konrad

I agree with lemur about the 4RT. The problem is that if you find a "cheap" one, it's going to require maintenance. At this point, I suggest trying to borrow a bike to see if you even enjoy the type of riding you are anticipating. P.S. I meant to respond about the new Michelins on the cheap 125. Any amount of road riding will rapidly degrade them. Tire pressure is another issue. It will need to be significantly higher for road riding than trials.

As a wise man once said, "The cheapest restoration you can do is with a camera." You won't know if it's truly a bargain until you ride it.

125 will only make half the torque of a 250. A 125 will be geared shorter than a 250, but maybe only by ~15%. P.S. My compliments on your English. Am assuming it's not your first language.

You have a lot of conflicting requirements. A 125 would be fine for trials. In fact, it would teach you a lot about how to properly use your body to get the most out of the its limited power. I would not want to try to chase down an enduro bike with a 125. But if you pick the gnarliest terrain, you would have the advantage. If your skills as a mechanic are very good, you may be able to find a bargain on an older bike. Each bike must be judged by a pre-purchase evaluation. But in general, you get what you pay for. There will be more wrong with an older bike that will require repair.

These spring rates are suggested by EM for the FACTOR-e, but they would be appropriate rates for the Race as well: https://www.electricmotiontech.com/home/em-factor-e/suspension Trials bikes require considerably less rebound damping than other forms of motorsports. Some information about the Tech forks here: https://www.electricmotiontech.com/home/em-epure-race/mechanicals/tech-39mm-forks

Hope it works well for you on Sunday! I am curious about how you measure timing in mm bTDC on a 2-valve 4-stroke. Are you removing the head? If you have a timing light, I'd be curious what the PVL-Electrex hybrid is actually doing.

It's possibly a faulty coolant temperature sensor. Your service manual should give a resistance value for the sensor at a specific temperature. A problem with the sensor can also make the fan operate erratically. Some information about the OSSA version here: https://www.ossa-efi.com/home/electrics/sensors#h.y0jvm4uz6jfk

That is a question you should refer to PVL and/or Electrex. It's possible the combination of two different systems will provide a useful synergy -- or far from optimal results. I don't know anything about the Tiger Cub, but a search revealed its stroke is 58.5mm. To calculate ignition advance in degrees from mm bTDC, you also need to know the rod length. Guessing the rod length to be twice the stroke usually gives reasonable results. 6.5mm bTDC equates to about 35 degrees of advance. 2mm bTDC equates to about 19 degrees of advance. Ultimately, the best ignition timing is the setting that yields the desired performance without signs of overheating and/or detonation. Experimentation, riding feedback, and looking at the spark plug will guide you. Very generally, advancing the timing gives a boost in low-RPM torque at the sacrifice of running well at high RPMs. P.S. Advancing the static timing too far can make the bike difficult to start as it promotes "kicking back".

All vehicles manufactured today use metric fasteners. Even US automakers started transitioning to metric in the 1970s.

What are you using to monitor that signal? I have a Hantek HT-25 "capacitive" probe, but never been too impressed with it. Supposedly 10,000:1, but I have doubts about the accuracy of that number.

Just stumbled across this: https://int.nonstopbikes.com/ossa-tr12-b1c8/ It a re-badged Mecatecno electric child's trials bike. The also make a smaller, cheaper TR8.

The motor came from China, did it not? Look on AliExpress.com for similar brushes.

Measuring the resistance of a brushed DC motor is not very informative. The resistance will vary based on rotor position. But I have a new Comex fan in my spares. Powered from exactly 12.0 VDC, it draws 1.8A. At 13.8V it is more like 2.2A. This is with the fan sitting on a bench. Impeding the airflow with something (like a radiator) will change the current draw somewhat.

The problem with these "simplified" wiring diagrams is that they don't really show the electronics involved. From my experience, the heavy-gauge yellow wire is for a stator coil that powers the fan. One end of that stator coil is connected to the bike's chassis ground. The coil's resistance will be quite low (maybe 1 ohm) and the wiring itself will contribute to the reading. Your symptoms / measurements suggest a poor connection to ground at the stator end.

Wow, I can see why you wanted to show that off! Sow's ear, meet silk purse. 🙂