Motorsport Studio - The Art of Racing

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Saturday, January 1, 2011

Goodbye CR500 powered NSR250

Well, it was time to face reality and take down the CR500 powered NSR250.  It was a 1991 MC21 Honda NSR250 modified by the previous owner to take the 1990 CR500 engine, needing only some minor refreshing in order to be race worthy again.  With my fresh race license I thought I would use this as a first racebike because a 'normal' (read: 'sane') bike wouldn't be interesting.  Yeah, not so much.  Here it is as bought from the previous owner near Kokomo, IN:

The engine was mounted using solid stock that was welded to the inside frame rails and the stock front mount downtubes were also modified the same way.  The rear crossbeam was modified for the rear upper mount, the front crossbeam was through bolted for the cylinder head stay and there was a small lower cradle to hold the lower engine mount.  All solid mount, the fabrication was workmanlike, not the prettiest but it worked more or less.  The OEM pipe was cut apart, reclocked and rewelded to get the pipe under the engine.

 The only things I did when I bought it were to replace the tires with Michelin H2 DOT's on the stock rims, size 110 front and 150 rear, and the normal fluid, plug, chain, etc on the motor.  First time at the track was at Putnam Park for a trackday, what I remember:

- If not for the vibration, it was an awesome package.  Very torquey, could be short shifted and preferred to be in the meaty part of the powerband.  Since power dropped off quick and hard up top with little over-rev, it really wanted to be run around the torque peak and not buzzing at high rpm.  In a lot of ways it felt alot in character like a TL1000 big twin, a nice torquey motor.

- The chassis felt great, never touched a clicker although the gearing was off and the shifter was very clunky and difficult to engage cleanly, so I mostly kept it in 4th gear and relied on the engine torque.  Compared to the TL it was like riding a bicycle, turned in very nicely, with the stock rearsets it ground the pegs easily and liked being on it's side.

- VIBRATION!!!  I had read and heard how bad the vibration might be, but it was beyond what I could tolerate.  My fingers and hands would be numb from vibration just a few laps in.  It was a stock engine so  the balance factor could have been adjusted to move the vibration to a different region (lower in the rpm range like the Tulda CR500 powered bike), but I never got far enough along to play with this.

- Pain in the A$$ to hot start!  Bike had a modified kickstart lever which reached through the bodywork, but it was oriented strange and the bike was a pain to start once it was hot.  I fouled the plug a couple times that day due to hot start issues, and when it was running my legs were so tired from kick starting that I was worn out before I got a few laps in.  Push starting required lots of speed, running and jumping on would just slide the rear tire, this big single had alot of compression.  This would never have worked as a racebike because I would either miss grid due to not starting, or I would overheat sitting in grid, and if it stalled, it would take a crew of fast running pushers to get me started again.

It wasn't a complete disaster even with those issues, so after this first outing I set about to correct some of the things that needed immediate attention:

-  Hot start issue - Bought powered starting rollers, basically a truck starter chain driving two rollers.  Starting was a little easier with the rollers, but they were small diameter so the bike easily rolled off the rollers.  I meant to make a new roller starter with the larger rollers at an angle, but never got around to it.

- Narrow powerband - I contacted a few places to look at a new pipe design which would extend the powerband, but quickly found that the main body diameter of the pipe would need to be on the order of 6" outer diameter, and there was no way I could package this under the bike, it would have scraped the ground with the slightest suspension travel.

-  Shifting and peg scraping - the linkage was quite a bit off axis between the lever and the shaft, so I ordered Tyga rearsets and made a new linkage for the shifter rod.

After all this I never got around to running it again, and lost interest, primarily because of the fabrication of the engine install itself.  Since all the frame work and engine mounts needed to be redone, it seemed the best course of action would be to take it down to the bare frame, and take it all back out when time allows for a proper rework.

So, here is the bike before it was stripped, and after.  I have a clean fresh MC21 frame and swingarm that will be used as the basis for the next version of this bike, it won't be a CR500 engine but it won't be a stock 250cc NSR or RS motor either.  Something interesting this way comes.  Eventually...


1 comment:

  1. Interesting build. I would really like to know more about what you did after?

    Please send me an email.