I am preparing a friends NSR250 MC28 for track work. I have not worked on 2 strokes for some time and whilst I find the mechanicals fairly straightforward, this carburettor systems is new to me. The bike has a recent TYGA 300 top end kit with expansion pipes. I am examining the carbs jetting following the previous mechanics assembly where he fitted richer main jets for the running in period. The owner said it was sluggish revving over 5,000RPM. It has MJ 175 and SJ 42 installed. The oil injection is disabled and sundry hoses blocked or looped back, which I assume at this point are correct.

One thing I am concerned about which I need advice on is this - I noticed a loose bolt on the flange of the reed valve manifold, thinking the mechanic has simply forgotten that. Its on the right alongside the throttle sensor plug. But then I saw on the forward part of the reed manifold, a hose stub at the rear of the left (rear cylinder) which has NO hose or block off at all. Given the recent TYGA kit build, I am suspecting a hose may have been left off by accident.

Could this be a problem? What does it do and how should I treat it?

Many thanks
Mike

A picture of the situation would help, atleast for me to try and get an idea of what you are talking about!

Follow to the end of the red screwdriver shaft, you can just see the hose stub


http://i884.photobucket.com/albums/ac47/ml850mk2/NSRcarbhose.jpg

Sorry maybe this image paste will work, otherwise you go to Photobucket and whole bunch of nice stuff with pushrod motors.

that's the vent from the transmission.
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Welcome to NSR-WORLD, "ml".

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Andy.
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I do wonder if a transmission breather is NEEDED on the NSR250.
Can anyone explain why it is needed?
Is there any way (other than atmospheric pressure and altitude combined with running/storage temperatures) that can cause significant pressure or vacuum?

Of course, in a four stroke (especially big singles) where the crankcase and transmission share the same volume, there is a lot of pumping going on due to the piston movement, and blow-by past the rings can pressurise the crankcase, but this is not the case in the NSR250.

Do atmospheric/operational temperature/pressure differences actually necessitate a breather tube?

Is it just the temperature changes observed between running the bike in the Saharan summer and then being stored in Northern Finland over winter that necessitate the use of a transmission breather?
OK, that is an extreme example, but you get the idea, right?

Is the possibility of developing a leak between the crankcase volume and the transmission volume enough to warrant a transmission breather on the NSR250?

Just plain curious. That's all.

I assume I am missing something here. But, someone can hopefully explain what.
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Well, I'll have a go at answering my own question:
Maybe it is best to have a breather so as to reduce the possibility of oil leaks due to pressure on any of the affected seals?
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[color=#808080][size=9]Yes,.. I too know how to waste Time and Money,...

Don't forget how hot the transmission oil will get during use, which will cause the oil to expand.

Heya

I asked the same question about 6 months ago when i brought my second NSR, i mentioned on my thread about the bike that it to had no hose off the gearbox vent, then theorised at that at some point if the crankcase became pressurised due to a sealure failing between the crankcase and the gearbox the oil/air/fuel mix would come out the vent, eventually covering the rear tire, but i'm not sure if this is physically possible. I do know on the motor i have that gearbox oil seems to seems to be escaping from everywhere else on the gearbox except the vent though. So tired of the permanently expanding green pool under it. The gold standard has go to be what honda did when they left the factory.

Heya

Seeing as i carry an NSR motor where ever i go, i decided to see what has to happen for the gearbox to get pressurized from the crankcase. There is a crankcase join between them, which is highly unlikely to fail. In this pic you can see the crankcase breather occupies the space between the reed valve mounts for cylinders 1 and 2, an example of economy of design. This engine has a hose fitted to the gearbox vent, but i don't know where previous owner had routed it to.



In this pic you can see on the left hand side of the engine, there appears to be no common boundaries between the crackcase and the gearbox.



On the right hand side however the crankcase and the gearbox share a boundary, there must be a seal around the crank, as the crank protrudes into the gearbox, so it appears the only way the crankcase could pressurize the gearbox is if this seal fails.



As remote as this sounds it is in my humble opinion that it should be vented to somewhere, if it was a race bike it would be vented to a transparent catch bottle visible to the rider.

Cheers