NS125R Advice Needed

[Symon]

Hi Folks,

I'm new to the forum and have scoured the archives looking for any info that may help me with a NS125R that I've recently bought. When I bought the bike I was aware that the engine needed a bit of work (primarily a big-end rebuild). The Small end and piston had already been done - but after getting the work done the previous owner realised that the big-end was gone aswell - and never got round to getting anything done about it.

Anyway - the engine is now rebuilt and installed back into the machine - but I'm having a problem trying to get the bike to idle properly. I'm also getting a bit of a "thump" from the engine at low revs (i.e. < 5,000) when the throttle is just partially opened. The thump turns into a very healthy two-stroke firing sound under all other conditions and there are no noticeable knocks or thumps when engine braking.

The engine starts from cold really well (always first kick), and will idle ~1,500 from cold with full choke. I can then partially release the choke as the engine heats - but I can never get the bike to idle with the choke fully released. I've tried adjusting the idle screw on the carb but this doesn't make any difference at all. I've also tried changing the setting of the pilot screw on the carb - but again absolutely no change (I don't even notice any preceived improvement, or otherwise, in engine running when adjusting the pilot screw). The previous owner provided me with receipts indicating that the carb had been completely overhauled and cleaned when the top-end was done - so I'm loathed to go this route again - but am a bit suspicious about the operation of the pilot jet.

Thinking that the "thumping" may be a symptom of bad timing, I've done some checks of the electrical system. The only anomoly I can find is that the resistance over the secondary windings in the HT coil is way lower than stated in the Haynes manual. The manual states that the resistance should be 1.6 - 2.0 Ohms, but the measurement on my coil is only 0.5 Ohms. I've checked all other settings - other than the final check with a timing lamp as I don't have one - and everything is reading exactly as it should be. I can obviously only do limited checks of the CDI but given that I am getting quite a power jump at ~ 6,000 revs, I'm assuming that the CDI is advancing the timing correctly.

So - in short I'm thinking it's a timimg problem at low-revs, or a pilot-jet carb problm - or a mixture of the two - but I don't want to go buying a new coil or dismantling the carb as I may be completely wrong!

Can anybody shed any light on the problems I'm having, has anyone experienced these kind of symptoms before?

I'd be really gratefull for any suggestions any of you may have gleaned from previous experience.

Cheers,
Symon.

[Symon]

Meant to also say:

Power delivery is really "lumpy" at low revs - just doesn't appear to be firing cleanly. When trying to get it to idle after warming it up, I can use only partial choke. With the choke fully applied the engine just "drowns". If I try to idle it without choke (by applying a little throttle) and then check the plug - it is very black. I kinda figured that it is running too rich at low revs or I'm not getting full combustion (timing?).

The bike is an NS125R (1993 - the faired kickstart model)

Don't know if this info helps any - just thought I should add it.

Cheers,
Symon
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VFR 750 (1994) - Anyone good with plastics?

[Kickstart]

Hi

I would check the carb carefully, especially if it has been stood with petrol in it since it was cleaned up. You do not say whether the previous owner got it running before realising the bottom end had problems.

Other than that check for air leaks, such as from a cracked carb mounting rubber.

Coil is a definate possiblity a well.

All the best

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

[0ddball]

Adjusting the pilot screw on mine made absolutely no difference either, not sure why though. Best to give the carb a good clean first, it's only really a 1 hour job (leave the jets soaking in carb cleaner for at least 30 mins).


btw, was is the bike a white one bought off of ebay?
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ZXR750L

[Fallen Angel]

[Kickstart]

Hi

Ah, so not sat for that long. Petrol might still be an issue (even more so if the stuff in the tank is what has been in there since before you blew the engine up).

Well worth cleaning the carb out thoroughly.

All the best

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

[Fallen Angel]

The petrol that was in it was just a little I put in it just after it was apparently 'fixed' the last time, probably only a tiny tad in it as I only put in about £3 to check it out.

