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NS125R hesitation around 3k - 5k revs

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KevTM
World Chat Champion



Joined: 15 Apr 2004
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PostPosted: 21:09 - 26 Apr 2004    Post subject: NS125R hesitation around 3k - 5k revs Reply with quote
Went out on my bike for the first time to day and enjoyed it very much.. got a nice powerband Twisted Evil

Only thing is though it hesitates around 3k - 5k revs. Usually dipping the clutch and letting it out again whilst keeping revs around 6k - 7k will do the trick. Any ideas on what the problem is?

It wouldn't bother me too much but it's a pain in the ass trying yo get to 30mph - 40mph without revving the bollox off it.

Cheers.
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Ian (GPX)
Brolly Dolly



Joined: 05 Aug 2003
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PostPosted: 22:30 - 26 Apr 2004    Post subject: Reply with quote
You need to have around 3k - 5k to keep it going, it's a two-smoke and not a 4-stroke.

I had an NS125R and it needed to be ridden hard to get anywhere, the power band kicks in at around 5.5K until it gets to 7k - 8k and then dies off.
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KevTM
World Chat Champion



Joined: 15 Apr 2004
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PostPosted: 23:10 - 26 Apr 2004    Post subject: Reply with quote
Ok. It just seems to be quite mince around the lower revs compared to my other 2-strokes that i've had (if i remember correctly).

I'll live with it! it just means i need to feather the clutch to keep the revs a little higher.. cheers! Thumbs Up
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G
The Voice of Reason



Joined: 02 Feb 2002
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PostPosted: 23:18 - 26 Apr 2004    Post subject: Reply with quote
Have you had another (road) sports 2 stroke?

They aint designed to be ridden below 5k rpm Smile.. rev the bollocks off it ... that's what it was designed for Wink.
Fiddling with the carb may well sort something... but the only time I took an NS carb apart I think I made it even worse.
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AcIdBuRnZ
World Chat Champion



Joined: 28 Jul 2003
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PostPosted: 23:51 - 27 Apr 2004    Post subject: Reply with quote
Below 6-7k there is nothing to speak of on the NSR.

As G said: rev the tits off it!!!

Just get used to it until you get a bigger bike, I'm afraid.

Mark
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Past: Honda NSR125R >> Kawasaki ER-5a2 >> Kawasaki ZZR-600e1 >> Suzuki GSXR-750Y >> Honda VTR 1000 Firestorm
"Chickity China the Chinese Chicken..."
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KevTM
World Chat Champion



Joined: 15 Apr 2004
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PostPosted: 00:06 - 28 Apr 2004    Post subject: Reply with quote
no problems. i was just curious in case i had some sort of problem which may have gotten worse, or could have been easily sorted.

It's running great and the hesitation is more or less between 4.5k and 5k. Anyway, atleast i know it's alright! Laughing

thanks.
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 00:51 - 28 Apr 2004    Post subject: Reply with quote
KeV6 wrote:
It's running great and the hesitation is more or less between 4.5k and 5k.


That is probably exactly half peak power.

The way exhausts work is that the pressure waves down the exhaust are tuned so that at peak revs the bounce back to the barrel just in time to bounce back again and draw the next lot of exhaust gas from the engine. The trouble with this is that at half those revs the pressure waves are doing their best to shove those exhaust gases back in to the engine, causing a flat spot.

All the best

Keith
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jimbo
Borekit Bruiser



Joined: 23 Aug 2003
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PostPosted: 14:21 - 29 Apr 2004    Post subject: Reply with quote
Kickstart wrote:
The trouble with this is that at half those revs the pressure waves are doing their best to shove those exhaust gases back in to the engine, causing a flat spot.


The reality is that the same pressure wave interference is happening anywhere outside the power band. So the power valve was invented in the 1980s to address the problem.
The power valve evens out the power curve by reducing the exhaust port area at low revs, reducing the effect of non-resonant pressure waves from the exhaust.
The flat spot on the NSR coincides with the point where the power valve starts to open. If you watch the servo while you rev the engine (warmed up of course), you'll see it tweak the valve just after your flat-spot. (the servo operates twice, around 2k and then between 5k and 7k depending on your set-up). The flat spot is the result of the cross-over region between low-rev range and high-rev range power valve settings. At this point the end of the low-range curve and the start of the high-range curve coincide, giving the worst of both worlds and a boggy patch.

The most interesting feature of exhaust-pipe tuning is not the removal of the exhaust by the pressure wave, but the compression of the mixture in the cylinder by a pressure wave node. At peak power, the pressure wave pulls unburnt mixture into the exhaust throat, and then pushes it back into the cylinder. This is like a turbo, increasing the compression and allowing tuned 125 2-strokes to burn more than their rated capacity. Some say up to an equivalent 180cc of fuel is burnt at resonance, but i don't know how that can be measured. This is one of the reasons why 2-strokes burn more fuel than 4-strokes.

The flat spot on my NSR is at 6.5-7k, which is ideal because it happily motors up to 60mph before the power valve opens. This means i can drive like a granny, save fuel and still make reasonable progress, or kick it down a gear and get into the power band.
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Kickstart
The Oracle



Joined: 04 Feb 2002
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PostPosted: 00:53 - 30 Apr 2004    Post subject: Reply with quote
jimbo wrote:
The reality is that the same pressure wave interference is happening anywhere outside the power band. So the power valve was invented in the 1980s to address the problem.


True, but then it is at its worst at exactly half the revs where it is doing its best.

The power valve systems were first used by Yamaha. Think they came out with it on competition bikes in the late 1970s (although I might be getting confused with monoshock suspension).

The power valves main effect is to alter the height of the exhaust port, which actually changes the exhaust port timing. Effectively it is doing the same job as variable valve timing on a 4 stroke (which Alfa used in a car from about 1981).

All the best

Keith
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jimbo
Borekit Bruiser



Joined: 23 Aug 2003
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PostPosted: 00:34 - 01 May 2004    Post subject: Reply with quote
The first mass produced power valve engine was on the Yamaha TZ in 1981.

kickstart wrote:
The power valves main effect is to alter the height of the exhaust port, which actually changes the exhaust port timing.

The power valve physically alters the area of the exhaust port.
This has several effects: an effective reduction in compression at high revs(reducing detonation), a reduction in airflow impedance at high revs (increasing the effect of resonant pressure waves) and an effective change in exhaust port timing.

Two-strokes are always tuned by Port-Time-Area, not port-time, unlike cars such as the
kickstart wrote:
variable valve timing on a 4 stroke (which Alfa used in a car from about 1981).

Karma
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coolfox1
Scooby Slapper



Joined: 14 Apr 2004
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PostPosted: 11:26 - 22 May 2004    Post subject: same Reply with quote
Hiya

Have a nsr 125 2000 and have the same thing, thing was when I first got it until I'd had ir checked out and was getting used to it I didnt take it too high on the revs and tho the take off was good didnt get too high, also I have a really rather ill sounding rattle that seems to happen when I'm between 3000 and 5000 revs so kinda dint want to do any damage, still have the rattle but remember the buzz I got when I did take it higher and the top revs, and good god, was I amazed, love the little pocket rocket now and that kick and hang on here it comes feeling when you get past the flat spot is just the best feeling.
I guess a lot of you guys may be thining geezz new biker, try a real bike and then you'll find out what real power is about but, well it puts a nice beem on my face !

Be safe !
Ian
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