'20 RXB250L - NC250 Motor Swap [Build Thread]

[Ursa_Adv]

Nibbi carb is in. I lucked out and it came with a 32 pilot installed. Small victories. Clutch cable and throttle cable have been adjusted with proper slack.




I need to pick up a 55mm hose clamp for the intake (the largest I have is a 52mm). With that everything is locked in place and I can start putting more of the bike back together.

I also managed to find a better spot to mount the coil as well.





Things to complete:

• Fill radiator
• Fill engine with oil
• Put new master link on chain
• Set chain tension
• Grease front axle
• Torque front axle
• Drain and refill gas tank
• Remount front brake
• Refill and bleed front brake
• Exhaust

*NOTE: please don't judge me too harshly for those terribly cut coolant lines. I didn't swap out razor blades until it was far too late.

[JerryHawk250]

You're getting close. Man, who butchered the hoses. Just kidding. Looks good to me.

[Ursa_Adv]

Test fitting (minus the exhuast):



Not gonna lie... She looks pretty damn good to me.

[Ursa_Adv]

Mounted up the front brakes this morning. Two things jumped out immediately.

1) The brake line is not long enough to loop over the cluster and down through the passage in the headlight assembly. I am going to look at picking up a CRF450X front brake line as I think what is currently installed is from a CRF250R. The 250R line has always been a bit of an issue, in older pictures you can see that it was running almost horizontal across the handlebars vs. looping up and over the way it should.

2) The speed sensor has two issues of its own. 1) the wire is wayyyyyyyy to short. 5 minutes with some spare wire I had laying around and I made a 3 pin jumper wire to go from the cluster to the sensor wires. 2) The sensor is a magnet type. It came with two little button magnets that I cannot mount anywhere on the disc without it resulting in clearance issue with the caliper. So I've decided to make a longer sensor bracket and swap to some magnet bolts for the brake disc.

In other news the exhaust ('12 CRF250R) header is on its way. Once it arrives I will start work on the exhaust. Engine is filled with oil, judging by how little it took, there is a some of the factory oil still in there. I will do a full change after the heat cycle tests. Rads are filled with distilled water for now. Same deal as the oil. Full change after heat cycling.

[Ursa_Adv]

Waiting on parts to arrive is like watching paint dry.

Brake lines get here tomorrow. Exhaust pipe pieces arrive today.

Hopefully I can get the header flange mocked up tonight so I can take it to be welded tomorrow and put this to bed. Weather has been pristine the last couple of days and I am chomping at the bit to fire this engine up.

[Ursa_Adv]

Iteration one of the exhaust is complete.






It bolts up, but requires some tweaking once its bolted on to make sure it doesn't sit against the frame and the water pump like it does in these images. I will revisit the exhaust later, but for the moment it is bolted on and the muffler mounts. It is good enough...for now.

I have run into a different issue, on the wiring side. I believe there is an issue with the stock Orion start kill switch (4 pin). It uses a really weird rocker inside to do the grounding out for the kill switch.

I tested the starter by bridging the relay wire (red) to battery positive and the engine cranks no problem. I have a feeling I am over thinking this and I need a sanity check from some of our wiring gurus out there. This is the plug setup:




Unless I am crazy this is how it should work right (see below)? Or have I just had a complete brain fart?



The problem is that the black wire on the starter switch plug isn't hot. It isn't passing any power. I am going to open up the engine harness and see where it goes tomorrow, but in the mean time any assistance is greatly appreciated.

[Ursa_Adv]

Based on this diagram (same CDI used on the NC250) the black wire should be 12v.



I made a mistake when I wired the ignition key switch. I, in my infinite brilliance, thought it would be a hot and a ground going into the engine harness. Nope. Red comes in hot goes into key switch comes out hot on black to power the CDI and let the starter switch do its job. So I'll fix that tomorrow.

[Ursa_Adv]

That was the problem. Not only did the black wire need to be passing power, I also had to replace a wire inside the engine harness and then solder it all back together. They used extremely flimsy gauge wires to pass power in this harness. I was loosing 12v every time the harness even slightly squished or bent. The ground wires are stout little bastards, but for some reason unknown to me the power wires were so small.

After fixing that issue, I bolted thing back up and hit the starter. It took some CRANNNNNNNKKKSSSS, but after about 20 minutes of adjusting, checking, coaxing, and swearing: SHE STARTED UP. Engine sounds good. No excessive ticks (so I managed to set the valves correctly). Idle was set extremely rich. Made some adjustments. Let it run for five minutes and now it's cooling down.

Lessons learned today:
1) A multimeter is an amazing tool. Learn to use one. It will save your sanity.
2) Iteration 1 of the exhaust design isn't going to cut it. The clearances are just too damn close right now. I will be fixing that before I take any test rides. The good news is I now have all the angles can take some pictures and get a better design in there.
3) I hooked up my tach wrong. -_- I was thinking like old school and just ran a wire to the plug wire, wrapped it, and called it quits. I need to run a wire to the hot side of the coil to pick up the pulses.
4) I need a battery charger.

[Ursa_Adv]

I just realized I started this project in October of last year (23rd was when I started tear down of the old motor). The days went by slow, but the weeks have gone by fast.

I'll post some videos later tonight of the engine running.

[franque]

That's really strange that the CDI passes the +12V to the ignition switch, are you sure that the CDI isn't receiving switched power from the ignition? Normally that's how they wire stuff... In any case, congratulations, I'm glad this came together!

[Ursa_Adv]

Originally Posted by franque

That's really strange that the CDI passes the +12V to the ignition switch, are you sure that the CDI isn't receiving switched power from the ignition? Normally that's how they wire stuff... In any case, congratulations, I'm glad this came together!

So yeah, my explanation was typed late at night and on my last trailing braincell. Probably not super clear. The CDI isn't passing power up to the switch. Just part of the whole circuit.

Battery > Engine Harness Positive 12v (red wire) > Ignition Switch > Black Wire 12v

The Black Wire 12v then splits into 1) CDI power and 2) starter button power.

So the CDI is getting switched power from the ignition key switch.

[Ursa_Adv]

Videos are going to have to wait. Pretty sure my in line fuel filter is faulty, I've got gas between the petcock and the filter. Bubbles in the filter, and massive dry spot in the fuel line.

I did the old open the petcock and pop the line on the carb (pouring a bit of gas on a tastefully placed shop towel) and plugging the carb fuel line back in. This got the air out of the line.

I am now going to wait a few hours and check on it to see if I air bubbles again.

[kevperro]

Originally Posted by Ursa_Adv

I just realized I started this project in October of last year (23rd was when I started tear down of the old motor). The days went by slow, but the weeks have gone by fast.

I'll post some videos later tonight of the engine running.

You are a warrior..... patience of Job for sure. I've had my bike for a week and I'm waiting on a few parts and I'm itching to ride!

[Ursa_Adv]

Fuel issue update: FIXED

I removed the inline Tusk filter (hairline crack). Then had to open up the bottom port on the carb to get rid of the massive air bubble that took residence near the floats. We're back on track. The bike fires up with the slightest bit of throttle when cold and no throttle when warmed up now.

Video Proof that I am not making all this up and the bike does run:




Things to do:

- set chain tension
- grease front axle
- Torque front axle
- refill and bleed front brake
- Resolve this issue with my front tire going walkabout without permisisions:

[Ursa_Adv]

Slow day. Learned that this bike NEEDS choke on a cold morning start. I was spoiled on my little CG, as I never had to use it.

Tidied up some wiring. Adjusted my coil mount ( now using two 3D printed pieces). Bled the front brakes. Thrills a minute on this build, folks.