NSR125 7-8k revs problem - not sparkplug. **FIXED!**

[Zen Dog]

Hello, I recently bought an NSR, and its in beautiful condition apart from this one thing...


The Symptoms

The bike revs freely throughout the rev range when the bike is at a stand still, and works perfectly when riding between 1-7k revs and 8-11k revs. But between 7-8k the engine just seems to refuse to ignite the fuel, with only occasional bangs as the unburnt fuel in the exhaust ignites.

Once 8000 rpm is achieved the bike is back to normal and goes like stink, and below 7000 the engine is perfectly usable, the engine only shows any sort of problem in this 7-8k range. The engine does not encounter this problem about 1 in 4 times, when it functions perfectly normally.

Additionally, the bike now cuts out when warm, and has to be left for a few minutes, when it will start again perfectly normally.

What has been tried.

The bike has been throughly warmed up on a number of occations. It makes no difference to this problem.

The spark plug and plug cap have been changed. No difference.

The air filter was changed and the airbox cleaned. No difference.

The carb was removed and cleaned. The bike seemed to be running a little rich so the needle was dropped to make it run leaner. No difference.

The carb was changed for one known to be working perfectly, off a friends NSR. No difference.

The exhaust was removed and cleaned, and the powervalve checked. It appears to operate and be functioning correctly.

The coil was changed for one known to be functioning correctly. No difference but the engine now cuts out occasionally when warm.

The engine was compression tested. The engine was cold when it was done but it still came out at 126psi or 9 (bar?) which was in the green on the gauge.

The alternator was checked and seems to be fine.

The inlet hose and reed valve were checked and are fine. There is no restrictor.

The CDI, rectifier and exhaust were put on to a friends NSR. They all worked fine.

Still left to try.

As far as I can tell, the things still left to try are -

We are going to try putting the alternator off my friends NSR onto mine, just in case this is the problem. I doesn't seem very likely though.


Additional Details
The bike is a 97' german import NSR125R, and its done about 18000 miles. I don't know if the engine has been changed/had rebores, although i suppose it must have by now.


Any ideas or suggestions greatly appreciated.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[Hex]

Could just be the flat spot that all 2 strokes have. Though you say it only happens 1 in 4 times etc which confuses me slightly.

Sounds like you have already thrown a lot of money at the problem (if there actually is one). remember its a 2 stroke so it likes/needs to be ridden hard.

Try adjusting your riding so that your always in the sweet spot just above 8k ish.
____________________
The BCF's very own Party boy! Though he's getting old and feeling it!
Monkey hanger, Born and bred
My little photo portfolio

[Zen Dog]

Its not that. i've had 2 strokes before (KMX125) so i know they are rubbish at low revs. It feels like between 7-8k revs that i've forgotten to switch the fuel on, that slow whirring to a standstill, with occasional whumph/bang noises. As soon as I'm out of that rev range (either below or above) everything is back to normal, all fine and dandy.

I haven't spent a huge amount of money trying to fix it yet, but i'm running out of cheap and easy things to try, and pretty soon i'm going to be left with difficult/expensive things to try

I would try to keep it in the sweet spot (over 8k) but the problem is right in the middle of the rev range, so if i want to go up a hill, i either have to go incredibly slow, or change down and slip the clutch and screw the engine, and then encounter exactly the same problem in the next gear. I think I'm just going to end up with a burnt out clutch and a dead engine from screaming it all the time if I do that.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[craigT19]

sounds to me like u got yourself a restricted bike mate
____________________
Bikes owned :- 2001 nsr125, 1999 zx6r, 2006 yzf-r1, 2009 xmax 250, 2012 yzf-r1, 2015 MT-07
Current bike : - 2016 MT-10
BCF member pops. sadly missed 1945-2003

[Zen Dog]

Well, I cant say for sure, but why would only 1000 of the 12000 rpm rev range be restricted?

And why would the bike be a screaming demon (for a 125) above 8000 rpm? Surely it would be the top end of the rev range that needs restricting?

And why would the restriction take the form of the engine cutting out and "backfiring"? I'm sure Honda could come up with a better restriction method than that. Its possible that its a CDI restriction, but i thought that just limited how high you could rev.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[alains]

probably a condenser gone (the cdi has 2 of 1µf) it happens very often on the NSR , i assume you can buy a used one because a new will cost at least £120
____________________
the worst day fishing is better than the best day working

[Zen Dog]

Aren't the CDI's specific to certain markets? It could be difficult trying to get hold of a second hand german import CDI

Thanks for the suggestion though, i'll bear it in mind.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[alains]

normally not but possible they change the voltage furnished to the servo-motor. friend of mine suggest that in germany according to german laws they put a second box to limit this voltage providing a lesser openning on the valves (people breaking the law might put a hidden switch to avoid the inconvenience . i you want further informations on how to control the valves , look on the topic i've put on today
____________________
the worst day fishing is better than the best day working

[Zen Dog]

I've edited the original post to reflect the fact that more tests have now been done (compression test, coil swap).

