Honda nsr 125 - No power band

[TheWrightGuy]

Hi guys, just ridden to work. My power band doesn't give me anymore power. It starts a 8k. It lets me going into the power band range but doesn't work! It's like a 4 stroke! I have a low battery which I'm changing for a new one after work. Could it be battery or power valve?
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[Suntan Sid]

Is the power valve still opening and closing as revs rise and fall?
Take the covers off and have a look.
When was the last time you decoked the head, piston, power valves and exhaust, and when did you last lube the PV cables and what's your plug like?

A power valve is merely a mechanical means of raising and lowering the ports in the barrel. If the PV is not functioning your "power band" will have moved up or down the rev range depending on where the PV IS stuck!
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[-]

Is the PV even enabled? Some are disabled with a jumper in the Ebox by the cylinder head.

As above, you can see the PV servo under the throttle side of the tank, rev the bike and see if it moves.
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[stevo as b4]

How many people on BCF over the years have brought a clapped out old two stroke and said:

No Power band, won't rev over 8000rpm, won't go over 70mph, keeps rattling and smoking but will it still do a ton? etc etc.

Am I massively missing the point here, but if it starts and rides and keeps up with traffic and does not break down, then why should you care that it doesn't do things?

Only the people with more money than sense to throw at these temperamental 2strokes should be trying to slove all these issues. I don't see why a cash strapped young lad, with little knowledge or experience, or money for spark plugs and fancy oil, should be the slightest bit concerned if his RS125 only does 70mph.

[Suntan Sid]

OP hasn't mentioned what bike it is, for some reason I had assumed it was an NSR125.
A change of or a clean plug will hopefully sort it, if its not the PV!

If it is an RS, I just assumed they all, ate plugs, spend more time off the road than on it and there merest whiff of rain sends the electrics into spazz mode!

Two strokes, in general, aren't any more or less tempremental than any other bikes, I will make an exception for the RS125 though!
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[TheWrightGuy]

Thanks for the help guys! It does say that it's a Honda NSR 125 in the subject. Someone said why care if it only does 70mph. I run it on Castrol Racing 2 stroke and have got a NGK plug that was changed a couple months ago. The bike is still perfectly ridable it just doesn't give me anything after 8k.
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[Fidge]

First check if the cables move when you rev the engine, take off the cover and it should look like this
https://i.gyazo.com/99e9a5477eec11b3e661a66bcf258670.png

if it doesn't move when you rev the engine then take off the nut and gold thing (reverse thread) and try move the valve manually. If it moves pretty easily then cables may be bad or the servomotor gears may be fucked. If it doesn't move easily then it would need decoking.[/img]

[stirlinggaz]

Hi,
theres obviously something "wrong" if you cant get it to go over 70mph or the pv isnt giving it that kick at 8000rpm.
The "powerband" should start at around 6500-7000rpm & if the "power-valve" (the rc mechanism bolted onto the side of the cylinder) is working, should continue right up to 10500rpm. If it isnt working, power may drop off at around 9000rpm.
first, make sure its actually fully de-restricted.. unless you done the de-restricting yourself, check.
dont trust any previous owners claims. seen a few supposedly de-restricted nsr's with exhaust restrictor pipes still welded in, wrong sized jets etc etc.
So go check.
your looking for a ci626 CDI & a tv78 pv servo. got them? Good.
Now make sure the pv actually works!
Look down on the right & with the bike in neutral, slowly rev it up to over 8000rpm. Does the rc move when you hit 8k? Yes? Good Good.
now, does it have the carb restrictor plate fitted, look for a plastic reducer plate, its quite obvious when you see it. Its a square plate with a hole in the middle. not there? nice.
carb jets? this is the bit thats often skipped. Dunno why really, its only 15 quid max for the 3 jets & its a 5min job to swap them over.
do you know what jets are in the carb? No? well go have a look! should be 138 main jet, 42 slow jet & 65 power-jet. If the jets are smaller, then the bikes not been de-restricted properly (although you might get away with a 135 mj but I would plug chop to check)
lastly, what exhaust do you have? If its the original honda 1 piece thing, make sure the restrictor pipe has been removed or better still, ditch it & get a decent exhaust
spark plug. whats fitted? I use a BR95ECS but Alains (bcf's resident nsr guru) favours the iridium flavour. Whatever is fitted, fit a new plug. Wont do any harm will it? spark plugs have a hard life on 2 strokes & should be treated as consumables.
compression? whats it at? not with your thumb but using a compression tester. Your wanting over 130psi. Once its below 110psi, whip the cylinder head off & at least fit new rings.
de- coking. Has it been done recently? If not, why not? when I first got my last nsr, I was intrigued as to why it wouldnt go over an indicated 80mph especially since I had checked all of the above. Upon closer inspection, it turned out the piston had a nice thick black layer on top. I removed it with a dremel & polished it up to a shine with autosol.
Same with the pv/rc flaps, I dunno if they were stuck shut , but it wouldnt have surprised me, they too were covered in black, hard carbon.
I also left the exhaust (oem) full of caustic soda solution overnight. twice.
the stuff that came out was as thick as soup!
after de-coking, the bike would hit over 90. nothing else was changed btw.
what else...you need a healthy battery to power the pv.
so I may have just typed all that out for nothing & a fully charged up or new battery will solve all your problems.
oh, & fresh fuel, splash out & get some premium stuff like bp ultimate or similar. why not, the detergents in it might help, certainly wont do any harm.
once you have done all that, come back & update us.

