`89 NS125 issues

[Marci]

Wotcha folks... I have an NS125R, `89 model.

It's exhaust was fubar'd and couldn't get a direct replacement so ended up with a Micron Performance Pipe off an MBX125... worked wonders for top speed... previously (despite being fully derestricted) the bike would only handle 70mph tops. With new pipe, top speed is up to 90mph.

HOWEVER, it runs like a bag o' crap.... kind of!

When accelerating, it stutters. Let go of the throttle then grab it again and it smooths it out again.

Trying to assess whether it's an issue of wanting more or less air or whether it wants either main jet or pilot jet looking at.

Have removed airbox and tried it like that - bike suddenly gained LOADS of bottom end and ran sweet enough, but still exhibits similar problem higher up the rev range, and more dramatically.

This a case of underfueling? Thinking of upping main jet to 125 or 128 to see how that helps things along...

Further examples of the issue, when it starts stuttering, you can open the throttle completely and it doesn't make a difference. NO acceleration, just constant speed with stuttering until you shut the throttle and reopen.

Have also noticed breather from bottom of float bowl (which is tied up vertically to the chassis) is full of fuel...

Too much fuel in float due to main jet being too small?

Just wanting pointers on what to look for or what you'd do if you'd had to fit a crosser pipe onto a sportsbike...

It seems like it's gonna be a bit of both - remove airbox and fit K&N Filter, and increase main jet... but I don't have a clue about bikes in the slightest...

EDIT: Oopsie! Can a mod shift this to The Workshop please!

[Marci]

Well, stripped carb this evening, cleaned a load of crap outta the float bowl... turns out float valve and seat really need replacing - float breather is constantly dripping fuel as float isn't shutting off the flow.

[Marci]

However, despite float valve issues, simple clean of the carb and removal of rust dust out of the float bowl seem to have cleared up 90% of the issues...

[Kickstart]

Hi

The needle valve assembly can be replaced. Try speaking to Eurocarb (in Reading) who are the Dellorto importers.

The NS was one of the slower 125 sports bikes of its day, but the only restriction you can really get rid of is the one in the exhaust. Without the exhaust the bike is not deresticted.

Surprised the MBX exhaust works so well, although the MBX engine is quite similar.

From memory the airbox on the NS was quite restrictive so replacing it was a K&N single filter can have quite an effect, combined with a decent exhaust (the stock NS exhaust was quite good but a total nightmare to derestrict). The other restriction was that UK models omited any way of closing the ATAC chamber (Hondas early attempt at a power valve) at high revs, but it is not practical to try and fix this.

All the best

Keith
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[Marci]

Spoke to EuroCarb yesterday and now have a set of MainJets, new float valve & needle, and K&N Air Filter on their way to me... superb company, accept payment by PayPal (always a bonus), very pleasant to deal with - ta for recommending them!
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[Kickstart]

Hi

Didn't know they took PayPal. Interesting one .

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Marci]

Got meself baffled here now... replaced float valve and seat and washer under seat... float breather still leaking. Checked float height - s'at 24mm as per spec... any pointers?

Fitted 125 main jet and have K&N Filter to one side to add later... s'gotten too dark out to be messing around further.

Have also stuck in a BR8ES plug instead of a BR7ES... purely cos everywhere I went in town, no-one had any 7's in stock.

Extra question - ATAC chamber... ages ago I was told this was a second exhaust port that'd been blocked off to further restrict the bike, so popped out the bolt and chopped the blanking plate off the end... seem to see conflicting info in TheWorkshop as to whether this was a good or bad thing to do...? and I can't honestly remember what difference it made to performance when I did it *shrug* we lopped it off whilst parked outside the challets at MCN/Butlins earlier this year... the resulting hangover restricts memory of the ride home...!

The bike certainly has no off-the-line speed at the moment... but soon as hits 2nd gear flies...
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[0ddball]

The valve still mustn't be seating. Did you replace the float or check it for punctures?

