Plug reading done right: https://www.youtube.com/watch?v=uHbhoX0s5_8

Strange but I don't do it this way.
I run the altitude corrected factory recommended main jet and call it a day.
Reason: I run avgas and it don't seem to color the plugs the same. They are rusty colored actually. So fuel type does make a difference in my opinion.

yep I agree. type of fuel makes a hugh difference in reading plugs.
With the gas having methanol in it here is the states, it is much harder to get a true plug reading. I go by color and shutting off the ignition at the throttle position I want to read but I mostly go by seat of the pants. start really fat and work your way down. as soon as it gets on the pipe and plug is not wet and has about the right color, I stop. longevity over performance is my goal. got ta keep'r lubed up.

If I watched and heard correctly. He stated 1 mm.
If so, ummmmm. Be careful. Just me and my opinion. I would recommend around 2 mm or .080. if I heard wrong sorry.

So does types offuel make a diff?
As far as smoke ring goes. I would say air fuel rtio is air fuel ratio. Will it effect appearance in other areas I would say yes.
Plus there is more to look at including the smoke rings thickness. Still good info other than distance.
I base this off of what I do. This includes load at time of reading. You do it with out load and using 1 mm you will go lean when additional load and increase in temps and loss of air quality. Just food for thought.

When it comes to plug reading I think we are just wasting our time.
I say this because the factory has already done the foot work. These carb engineers spend a lot of time on engineering these units and I bet some spend their whole career on this. So I bet my rent money they are close. Just for fun try checking the jets in your carb right now and compare it to the stock jetting chart. I bet it's goddamn close (within two jet sizes). Even if your jetting is out (a small amount) it won't make a noticeable difference and it won't make any real difference. I believe this because, how many dirt bikes have you owned and just rode the hell out of them with no re-jetting?? You just went to the dealer-bought a bike-climbed on-then rode the hell out of it and it never blew up regardless of the outside temp or elevation. It did not matter if it was liquid cooled or an air cooled 1970's machine. The only thing that seems to matter to drivability is the pilot jet. When you stab the throttle and it just sputters then it becomes hard to ride.
Now we come to the oddy's. Why are these machines so special ?? Why do many blow up ?? Unlike the dirt bikes (of any year you choose).
Reason: Because the oddy is air cooled and the dopes put it behind a seat. So it had limited cooling. This elevated Engine temp causes detonation of the fuel. My brother and I have found out that the cooler you can run your oddy the better it runs. Our Engine temps are about 120'F (liquid cooled machines). You could feel the power loss when the temp got over 140'F. I believe there is a thread on this temp thing somewhere here but not sure what it was called.
Just my opinion.

Wow, Co I must say I disagree with you on that one or at least most of it. Mine not even close to stock jetting scale. I'm way off the chart. Maybe more when I get home. Let's not send some one done a distructive path with the stock jetting chart with highly modded Engine. Maybe use it as a tool for the right path instead.

Well I can't argue the point on a "highly modded Engine" comment. I am basing my opinion on a stock machine with --- maybe a pipe.
If you look at a chart in a manual and also use an altitude corrected chart, I bet you are within two jet sizes --- UNLESS you have some "highly modded Engine". Port design, ignition advance etc would make a difference but when you look at a mukuni or other manufacturer site, they sell their carbs pre-jetted, and this pre-jetting would be based on flow through the carb.
What I am really talking about in my first statement was that if you are out a bit on your jetting you won't explode your Engine.
If you want every HP you can get, then yes you gota re-jet.

Edit: If you explode your Engine and then blame it on your jetting because you were two/three jets high or low then I call bulshyte. Something else was very wrong. Not too many brand new dirt bikes blow up regardless of riding condition and you don't have to re-jet.
Highly modded Engine a whole different animal.

I agree. If you have a stock setup, then stock jetting will get you close enough to not burn up an Engine, I think the stock jetting is slightly on the fat side. not a bad thing unless you want maximum hp.

If someone has modified their Engine they cant expect to have stock jetting. most mod's leans the fuel ratio. pipe's, airbox mod's, changing oil ratio's, condition of air filter, hotter plug and boring to mention just a few things.
my thinking. Don't get horse power greedy unless you want to sacrifice longevity.

So, do I get your rent money? I'll pm you my address.

So enough of the funny stuff.
The big question is #1
Plug chop and read at what mm smoke ring or not.
#2 waste of time or not.

I think the general consensus would be better safe than sorry.

I personally think good to do untill one gets a handle on jetting with modded engines. Stock not so much.
I also have help others do this as a learning tool. Helped or not don't know. I like to think so.

Maybe we should also explore what mods have what effect. I think that would helpful for others.
Also some mods with fatten up the mixture Wich require a leaner jetting. We should not assume every mod will require richer settings.

CO touch above on this so let's explore that. Two to three Jets ummmmm. That may need to be covered a little closer. Even in stock trim. The stock jetting chart even covers this. Now for the pilot the 90 is better than the 89 in way of this.

BINGO!!!!