TL/DR? fit a new battery if your bike has been de-restricted & its all working properly.

cheers,
GAZ

[skatefreak]

From your origional post it sounds like you had the power band working before and it has just stopped working.
If this is the case the its safe to say that the bike was de-restricted and something has stopped the bike from working properly.
First port of call would be the power valve servo.
When you turn the bike on you should hear/see the servo move and when reving above 3000 it should move again (it is open from 0 - 3,000 rpm, closes from 3,000 - 7,000 rpm and opens again above 7,000 rpm).

This usually accounts for the majority of issues where the power band disappears in an instant.

If this is not working its your first port of call.
If it is then more investigation is required.
Its quick easy and free so again. The best option to check first.

As to why fixing clapped out 125 2 strokes?

The NSR I picked up had the power valve pinned open. Made the power delivery horrendously non-linear.

Tank off, learnt a little bit about the bike, took the wood screw out that was holding the servo/valve open, plugged the servo back into the harness and bobs your unkle. I had a working power valve making the bike far more ridable

They have a reputation for being un-reliable because many youngsters see you can ride them on a CBT and de-restrict them so many end up in young, enthusiastic hands who generally abuse them and aren't very mindful of general maintenance.

Having used an NSR125 for 4 years as a main form of transport I would go as far to say it was actually very reliable engine wise

-Jvr

[stevo as b4]

There's nothing wrong with making a bike work perfectly the way it was designed to from the factory when everything was new and in good condition and without wear.

I don't dispute that an NSR that's pampered and looked after at home by a careful owner could be a reliable machine.

Maybe the OP is nothing like a typical NSR other 125 sporty two stroke owner, and so then yeah he should spunk a load of time and money into it and make his bike work perfectly.

My post was more aimed at young people that buy sporty 125 cheap that seen better days, then mess with it and find it won't go well fast innit like mate's one that he clocked at 110mph down the bypass after 4 cans of cider etc.

If I was using an old 125 daily for getting to work and back, and didn't have the money to spend on sorting it out and making it mint, then if it wouldn't go over 60-70mph for example, then I wouldn't care. The reason being is that it would be a work horse that's needed, not a mess around and with a try and see the ton down the bypass every night machine, while stopping off at the garage for a bottle of cheap lawn mower oil to chuck in.

Yeah I'm having a go at the typical 17year old 125 sports 2stroke owner, but if there hadn't been all those twats that wanted fast as possible with no license, had not money to service them, and no idea of how to work on them, then there would be a lot more older nice non learner legal 125's around today, instead of them all being scrapped/blown up, crashed and bodged to the point of only being good now for the odd part, unless it's mint plastics your after and you might as well forget it then!