'20 RXB250L - NC250 Motor Swap [Build Thread]

[Ursa_Adv]

Project is still alive. Engine is in place. Cradle is welded. Waiting on the radiator mounts.


Current Progress pictures:

[JerryHawk250]

Nice work!

[Ursa_Adv]

Guess who's got their frame back?

I spent last night sanding, priming, and spraying the frame.



This morning, I put the engine back in, reattached the swing arm, and tested the engine position/alignment.




First order of business is to make spacers to keep the engine locked in place, the spacing we decided on allows removal of the engine without throwing your back out or having an extra limb, but as a result I have 8-10mm gaps that will require a spacers.

I am currently trying to decide if I want to go aluminum, delrin, or polyurethane. I know aluminum and delrin will lock the motor in but transfer vibration into the frame. Poly will absorb some vibration, but will flex and require replacement at some point. I've got a few local contacts I can discuss this with and should have them fabbed up by next weekend.

[franque]

I'd do aluminum or steel. Honda used some big old steel or pot metal spacers for the CB/CG engine family for the upper rear crankcase mount. My thought would probably lean towards mild steel, because if the engine is otherwise bolting up to the frame, on steel mounts, I would think you wouldn't run into any durability issues relative to the other mounts, but that's my (non) professional opinion.

[Ursa_Adv]

Measurements for the spacers:

Upper:

• Left

  OD 23.50 mm
  ID 9.70 mm
  Length 6.35 mm

• Right

  OD 23.50 mm
  ID 9.70 mm
  Length 11.40 mm

Lower:

• Left

  OD 23.50 mm
  ID 9.70 mm
  Length 17.75 mm

• Right

  OD 23.50 mm
  ID 9.70 mm
  Length 20.90 mm

Swing Arm:

• Right

  OD 25 mm
  ID 16.75 mm
  Length 6 mm

• Left

  N/A

[Ursa_Adv]

Rough cut spacers for the engine. As suggested, I used mild steel.



Test fitting these tonight and then starting on the swing arm spacers.

[buzz]

Hey looks great,will be interesting build. Keep up the good posting,just might do the same.Thanks

[Ursa_Adv]

Slow day today. Spacers have the engine locked in. The lower bolt needs to be a bit longer than I anticipated so I will need to make a run to Fastenal or Home Depot tomorrow to get a 3/8 bolt that is long enough.

With the engine in place, I can start re-assembling the rear end and test chain alignment. One thing I noticed is that the rear brake pedal will require some bends to accommodate the larger engine case, but nothing some time with the torch can't solve.

[Ursa_Adv]

I refitted the back wheel to test the chain alignment this afternoon.



The feeler gauge I need 0.08mm is on its way and should be here tomorrow. Once I've got the valves set, I'll start plumbing the radiator and hoses. Then it's on to the body panels and wiring.

[Ursa_Adv]

Re-mounted the rear end. Slowly starting to look more like a motorcycle again.



Valves are getting done today and then on to wiring. I've decided to hold off on the radiators as it is becoming apparent I will need to do some fabrication the exhaust. The CRF230F flange needs to be adjusted to fit the NC250 and the bends are now no longer aligned with the muffler.

[Ursa_Adv]

Valves are done. Word of advice for anyone who has one of these motors: the flywheel magnets are POWERFUL. They will throw the flywheel each time you get near TDC. If you can avoid using a rachet, do so.

Also you won't be able to see the cam while you are doing the valves, unless you take apart the head, so just watch the valve rockers to find the "right" TDC. All the valves should be closed.

Rear valves, as we all knew, were a little bit of a pain but not awful. No worse than other bikes I've worked on.

They are now all set to 0.08mm

[Ursa_Adv]

Lighting harness is mounted and complete. Gauges and all accessories work. I have one remaining wire to run for the tachometer, but I won't do that until I am closer to starting.

Engine harness is 90% ready to go. Currently working on mounting solutions for the CDI and Coil. Confirmed the harness is powered properly.

Radiators are at about 80%. I have all the hoses, just waiting on the couplers so I can finish.

Remaining items:

• Determine the correct length of clutch cable. Stock is too long. CSC RX3 is better, but still too long.
• Order a masterlink to reattach the chain
• Order a better carb. Most likely a nibbi 34mm. The carb it came with is "gritty"
• Figure out the exhaust

[Ursa_Adv]

Rads are now plumbed. I used the RX3's coolant loop. I got a new Masterlink for the chain, but I need to get the clip on still. Clutch cable has been figured out (I just needed to slow down while adjusting the tension). I made a bracket for the coil to mount above the cylinder (no pic yet) and I have decided to mount the CDI on the underside of my seat to save some space.


Some in progress pictures:

[JerryHawk250]

Looking good!

[Ursa_Adv]

I ordered a 34mm Nibbi PWK carb today. Oddly, 33 pilot jets are a lot harder to find than I thought (might just be where I am located).

For now I will be going with:

Pilot: 35
Main: 135
Needle: 2nd clip from the top

Once the carb is in, I can start working through draining/remounting the gas tank, remounting the seat, and finally getting to work on the exhaust.