Blow through the petrol intake while pushing the float up and make sure the valve closes. When the valve is seated you should be able to blow the inlet till you turn purple but no air should get through.

As for the atac thing. I never really looked at mine but from what i read i concluded the best thing to do would be to leave it as it is.
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ZXR750L

[Kickstart]

Hi

heck the floats are not punctured.

The ATAC chamber is meant to be open at low revs and closed at high revs. It is just a chamber which is meant to increase the volume of the exhaust port, which was meant to give more power at low rev (but looses power at high revs).

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Marci]

Floats are fine... hung em in a glass of unleaded and no sinkage whatsoever... sunk em down and they flew straight back up... will give it a blow in the morning and see what I get...
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Marci]

Got bored, found a torch. Have carb in me hand in front of me... when floats are up, can't blow down fuel inlet. Floats down > can.

Soon as I hook it up to fuel tank, breather pours fuel straight out.

Have just rechecked floats.... they're still fine and float fine.
____________________
FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Marci]

Right. Sussed it. Problem isn't anything to do with the floats, or the float valve or seat. It's where the brass breather has been inserted into the floatbowl... If you block up the internal end of the breather and blow up the external part, you can still blow... and feel the draft from the base of the internal tube where it meets float bowl body.

New float bowl required Anyone got one lyin' around for a PHBH26??
____________________
FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Kickstart]

Hi

Probably the same float bowl on most bikes using the PHBH carbs, even the slightly larger 28mm ones used on the NSR, Mito, most versions of the AF1 and the later RS125s.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing

[Marci]

Hmmm.... got a spare 28mm in the sidecar's toolbox... will check it out for size next time I'm in it's vicinity...!

Cheers fer all yer help so far! Will let u know if she actually rides any better after all this tomorrow...
____________________
FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Marci]

Well... she does NOT like a BR8ES plug... too cold... rough as hell. Put the old BR7ES back in again and much smoother... am wondering what effect going to a 6ES would make as at the mo hotter = smoother below the powerband.

No real difference from floatvalve and jetting up to 125... Get to 75mph and kick up to 6th and it's as tho it cuts out... engine dies for what feels like an age (but isn't) and picks up and cuts out and picks up... give it all the throttle ya like and no difference... gotta cut the throttle, down to 5th and carry on...

Recommendations? Still got the K&N to whack on... and a 130 jet if needs be *shrug*
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Marci]

W-O-W!!

K&N on, still on 125 jet... just shy of 100mph, perfectly smooth - absolutely blisteringly wonderful tbh. Still a bit slow off the line... but cooorrrr... the NOISE! Never have I had so many heads turn as I drive up the road... suddenly folks on much larger bikes are nodding back at me... whereas before I was getting the L-plated blankness!

Mixture's still a touch off, will play with that thru the week, and see if 130 jet gets me just past 100 instead of just under...

Altho I think this is the bike at it's limit now... s'too light on the front soon as u get past 90mph to be rideable...
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[0ddball]

100mph is good going. The best i managed was about 85mph @ 9000 rpm (wouldn't rev any more than that, even in the lower gears)
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ZXR750L

[Marci]

Well... 100mph at the speedo is probably more like 95mph actual... mine revs off the guage... seems to have a 2nd powerband that kicks in from 10000rpm thru to 12000rpm, or feels like it...

Mine does similar at 85mph... appears to rev out and go no faster at around 9k in 6th, but if you hold the throttle exactly at that point, after a few seconds it picks up and keeps going... slowly. Takes a long stretch of road but manages it... soon as it gets to 10k it takes off again with a spurt up towards late 90s.
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[stevo as b4]

It could be over 10mph out like the AR125 is tho, which is why they have been know to clock 90mph down hill, in 12bhp form!
Either way it's way better than the std restricted spec bike which struggles to top 70mph and is the slowest of the sports 125's over the 1/4mile.

[Kickstart]

Hi

From memory the NS had one of the more unrealistic speedos.

All the best

Keith
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Traxpics, track day and racing photographs - Bimota Forum - Bike performance / thrust graphs for choosing gearing