I have been visiting this site for over a year and really welcome the new forum, congrats
My question is in regards to compresion testing the engine, I am looking at either picking up a 84 ns250 or 88 nsr250 as a second bike and would like to compresion test each one, would someone tell me what should be the range for each, i know both motors are a little diff but should the compresion be relatively the same and what should it be. thanks

I had a couple of Aprilla RS250's on the dyno a few days and ending up doing a simple compression test..

both read between 45-55psi depending how fast i kicked and how knackered i was feeling. The throttle was fully open when i was kicking too. Both bikes made about 57bhp and were stock so the motors must of been ok.

I guess a NSR engine should be something like the same compression but dont hold me too it.

Hope thats alittle help

Stephen

Miles out! My MC21 made about 120psi on a freshly rebuilt barrel / piston.
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

Right, a few months back I tested my MC21 and got 120 on the front and 115 on the rear...or was it the other way 'round? I remember the numbers though!
-Jim
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-Jim
'93 MC21

It makes a big difference if you have the throttle open or closed. Closed means you're trying to suck air through a tiny gap, and are unlikely to have sufficient strength to get a decent figure before you collapse gasping!
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MC21SP Plaything
BMW F800GS Bumblebee
Triumph 9551 Daytona Big boys toy
FJ1100 Sporting relic
GTS1000 oddball

lol well these both made 55 or so with the throttle wide open... i guess its down to port timming. Both ran perfect tho and made good power...

It was probably your compression tester that was the culprit.

The cheap ones use a heavy spring in the guage checkvalve that requires about 40psi to overcome the load of the spring and open the port to the guage. That would make your reading 40psi lower. Also, the Aprilia and Suzuki (same engine) run a lower compression ratio that would make your readings lower. Most I have tested are around 90-100 psi.

My NSR pulls about 115-120psi with the overbore. It was about 110 on the stock cylinder setup.
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Paul Herr
2000 Honda RS250
1994 NSR250SE, far from stock.