DT125R FORUM

@SpookDog cover the gas tank cap with electical tape, I did that as soon as I got my bike, the keyhole can leak. It's not likely someone will try to steal your gas and it's inconvenient to lock and unlock the tank every time you need to fill it up which happens too often with this thing for some reason

@Calum won't work as well without a new pipe with different timing and BMEP and all

@HOTSHOT-III is DT80 wheel compatible with DT125R? It looks identical from the photos, maybe I could swap the entire wheel and be doje with it

@SpookDog 0 wheels in my area, you have to get lucky and find a totaled DT to scrap for used parts. Besides my rim and tyre is in good condition, I could gamble with spokes that could break or buy some spares just in case they break. Not sure if heat gun would be effective at loosening the threaded nipples

I had issues with the front wheel, at least 3 spokes loose to the point I could bend them with one finger, several more not as tight as they should, tried to tighten one spoke and it snapped at the thread. For some reason, wheel spokes for motorcycles are almost unobtainable in my area but I've managed to find a few replacements of the same length, they're a mismatch in width and I did a botch job, the wheel is not perfectly aligned and the bike jerks when riding. Hope my axle or bearings didn't get damaged so far, I'll probably try to find a complete 21 inch wheel on a junkyard and replace all of the spokes

Mine's the same, runs about 1/4 of the gauge at max with no issues. You should avoid revving too high while the needle is still in the thin green line but don't let it idle for too long, it's generally a bad idea for 2 strokes. Depending on your mixture, the engine is warmed up when you see significantly less smoke coming out of the exhaust, trying to slam high RPMs in lower gears when it's cold will create misfires and janky throttle response.

@Rallyfinnen I think you might be right about the timing angle, one of my repeated measurements came out as cca 185°. I could have used the wrong technique when measuring; Shined a flashlight through spark plug hole and basically measured between the times I can see any light coming through, used the magnet rotor to set marks. A solid, definite number would be useful; my machine has a 3MB00 cylinder and a standard thin gasket, it is not marked with "P" and has never been bored or ported.

As for the temperature, I didn't get reliable numbers with a multimeter probe at the flange with the exhaust off, so I opted for calculating it via BMEP which turned out right around 7.10
The temperature would be lower at the middle of the pipe and even lower at the convergent cone, the highest temp is supposed to be right at the exit of the cylinder and if I'm not wrong, that's the part that creates the pressure wave.

From what I've gathered and managed to measure regarding my 1991 DT125R with 3ME00 stock cylinder;

Port timing: 174°
Port diameter: 34.5mm
Port length: 45mm
Flange diameter: 32mm
The temperature should be around 450°C making the BMEP right around 7.1

I'm not too confident in those measurements so if anyone can confirm or point out any mistakes I'd appreciate it

Here I will list all useful resources I've found so far regarding the design and fabrication of a tuned pipe for a DT125R (and it applies for other 2 stroke motorcycles as well)
I am not an experienced fabricator nor do I have a lot of experience with 2T tuning so anybody with more knowledge should feel free to comment and correct me if needed.

Free calculator (2T exhaust calc) http://birdcagesoft.com

The guy who wrote this software also made a series of videos on how to acquire needed measurements and use his program https://youtube.com/playlist?list=PLIxC_wjJqk0rYVOG6EpLU2EaAVWZ2saWA&feature=shared

Cone layout software (it's a free demo but you can use it even after the trial): https://www.conelayout.com/

Simple spreadsheet, likely not very accurate: https://docs.google.com/spreadsheets/d/1nksxtd8M9KeJJ8l10ZHtq8yk8W6gZQZhXmFXj4fGZC4/htmlview

Video of a guy using the above mentioned to fabricate an expansion chamber for a DT360 using the pie cut method: https://youtu.be/TgoHIF2boPE?feature=shared

Jambros' calculator topic on another forum, this spreadsheet is much more detailed and useful to someone who knows how to use it: https://www.rzrd500.com/phpBB3/viewtopic.php?t=16273

Another video where someone fabricates a resonator for an older dirt bike using cone layout and a wire to determine the path which the curves go along (it's in Finnish and there are multiple videos on this topic from his channel):
https://youtu.be/Q8k5hFfUA_o?feature=shared

Fabrication of a scooter exhaust using the hydroforming method: https://youtu.be/DVeoDNHGEc8?feature=shared

Older post about hydroforming of 3D curved exhausts https://web.archive.org/web/20200221121820/http://www.eurospares.com/frame8.htm

Making shapes for sheet metal cutting is most accurate using CAD software and exporting them in DXF format, I know how to use Solidworks in case anybody needs help.

Generally, sheet metal thickness of cca. 1mm is adequate and regular mild steel is cheaper and easier to work with as opposed to stainless, rust can be prevented with temperature resistant paint and burning in the inside with oil.

Tig or oxy fuel welding is the preferred method since we can make autogenous welds with minimal heat input; Dedicated tig welding machines are expensive but a cheap inverter welder can be easily converted to a scratch start tig; https://youtu.be/aGDHX5JdOos?feature=shared

@SpookDog It's important to bend edges and weld around them autogenously because thicker welds can lead to kinks, this guy did that for a scooter pipe, he just screwed up the measurements https://youtu.be/DVeoDNHGEc8?feature=shared
Yeah, and it's important to make sure there's no air in the pipe so it doesn't turn into a bomb

@SpookDog I'll inspect the power valve from the exhaust port and try to post pictures later, just to be sure
The bike feels fast and has good acceleration so there's a possibility that it's just my speedometer that's screwed up LoL

@SpookDog It's not a big deal to rent a slip roll, it's the fact that there's a lot of opportunity to screw everything up with that many welds. Think I saw one guy make a hydroformed pipe out of 2 parts then he cut the forms in a couple of pieces to rotate in position. Could be easier with less welds

@SpookDog I can tig weld but I use scratch start since I don't have a real tig welder, this one works fine with steel, it's just harder without fancy controls. I'm not really good with stainless and it's more expensive, my friend works in a nearby factory where they only laser cut mild steel.
Cones aren't a bad idea, there's a software called cone layout, it might be worth trying

@SpookDog no, my revs at top speed are around 8500, i haven't messed with the power valve much but the marks are aligned as it should be and servo is working
Sprockets are 16/57

Has anyone made their own expansion chamber? I'm planning to make one myself, using free software and sheet metal hydroforming but this is not gonna be an easy job with all of the diffuser curves

Mine is also strange, it's marked as 3ME and DT125E, but otherwise it's identical to DT125R. I did some digging and apparently my bike is Swiss version and it's made in 1989. So far I haven't found any restrictions on the bike (reed switch doesn't even exist) and it easily revs to 10k, however the top speed doesn't go over 85-90 kph with stock gearing of 16/57

I haven't torn apart the engine yet so I can't compare it to other models, but it is marked as 3MB-00
Then again, I've noticed the expansion chamber is leaking on multiple spots and is generally in a bad condition, someone installed a 4 stroke silencer so the faulty exhaust system could be sapping power