What am I not understanding?

This is not the first time my opinions/experience/definitions have been different than yours...  I have great respect for experiences of others and I'm trying to understand...  I certainly don't know everything. 

P.S. I've always thought the part throttle, low load "pinging" sound we sometimes hear on these bikes is explosion of unburned fuel in the pipe. 

@MarkT

I don't see how these statements contradict each other.  Detonation/pinging/knocking is a spontaneous ignition of an unstable pocket of air/fuel mixture caused by the heat and pressure of the incoming flame front within the combustion chamber.  This pocket of fuel literally detonates, in contrast to a controlled deflagration that occurs in a relatively slow and gradual manner as the flame front propagates through the chamber.  This explosion causes a rapid spike in pressure that generates an audible "knock". 

Pre-ignition occurs when a hot spot develops inside the cylinder and acts like a glow plug in a diesel engine, causing the air fuel mixture to ignite while the piston is still traveling upward on the compression stroke.

I did say det is unlikely to occur at part throttle.  That's just a general statement.  These big air cooled singles are unique in that regard.  They will ping even with the throttle closed on overrun.  Being a two stroke there is never less than atmospheric pressure present in the cylinder when the exhaust port closes so compression is there regardless of throttle position.  With a hot engine, lean jetting and I'm guessing a poorly setup factory squish band, these things love to ping.  Opening the throttle improves scavenging the chamber, exchanging more of the hot exhaust gases for fresh, cool air fuel mixture.

What you hear in the video is detonation without a doubt.  Sporadic and not likely to cause damage, but still undesireable. 

turbodan...  let me be more specific quoting you...  "I've never seen a two stroke exhibit either condition anywhere other than full throttle"


That was said right after you described your definitions of detonation and pre-ignition.

And once again, especially in your last post, you've said a lot of things that do not agree with my experience and knowledge...  I studied and experimented with engines and tested fuels at the University level and was a member of the SAE for a few years...  but that doesn't mean I'm right and you're wrong.  Some of the best porting used to be done by guys who experimented and found out what works...  I have a lot of respect for "street smarts"..  Some of the things I've done "shouldn't" work but they do work very well even though my "book smarts" can't explain why.  

Thanks for sharing your perspective even though I don't quite understand/agree with all of it. 

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Just got to keep those motors on the point of rich everywhere to survive.

It does sound like is pinging/knocking a bit to me in the video, my RTs have has a lower popping sound normally.
Id try to retard timing a little bit and maybe raise the needle...

I really appreciate everyone taking the time to offer information. I'm completely new to 2 strokes so all the replies are highly appreciated. 

Here are the things I've ordered:
Motion Pro pressure leak down test kit so I can check the motor for air leaks
New o-ring for the needle jet and the proper 230 main jet (currently has a 220), I'll check the other carb components while I have it apart
Gas Cap rubber rebuild kit
Flywheel puller 27x1 and flywheel holder to check timing  

Is there a easier way to check the timing or will I need to remove the head? 

Thanks again for all the replies.