It is looking increasingly like the CDI unit, but hopefully the inlet manifold will be checked today, and the exhaust swapped for a working one to check that. Once those have been tried, hopefully i'll be able to get a used CDI having tracked down the problem by a process of elimination.

Thanks to everyone who has supplied suggestions so far.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[Robby]

One little thing to note. Leaning out the mix is never a good idea. The plug may may make it look like its running rich, but if it was idling for a minute before you got the plug out it will have oiled up a bit.

Try your reed valves, they could have got a bit chipped if they are original and the bike has done 18k.

[Zen Dog]

Yeah, i know that leaning the mixture runs the risk of seizing the engine. I was just one of the things that was tried when trying to isolate the problem, and the mixture will be put back to its previous settings now.

Checking the reed valves is on the list of things to do, hopefully later today.

Thanks for the suggestions.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[Flywheel]

hi

i have an ns125r and sometimes get the same problem. Dont know what it is and dont think it would be a restrictor so mabe is the CDI.

[Zen Dog]

I've edited the original post to include the fact that the inlet hose and reed valve have been checked.

Flywheel, i'm not sure how similar the nsr and the ns are, but if/when i find out what it is thats wrong with my bike, i'll certainly tell you in case it helps with your problem.

I managed to find a second hand CDI for sale on-line, but the numbers on it, and the numbers on mine are completely different. I also found a place which says they will sell me a new one for £65, but i have to give them my frame number. Whats that about?

I'm not sure if i'm going to be able to try my friends CDI on my bike, as his is a UK one, and i don't want to break his CDI if they are significantly different.

But i'm seriously running out of other possibilities. Some kind of exhaust obstruction? Weird wiring?

(bangs head on desk)

In order to solve a problem, you need to find out what the problem is.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[Kickstart]

Hi

I would be tempted to go somewhere away from houses, and then check the ignition timing with an inductive strobe light all the way through the rev range.

The only odd thing I can think of is that those revs are about when the power valve is opening which will be putting a fair bit more strain on the electrical system. Give the battery a charge, then run the bike with the lights off and see if it still causes a problem (ie, when it has pretty much maximun electrical power). Also clean all the connections, and check the voltage over the coil.

All the best

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

[alains]

hi kickstart,
do you know that the power furnished by the alternator is 276w model R (168w for the F) at 5000 revs , so it's quite impossible having a lost electrical power . calculate .....
cdi (and satellites) 80 w , headlight 70w , leds 12 w , park lights 14w and temporary blinkers & stop lights at max 62 w , so the battry take the remains : permanently 100w , occasionally 40w
____________________
the worst day fishing is better than the best day working

[Zen Dog]

I've updated the original post to reflect the fact that the CDI unit, rectifier and exhaust were put on a friends NSR, where they functioned perfectly.

We are seriously running out of ideas now. Aside from changing the alternator (hopefully later tonight), and checking the ignition timing, we are completely stumped.

The battery seems fine for charge, and the bike runs the same with the lights on or off. My friend has a voltmeter thing, but hes not sure where it is, we'll have to dig it out. Its seriously disheartening when everything seems fine and it JUST DOESN'T WORK!

I'm going to end up having to take it to some Honda approved spanner monkey, and he'll probably start by trying all the stuff we have already done...(sound of head/desk interfacing taking place)....

If anyone has any other ideas, they will be gratefully accepted.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011

[Ste]

Has it always done this, or did it just start doing it recently?

I believe that a CDI unit off any NSR will be okay. I'd be tempted to try your mates CDI in your bike and see if that makes any difference just to be able rule the CDI unit out completly.

[maza1]

you really tried a lot stuff,but maybe you went the wrong way.if a condenser was bad,it could not go over 7k rpm.When a condenser is bad,motorcycle doesn't work good on any rpm range.
here is what i think:
as much as i get it,i understand that your bike small throut on 7k rpm.(sorry for bad English).He might be choking there.I reject the current problems,cause you seemed to check them all.
Here's what you do:
1.start the bike
2.let it get some normal temp,(maybe driving it for a few minutes)
3.park the bike(still working)
4.rev it above 7k (let's say about 10 000 rpm)
5.when it reaches 10k rpm,instantly turn the power of LEAVING YOUR HAND ON FULL THROTLE until the bike shuts down.
6.the bike is off.put your head near the RC valve(it should be on the right side of the cylinder)
7.turn on the power and listen carefully

-If you didn't hear the servo-motor work then you contanct me on maza1@ptt.yu or send me a message on cel phone +381638396471.

