lemur

Then call it a rotor. We know it has one of those because yours has a crack in it and if you search for 'KTM 50SX rotor' you come up with something that looks very similar to your broken part.

Unlikely but to know for sure you would need another same model bike that is working.

I'd pull it off to determine exactly why it split, see if the crack can be reduced back to zero and hit all the cracks with superglue to reinstall and see if it splits open again.

Owned 5 4RT models from 2005 to 2017, 250, 260 and 300RR, owned 3 at the same time and swapped parts between each to trouble-shoot. Model makes almost no difference because the 4RT didn't change much to the point where most parts from one will fit right onto the next and work just dandy. I have the header pipe with the Cat converter and the O2 sensor from my 2014 260 4RT sitting in my spare parts box. That was my only 4RT that came in the crate straight from Spain unopened.

You're right, sometimes it's better to direct the question to the source of an answer and let the reader figure it out from there 👍 https://www.nature.com/articles/s41598-023-43695-6

When I installed the race kit on mine and removed all the street stuff the only thing I had to alter was the idle screw after letting it come up to operating temperature. It came with the catalytic header plus the straight header plus heat shields for both, ECU seemed to know how to deal with either way or with restrictors in or removed. PGM-Fi works brilliantly, only way I could make one run rich was if the air filter was plugged.

Did your bike come in a crate straight from the factory or was it unpacked by a reseller? On my 2014 4RT the race kit parts and the supplementary instructions came included with the homologated bike in the crate. Lots of extra parts for the dealers to scoop prior to resale. The bike was initially fitted with all of the restrictors required to make it street legal for a beginner limited license and all of the race parts were in boxes. An expensive ECU was not included at that time but likely the purchased one can be user programmed beyond the 2 position switch presets, & not something the average rider will ever need.

Manual for the 2020 ST250 shows a drain hole plus an inspection or fill plug they tell you to remove to drain the oil. Odds are the plug is at the same height an oil level window would be if it had one. First check would be to measure the quality of oil you drain out and go with roughly the same volume and then see where that fills to relative to the inspection/fill plug hole. Search for "Oil comparative viscosity chart" and you will find a chart that details the relationship between ISO, AGMA, SAE auto and SAE Gear oil viscosity. Personally I use ISO 46 hydraulic oil in all my 2T and 4T wet clutch/transmissions and never have a problem, same oil they recommend for all season excavator and farm tractor applications which use virtually the same type of gears, seals, shift forks and friction plates. ISO 46 viscosity equates to AGMA 1, SAE 20W auto oil, or SAE 75W gear oil. ISO 46 can be purchased from any heavy equipment store in quantity and costs about half the price or less, so you can change it out twice as often. Too much oil in the case can cause clutch slippage problems and robs power, the oil level is relative to the clutch basket, the clutch should never be completely submerged or the plates will not compress because of hydraulic pressure on the inside of the clutch plate friction pack. Your transmission and clutch do not require a multigrade engine oil which is formulated to accommodate cold startups and a much hotter internal operating temperatures.

It's a permanent magnet is it not? There are a lot of iron filings stuck to the magnet in the photo, that might cause shorts and indicates a wear problem, I would clean it all up, test the coils and try again.

The 4RT is a superior all around trail bike, they have a broad powerband, start easy, cheaper on fuel, tough to break and hold resale value better than most. Easier to ride at non-competitive level trail riding, if you can still ride a motorcycle I would think 4RT is the one. I ride TRS now but would go back to Montesa 4RT in a heartbeat if sales and parts dealerships still existed where I reside, I rode 4RT' for 16 years. Montesa/Honda fuel injection makes the 4RT far superior to other 4-stroke trials bike models imo it would be the only 4-stroke trials bike to consider. As for servicing the 4RT, you will need to be able to adjust valve clearances accurately after 4 or 5 years of riding and it becomes harder to start, otherwise they are not much different compared to a 2-stroke to service. Replacement spark plugs are way more expensive but you will rarely need to replace one. Air filter is way better compared to some and the plastics are expensive but durable. 4RT works great in winter as long as you don't let the oil get thick in sub zero temps, sub freezing temps makes it near impossible to start unless you build a fire under it. 4RT Header pipe runs very hot and needs a shield upgrade or you will melt your riding pants on it regularly.

