I finally had some time to watch the video, and enjoyed it all. Well done. Back to the original issue - hard to tell, but it sounds to me (hard to know without being there) that it sounds actually rich on the mix. If that is true, even with the pipe open and no air cleaner(?) that would explain plug fouling.

I assume you could have the choke/enricher on for the cold which could affect things too. Again, I am going exclusively off the sound based on my own experience.

Keep up the video work!

I can't imagine riding in those conditions. Yer a braver maun than me!

Did I see 80 mph? My RT won't go that fast ......

Thanks for you interest. The fastest i went was 90 km/h on pure ice. The speedo in in km/h, the bike goes 100km/h at full throttle which is 60 mph i believe.
I generally use my winter tires when i ride in those conditions.
There is a picture of those in my previous post.
I cant say i wasnt scared at some points though. But im still young and strong, i can take it.

Thanks for your interest

Yes, now that people talk about it im starting to think it runs rich too, because as you say, the air filter and the exhaust delete should make it leaner but it still sounds rich.
That ride was with the choke off.
Oh, well, ill have to take a look at that.

When i crashed, i first killed the bike.
Then i turned off my ipad because ive got limited space on it and i wanted to film the RM ride as well. (what was i supposed to do, who cares if the bikes lays on its side, not me)
The speed was keeping me up as i was riding at high speed on pure ice.
At the time i crashed, i was giving some bursts of throttle and that's what got me.
I could drop 5 gallons of snow into the exhaust and it wouldn't matter. Do you own snowmobiles ?
Running without the muffler only makes it leaner which isnt a problem because the bike still runs rich, and i cant re-install the muffler right now because its clogged and ive got to unclog it.

If someone is interested, this is the ride i did with the mighty Rm 250 1990 after i crashed the enduro.
I managed to keep the RM on its wheels but there was a few close calls.
When i install those winter tires ill make another video.
In this video i show both camera angles.
I also made a guitar song to introduce the beast at the beginning of the video.

I'm thinking it is one of three things, or a combination of any combination of the three.
Timing, running stoopid rich , or a build up of sludge in the bottom of your crankcase from the leaking seal on the wet-side (and a possible leak between the crank and transmisson, a leak-down test would show it).

A side-note on spark-plugs:
Back in the day of point ignition, carburetors and crappy oil, spark-plug manufacturers added a coating to the ceramic on plugs to resist fouling. Now that fuel injection and CDI or MSD ignitions are the order of the day, manufacturers have dropped this step to save a few cents a part.
This is the reason that all service stations and dealerships had a little soda-blaster for cleaning plugs, and no one does that anymore.
You would think that on plugs for old bikes, chainsaws, weedeaters etc. they would still be coating the ceramics, but they don't. Most people just accept that plugs foul easily in these applications.

I checked the jetting when i cleaned the carb to get it running when i got it after it had sat for a LONG time.
Its not a complicated task and im going to check the jets and tell you the numbers.
I havent checked the timing yet since i work today and yesterday i was doing these videos.
I was planing to take the engine out this winter anyway.
I wont run the Enduro this winter, i just did it once to entertain you guys.
I will ride the RM this winter though.
Once the engine is out im going to replace every seal and gasket just like i did with my RM.
Last time i checked , the piston and rings were in perfect condition so i might leave them alone, or bring them to a machine shop to have a second opinion.
Ive never done a leak-down test because ive never worked on older bikes and the bikes i rebuild always run perfectly (as you can see my RM)
Ive touched nothing on the Enduro yet except the carb and spark plug.
I dont really want to put too much money into it right now because it still works ''fine'' as you saw in the video.
Maybe ill do a complete restoration on it later, but ive already spent loads of money on the RM this year and i also need some money for my current project car.
Interesting fact about the spark plugs, but if anybody own a similar bike, your spark plugs last for how many hours before they die ?

In my '72 Yamaha R5, I can sit in stop and go traffic for 15 minutes to a half hour, and have the plugs load-up and the bike die. The bike runs clean, so long as you are out on the road. If I know I am going to spend some time in traffic, I put in the next hotter plug.
EVERYONE back in the day, carried spare plugs on the bike somewhere (tool bag, battery box, etc.) It is just a fact of life on a carbureted bike, two stroke or four stroke.

In fact i had a few minutes before i had to go to work so i decided to pull the main jet out of the enduro (because the other one dont really matter since i pretty much always ride full throttle)
And im going to check the other one anyway but i didnt want to pull out the carb and put it back together in 2 minutes.
Its got a 40 main jet and if im not mistaking its quite small for a 250.
Might be the right one for the bike but its not a big jet.
My Rm runs a 55 or 60 right now i think.