Nsr Stoopid newbie questions

[swampy]

Hi folks, I bought an NSr 125 '99 a couple of months ago that needed some work doing on it, and have a few questions if anyone will be so kind as throw some advice my way .

1) I don't have the aero tube things, I am guessing they will cost an arm and a leg to replace. What do they do and are they really necessary ?

2) What does the power valve do, and how does it work in a nutshell ?

3) last but not least I was giving her a run the other day (stationary). I let her warm up for 5 mins or so and then gave it a bit of a rev. Instead of the revs dropping back to normal they stuck at about 4000. I killed the bike, and it wouldn't restart (but smells strongly of petrol) I am guessing that the throttle has stuck open and is flooding the engine - what do you guys think ?

Thanks for any help in advance

[The Tot]

2) The powervalve is a feature of all two strokes. It fuctions by opening and closing allowing the power of the engine to be "spread" throughout the rev range since two-stroke engines generally tend to have a "powerband". The powerband would be the "sweet-spot" where the bike picks up its pace- usually past 8000rpm. Essentially, without a functioning powervalve, one would have bugger all power past 8000 rpm as most of the engine power would be delivered below that. But by having a powervalve, it allows power to be maintained at higher revs thus more peak bhp. I may stand corrected, but that is what a powervalve does to my knowledge
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[mr jamez]

I shouldn't worry about the aero intake tubes, the airbox is behind the tank facing the back of the bike, so don't see how they do anything really. More of a cosmetic/gimmick touch. As for it sticking at 4000rpm, probably a stuck throttle cable or the slide might be sticking in the carb. Oil the cables a little with some thin oil, and check them over. Check you don't have the choke on as well.

My understanding of a powervalve is that 2 strokes have a pretty tight power band and have little power lower down the rev range. It is just two valves that sit just after the exahust port, controlled by a servo motor, that open at certain times. On the NSR below 7000ish the valves are closed, when the engine starts to produce peak power the valves open fully. It was just designed to make the power from 2 strokes more useable.

I think

[stinkwheel]

1) no idea, sorry.

2) A two stroke engine relys on back pressure (resistance to the exhaust gas exiting the engine) to run efficiently. The lower the rpm, the more back-pressure that is required to get the most efficient burn.

This relates to the power of the bike, a narrow exhaust (lots of back pressure) will make the bike pull away well at low revs giving good acceleration and the ability to ride up hills but a lower top speed. A wide open exhaust (hardly any back pressure) will make the bike pull well at high revs and give good top speed but behave sluggishly as you try to pull away.

The powervalve is essentially a rotating bar with a cutout in it located just before the exhaust pipe. At low revs, the cutout is positioned so it partially obstructs the exhaust making it narrower and increasing back pressure. As revs build, it rotates until the cutout lines up exactly with the exhaust giving progressivley less backpressure. Makes the bike produce the maximum possible power for top speed while still being rideable at lower speeds.


3) Sounds like the choke is on. Did you switch it off? Is it sticking?
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[swampy]

Thanks a lot folks for the info, has been really useful. I will have a look at the choke/throttle later on today and let you know how i got on.

[ZaphodBeeble]

I wouldn't worry about the 'aero intake system'. They are mainly cosmetic and probably only just let the smallest amount of air reach the airbox anyway. If they are cheap you might as well fit them as the bike looks odd without them. But, bikes are for riding and not for looking at in my opinion.
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Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[Zen Dog]