"Stator looks OK, noting obvious." Are you testing the stator coils with test equipment or by eye 🤔 looking at coils tells you nothing unless they are burnt to a crisp, you can meter test coils for resistance or you can oscilloscope test the AC output and confirm it is fully functional. Sparkplug cap and wire has been changed? They don't last forever. "Plug is gapped ok, looks ok, not fouled. " Sometimes the insulation is cracked and the defective plugs will spark way up inside the plug body and not at the electrode gap. Try another new plug. If you can't narrow it down to being either a carburetor or ignition issue there is not much anyone here can help you with. Do you have access to another GasGas TXT you can swap parts with, that is what a dealer would do to eliminate the carburetor as being your problem. Parts swap is also the only way you can reasonably test the ignition module.

Holy that looks expensive, I hope I can own one 😎

You act like a thermostat switch is a totally non-required 20$ part that every manufacturer puts on their machines without reason. What's the big deal with just fixing it! If he leaves the fan run 100% and fries a 90$ rectifier he can thank you for the recommendation to ignore it.

Wait a minute, so if you never had a thermostat switch fail either 🤔 the only thing self induced is if he doesn't fix it after he bought a broken bike.

That's why you still have a thermostat switch on there, the bypass switch is wired in parallel to the thermostat switch not in series. op Diodes can't take heat, it's the rectifier/regulator that will fry first, followed by the coils, followed by the fan motor that makes its own cooling

Must be why I never had to replace a fan, I clean most parts like that with a toothbrush and some spray cleaner 😎 op. When you run a generator and plug in a toaster you can hear the engine work way harder to make more electricity, bike alternator is identical, making electric power takes energy, making more power all the time needs more energy and making energy produces heat which is damaging to electrics. It's not fuel injected so the Fan motor is the highest power draw on your bike, it would be normal for it to draw down on the idle speed some when the fan kicks in, I added a toggle on my 315 Cota to deal with that exact issue, idle dropped noticeably when the fan kicked on.

Can't understand why anyone would permanently bypass a thermostat switch with anything other then a switch 🤔 they saved a whole 4 bucks on a switch and I imagine those same dealers have no problem selling additional replacement stator coils, rectifier/regulators and replacement fan motors as a result of that downgrade.

Can only think of two reasons to do that, they didn't want the engine idle speed to change when the fan kicks on or the thermostat went faulty. The better way to fix it is to install a working thermostat and put a bypass toggle switch on it to manually control the fan. I guess the question is does your engine speed change if the fan is connected and disconnected, if idle speed does change significantly that relates to the power load increase on the alternator.

Carburetor can't work great if the bottom end seals are not great. I'd be inspecting the bottom end seals for signs of leakage on any 2-stroke engine that has constantly changing carburetor type issues. ... when it runs does it fire with a nice clear compression pop-pop-pop or does it have a ring-ying-ying engine sound ?

Once you rule out everything else, it will end up being the need to adjust the valve clearances very carefully and accurately. Same thing happened with four out of four 4RT's that I rode from 2005 to 2020 they just didn't want to start one day and a valve clearance adjustment fixed every one of them. Owned two of them at a time which made trouble-shooting simple.

Better watch out it makes me feel invincible 😆

👍 Vest arrived, first impressions are it comes very lightly padded and some spots are not protected at all but it is Way more comfortable to wear compared to the flat board back protector I wore before. I like it 👍 Might add a tensor bandage for the 2-day trial this weekend but I think I'm good to go, if it gets too serious I can just drop down a line and ride as clubman.

Did that about 50 years ago but it was to prevent additional damage to the engine covers in the event of another fall.

They all stall if the idle is set too low. Personally I like to buy them in pairs, makes trouble-shooting service much easier and you are never down because something busted and needs to be ordered before you can ride.

It says 'Ride to the next level' right on the TRS saddle 👍 must be so 😄 Personally I find Beta 2T and TRS both exhibit more lively suspension compared to Montesa which rides very planted and the 4T engines don't like to be blipped, they respond to smooth throttle and clutch control is where you get the leap. Kickstart on the wrong side makes Beta 2T a problem for me, so TRS is what I ride now. The TRS 300 is a blast to ride spirited, even has electric start, the Montesa 4RT 300 is the absolute best bike for winter riding on studded tires because of the engines power band and that planted suspension. Beta castings and plastics imo have never been up to par with the Honda or TRS materials. add: I just read an add saying Beta is doing a major rear suspension change on the latest model, but imo that could complicate parts replacement.