-If you heard the servo-motor:
Check the THING (i don't know how you call it,it's going from the s-motor to RC-valve and moves the valve),it might be broken and doesn't turn RC valve properly.
If the THING is good and working,your RC valve might be stucking in some position,not giving engine a nice breath (it doesn't open enough,or it is heavy to be opened).
You should take of the cover of the RC valve and give it a good clean,and see for any deformation.Someone might damaged it on some earlier fixing)

You got my e-mail and cel phone number.let me know what you have done.

[ZaphodBeeble]

Hi Guys,

I've been helping Zen Dog for about a week with this bike. I've tried everything that I can think of so far but nothing seems to be wrong. All the components that I remove seem to be in perfect working order.

I have just fitted an alternator to the bike off another nsr but it still has the same problem. I need to know what circuitry is between the timing pickup thing in the alternator and the CDI as I can't see where the wires go (don't really want to rip the whole loom to bits). There are 2 little 'boxes' about 2cm by 2cm (labelled 3 and 4 on the attached wiring loom image) which I think are either relays or maybe something to do with the ignition. If someone could tell me how the timing pickup gets to the CDI I would be a happy man.

Cheers,

Monkey

https://www.nsr125.com/schem45.jpg

EDIT: Forgot to attach the picture
____________________
Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[ZaphodBeeble]

We've now also checked the wiring for shorts or bad connections but we can't seem to find any problem. I think I've worked out what one of the small 'boxes' does. It seems to be the bit that stops the bike running if the stand is down and the bike is in 1st gear. I've tried changing the CDI, the exhaust valve electronics and the rectifier but the bike still behaves the same.

It seems like it's an electrical fault as the bike misses and then ignites the fuel in the expansion chamber causing a loud pop. I considered that it was a mechanical fault (ie. head gasket leaking water) but there would be white smoke. As for the smoke, it looks normal once the bike is warmed up.

When the CDI off another NSR was fitted the bike ran ok for about a minute, it pulled straight through 7000 to 8000 rpm without a hitch, but it just went back to normal after that.

I'm pretty much stumped now, I'm going to drain the oil out of the reservoir and replace it with TTS. I think the stuff that's in there is this cheapo strawberry scented crap. It's worth a shot.

This thread is getting very complicated now but don't be afraid to post your suggestions, we could use any idea right now.
____________________
Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[alains]

here is the wiring , if you position of every bits , ask me
____________________
the worst day fishing is better than the best day working

[ZaphodBeeble]

Thanks for all the replies guys. It's fixed now, going really well. It was so damn simple; the control cables for the r/c valve were on the wrong way round.

I took the cables off the top of the servo and put some tape around them to hold them still in the closed position. This made the bike run smoothly but with a virtually non-existant powerband (as expected). When I moved the cables to the open position the same problem occurred. I stripped it all down, radiator off etc. and checked it all out. Someone had (for some reason) taken the cables off the r/c servo and put them on backwards.

Attached are some images of the bike while being worked on and after it was fixed.

Bike in bits:

https://www.users.totalise.co.uk/~billcar50/bikeinbits.jpg

Valve cables with tape:

https://www.users.totalise.co.uk/~billcar50/valvecables.jpg

Me getting stuck in:

https://www.users.totalise.co.uk/~billcar50/meunderbike.jpg

R/C Valve and piston:

https://www.users.totalise.co.uk/~billcar50/valveandpiston.jpg

Bike after a weeks work

https://www.users.totalise.co.uk/~billcar50/fixedbike.jpg

Phew.
____________________
Current bike: VTR1000F Firestorm. Previous: Firestorm (written off) - XJ600 Divvy - NSR125 - DT125 and a load of offroaders.

[Zen Dog]

Hi Guys. Yep, the bike is now sorted thanks to Zaphod "The Man Spannertastic". Some gimp reversed the control cables for the RC valve, and we probably wouldnt have noticed if we didnt have a working NSR to compare it to.

Still, looking on the bright side of a week of frustration, every part of the engine has been stripped and had a complete overhaul now

And i'm happy to state openly that Zaphod did all the work, and I just made sandwiches, took the photos, passed the tools, and generally whined at Zaphod to get out of bed and help me with the bike.

Thanks to everyone for the suggestions and help.
____________________
Current - '94 VFR750FR, '00 VFR800FI Previous - '10 Street Triple R, '92 MZ ETZ301, '05 TTR250, NSR125R, KMX125, "Honda" Win (chinese copy of an old Honda design with a C90 engine)
My bike trip around S.E. Asia 2010/2011