Yep
changing fuel ratio from 20:1 to lets say 32:1 will make you run richer and higher altitude

CO, I will share something with you I have never shared before.
This is some base line stuff so use wisely.
Fun with math time
Base line main jet mmx5x .90 one can use this and change the last nber according to adders.
Then add in for velocity.
Then make altitude and temperature adjustments.
Enjoy
Example
39x 5= 195
195x .90= 175
Care to guess what come stock in the 39 mm keihn
See why adnoh started with a 195
It's not rocket science
Play with it and compare to stock numbers
Use velocity and adders for secondary nbera to fine tune for first jet quess.
Then use a ratio for needle
Adnoh

I know your a math guy adnoh but the reason I believe what I believe is because of what happened to me a couple of yrs ago (June 2015). We were riding up the sourbie mountain area. It's about 5100 ft and we unload the machines at about 300 ft level. We weren't really racing up as I remember but we did have some speed going. We usually stop about half way up (at the bridge) and have a coffee. At that point my brother wanted to drive my machine and I drove his. He commented on the power my machine had. He said it had as much power as his and I did agree. My machine was a rocket that day and on the previous several rides. His machine has mods -- Pipe, carb, porting and a shaved cylinder (2mm), has 175-180 psi compression. My machine all stock except for a carb, has 140 psi compression. So now we get to the good part. We traded machines again and the race was on. Near the top I felt a sudden power drop, a backfire and she was ded. My brother came back to find me about 15 minutes later and towed me home. The autopsy showed that I had a cracked intake and a blown bottom rod bearing. The intake was cracked on the bottom side so you could not see it and it was cracked HALF way around . This intake must have been cracked for some time. That's a fact because my machine always had great power. Who knows how many rides on this crap stock intake. So in other words my machine was running incredibly lean for a long time and ran great. When I pulled the Engine the piston had no hole in it. In fact I would have used it except of all the metal rod bearing pieces embedded into the top of it. We were running Avgas. This thing ran ultra lean for some time and the piston would have still been good. The only reason it blew up was because the rod bearing let go because of lack of lube that's how fn lean it ran.

Edit: Here is one of the rides there, one month before it blew up: viewtopic.php?f=2&t=15702&hilit=sourbie

So here is an ultra lean running Engine and any plug chop would have shown that. Yet it ran for a long time in a lean condition. It did not hurt the piston. It lost a rod bearing due to lack of lube.
So my belief is you can run lean with no problem as long as the fuel don't detonate.
This is why I believe a jet size or two or three don't mean shyte.
My question is: How lean can a guy run ??

If I had a dyno and more money I would keep lowering the main jet until poofkaboom. This would be a great experiment. I bet the factories do this stuff all the time. Lean is mean. (My machine was a rocket. It was stock and kept up to a high modified Engine). I would run each jet at full throttle full load for half hour, then down one more jet size until kaboooooooommm.

Edit: I would sure like to know what the jet size equivalent ratio would have been with that cracked intake.

Comments ??

You were lean poofkaboom. Your brother's machine likely isn't set up right if its running the same speed of your lean mean machine. Something isn't right, and when modifying a 2-stroke everything needs to work together and compliment each modification in-harmony in order to work best. A (Insert modification here) isn't going to work well up top if its design for bottom hit - and vise-versa. Mix and match and you've got compounding issues.

The plug read from that video - the one thing I noticed is the size of the plug he 'read' had a very short insulator. On a plug with a longer insulator/threads the smoke ring should be a bit wider - aka 2mm.

why is the 90 pilot different from the 89? don't they use the same parts as in Engine, clutch and finale drive?

i have thought for a while now that i am still lean on my oddy, and this makes sense, i have a 190 i believe with my 39mm pwk. its seems good at low and mid range but full throttle, i won't hold it for long, not only does my egt spike, but the plug looks lean on WOT (Wide Open Throttle) runs when i was originally doing my jetting. i just don't have a bigger jet, that's why i have never changed.
since i got my egt i try not to go over 1250 and haven't burnt a piston up since then

Glad you found that helpful.
You do know that you pipe at around 1200 becomes In tune
If your interested in more cool info hang around this subject is going to be a good one I hope.
Its jetting related to adders. I just do not want to just dump info rather maybe better explain the why's and how's.
I always say if your Engine wants more by all means let it have it.
Feel free to add your set up for further discussion. I'm very interested .
Adnoh

not sure what you mean by this, but interested in what your thinking.
as far as my setup goes. i have a liquid cooled head and cylinder made by BV mild porting, hoping to maybe have that chagned. 88 cr 250 fmf reeds, no stuffer, power pros rev pipe, usually leave air filter box on, i am set up for the dual pre filter, just don't run a pre filter. 102c engages around 5K. compression last time i check was 170ish, running avgas 40:1 with motul oil

Thx for sharing that Adnoh.
That formula don't work for a Mukuni carb though.
We got 310 and 320 jets in our TM38 Mukuni flat slides.

Edit: Just so everyone is aware, and I did mention this in a previous thread, there are at least four different TM38 Mukuni carbs . There is a 100 point difference in the baseline main jet size between a couple of them. So you better know which carb you got.

CO, for base line use 9.8 and 1.96 see how close you get.
Fun with numbers.