The guy that was apparently fixing it put a little in it when he rebuilt it, but if there's any in it just now, then yeah, it's probably been there for a couple of months.
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[Marci]

Mine had similar issues when I got it, turned out the oil pump lever / arm had been bent so wasn't returning to it's proper zero-throttle/zero-oil position (closed). Permanently over-oiled and would never idle. Straightened the arm out, resynchronised the oilpump.

These days, full choke, kick her up, soon as she fires hold throttle at 4k rpm and whack choke straight off. Won't idle til it's nice n' warm... just sit n' hold throttle at 2k rpm for 5 mins or so and she should hold then hopefully.

If petrol has run low, with no inline fuel filter, then possibly lotsa rust dust in the float bowl... this can also cause stuttering and other issues both above and below powerband from my experiences...

Pilot screw - tighten it up then back it out 2 n 3/4 turns (I found it better than 2 n 1/8 turns), take it out for a run n keep it reving at lights etc, then when you get back home set idle screw for about 1600rpm. Now when cold starting it should idle nicely at about 1400rpm, and when warm idle slightly higher.

S'no real biggy dismantling carb - just take float bowl off - don't really need to do much more than that. Once float is off, rinse any crap out with unleaded, then pop the pilot jet out whilst you've got it open and give it a blow thru just to ensure it isn't clogged. 14mm spanner and a flathead screwdriver is all ya need...

Downside is often means rear panels off, seat off, tank off to get at it. Did on mine.
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FAB-Racing MiniMotoSidecars - Back with a vengeance - F1 Class 2011...
CBR170 engine, custom chassis - www.minimotoscene.co.uk

[Symon]

Thanks for the info.

I've already rebuilt the bottom-end and have checked and aligned the fuel-pump - so I know there's no bends in the fuel pump actuator lever so shouldn't be a problem there. I've also monitored the oil output to the carb from the fuel pump when I was priming the oil feed to the carb after the rebuild (running pre-mix from the fuel tank!) and it appeared to be operating as expected (i.e. increased flow when throttle opened and reduced with throttle closed). I didn't do anything clever like check the amount of oil delivered to the carb from the pump - but no doubt there are specs on this flying around somewhere to ensure the correct mixture ratio.

I already convinced myself (perhaps incorrectly ) over the weekend that the problem was with the carburetor - so I've dismantled it already. What I found was that the clip on the Pilot jet needle has been moved right down to the lowest position. None of the jets have been changed - but the face of the main jet looks a bit mangled (like someone has tried to remove it with a small spade screwdriver - kinda unsuccessfuly I might add!). Having now read what seems like an encylcopedia's worth of info on Carburetor tuning - I'm completely convinced that just moving this clip to the bottom position on the needle is what's causing the problem - resulting in a rich fuel mixture at idle and low revs. From what I understand, the result of moving this clip would effectively be the same as applying a little throttle but not increasing the slider gap to allow more airflow through the carb - hence simply increasing the mixture at low throttle openings. I know that the owners intention when they got the carb work done was to have it re-jetted given that the machine had been partly de-restricted - but from my understanding this isn't really the way to do it.

I've ordered a new main jet (as the current "defaced" one definitely needs replacing) and increased the size given that I reckon a richer fuel flow will be needed with higher throttle openings (given the partial de-restriction). Given that the carb is now in bits I'm also going to thoroughly clean the lot in carb cleaner and fit a new O seal around the top of the float chamber. Hopefully this will sort the problem - but if anyone has any other suggestions (constructive only please ) then please give me a shout.

I'll post if it solves the problem just incase any NS125 owners are scouring these posts in the future for a similar problem.
____________________
VFR 750 (1994) - Anyone good with plastics?

[Kickstart]

Hi

The needle is meant to be adjusted by moving the clip, but it should not really need moving much (if at all) for partial derestriction. There are numerous other shapes of needle available to fit these carbs.

All the best

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

[Marci]