my 2c worth,
years ago a certain tuner advised me that plug colour is no way to accurately assess jetting, Terry Becket got me to jet bigger and bigger until it splutters at WOT and then go down a size. it has never once let me down, it is exactly how chainsaws etc. are tuned and they run permanently at WOT.
hope this may help in the future, as you will need to know that it is correct next time.
cheers
les.

Elliottess1,

Absolutely correct.  Exactly what I did when racing.  There's a lot a BS about how air cooled two stroke engines are fragile but I've raced air cooled two strokes in high ambient temperatures in deep sand (high load) at WOT for miles and miles without backing off.  Even won a 24 hour desert endurance race with an air cooled two stroke. 

Dyno tests also prove that maximum sustained WOT HP is obtained one or two main jet sizes below the "blubber point" on one of these engines.

Plug reading is also an art.  The color of the end of the insulator tip doesn't tell you a whole lot except that the center insulator of the plug (not necessarily the engine) has been operating at about the right temperature.  (It's not "useless" to look at the tip color, especially with a known engine combo and recommended plug heat range.)

Spot on! If you choose to set jetting on plug colour, could be a good way out.

do you think that the main jets specified for my old Yamys '71 - '78 were based on blubber minus 1 or 2 sizes, or EPA or some other rationale?

How hot is too hot? I've checked the head temp with a temp gun after a ride and it's usually 220 to 240 or so, moments after stopping. I ordered a spark plug temp meter, but just canceled because I noticed it only goes up to 248 F/120 C, then just blinks. It seems to me it would be blinking a lot...

If you are using ethanol type fuels, you might find a need for larger jets?

Lean and hot at hi rpm

In my opinion , for what it's worth . Most of the japanese manufacturers would have set their stock jetting
on the rich side . They didn't want warranty comebacks from hamfisted customers .
Having said that , and fifty years down the track , the fuel we have available is somewhat different  ,
so stock jets are only a starting point , and may need adjusting to optimise for todays fuel .

Agreed. When you hear the random motor, be it a 4cyl tuner or a bike off the throttle and back firing away that is a lean condition. You need to re jet a little fatter to allow the fuel to get to the exhaust and prevent the oxygen in the pipe from igniting. If they had a wide range o2 meter from a bung they would certainly see +14 levels. Stochiometric is what??? Bang Baby Bang!!

Tom P

Spark plug temp meter should read to at least 550F.  Most of us have used the Trail Tech.www.trailtech.net/en-us/shop/accessories...er-head-sensor-14mm/

Infrared gun is going to give you a bit lower reading than a proper washer thermocouple under the spark plug. 

Cylinder head temp is great for general use but it responds way too slow to catch every problem.  So if you go down a few sizes on the main jet or go lean for some reason and make a full throttle run, about the time you see the gauge start to climb the engine quits.  Don't ask me how I know this...  experienced it more than once.

But it's nice to have cyl head temp.  You get used to "normal" and/or you know you're running safe.  So you can see if it's running hotter or colder than normal. 

(Best temp tool for jetting and prevent meltdown is EGT...  exhaust gas temperature.  It reacts fast enough to save you from a meltdown if you can keep an eye on the gauge...  I used to run an analog EGT.  A lot easier to see the needle position at a glance than trying to read digital numbers)