Does anyone run cooling system without the thermostat if only running on warm days? MC18 radiator does not have as much cooling area as MC21. Or is thermostat recommended? Your thoughts?
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MC21R9N track
MC18R2J track
CBR954
CR250

The Honda RS250 runs without a thermostat. You may find you need to manage the airflow as it can tend to get too cool.

Removing the thermostat definitely removes a potential point of failure, so it can't be all that bad.

The other option is to get a lower temp thermostat.

As I have recently found out, removing the thermostat can actually reduce the cooling efficiency in the sytem as removal can affect the flow and pressure within the system.

Best thing is to take it out and try it. If it doesn't work, or makes no difference put it back in.

I've been running with a thermostat locked in the open position on my 21. Even in the summer (25+ºC), the engine hardly warms up higher than 60ºC when driving on the road.

Once I installed a new one, I can confirm that the engine operation is nicer. I guess we all have noticed at some time how engines behave better when well warmed as compared to 'cold'.

On top of this, there are quite a few comments on the forum wrt exhaust bridge cracking on cold engines...

You mention that you'd only run on warm days but, can you guarantee yourself that the bike will never be used on a cold day?

So I'd definitely use a thermostat. It's not expensive nor difficult to install.

Hope this helps.
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If the problem you are having is that the bike is running too hot, (and I'm not sure if that is your problem), then I would suggest that removing the thermostat is not going to achieve much, unless the thermostat is faulty.
The reason I say this is because if the engine is hot, you can be sure that the thermostat will be open anyway.

If you really need to improve the cooling abilities of the cooling system then maybe try installing a fan on the back of the radiator?
I have seen a bike with a 120mm, 12 volt computer fan attached to the back of the radiator. I have no idea how effective it was.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

What bike was the fan installed on? Had the owner tuned the motor and found it ran hot; necessitating the addition of the fan?

I've never seen a fan needed on an NSR, and I've ridden and worked on them in exceptionally high heat and humidity, in several countires. That goes for MC18s too, not just 21/28s.

Optimal temperature for a 2-stroke is around 62~65 degrees. I've seen NSRs sat stationary in traffic reach around 100 degrees, but will drop to 80 degrees almost instantly the moment they start moving and there is airflow across the radiator. Normal NSR250 operating temperature should be around 70~75 degrees.

Running without a thermostat wouldn't be my recommendation unless racing. As Dave says, correct warm-up is something most people don't even do when a thermostat's fitted!
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Andy.
NSR-WORLD.COM

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It was a few years back now so my memory is a bit hazy, and the bike that I saw the computer fan on was an RZ350.
It was just parked in Auckland city on the side of the road.
I suspect (but can't say for sure) that the owner had made performance mods to the bike.
But maybe the owner did it just to keep things a bit cooler while "sitting at the lights" on hot summer days?

I think these days there are better options available, as a quick Google soon revealed.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

so is it ok if temps reach around 100deg momentarily??.

i find my mc18 runs at around 60-65 on the open road but it shoots up when driving around a town.

so i never drive it around a town makes me too nervous.

my thermostat is functioning fine, its opening and closing i have checked it.

For what it's worth (not much) my 1996 GSXR750 has to get up to 106ºC before the electric fans come on.
The fans turn off again when it gets "down" to 100ºC.
I would have thought "they" would have set a lower fan activation temperature.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

The temperature a fan will set on will be very much dependent on how pressurized the system is. I guess our NSRs cooling system works at atmospheric pressure (see the expansion tank plug), but modern engines can keep over 2 bars in the cooling system allowing a higher fluid temp before it boils.
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JHA did a cooling fan for MC21 and MC28. You can still pick up the odd one on internet auction sites.






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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...