Help with NSR 125 Carb set up, Jet sizes etc and tips

[Adrian Phillips]

I’m hoping that someone may be able to shed some light on settings for my sons NSR125R, I believe it to be a 1991 model, it has the twin round headlights.
Bike was bought as a non runner but is now up and running.
I believe the bike to be restricted; the exhaust pipe does not have an insert welded into it but there seems to be a restriction about 5 inches into the pipe, just round the first bend.
There is no restricting plate between the inlet manifold and the reed valve block, the reed valve does have a white plastic insert and I believe this may be to aid the airflow??
The question I have regards the carb jet sizes, the sizes fitted were; main 135, pilot (slow) 40, power 65. This seemed to work at low revs but bike lost power when revs built up. Was advised to use a main jet of 110, this seemed to make the bike run better and is what it’s running on now. There seems to be a lot of conflicting info on jet sizes, have tried dealers but they weren’t much help. I have a number of jets, due to spare carbs, and they are; main 110, 132, 135. Pilot (slow) 40, 50, 52. Power 65, 70. Any recommendations on what to try??.
Some info on the bike is
Frame number H1-JC20*217679* , Engine H1-JC20E*92085 [not original engine]
Carb PHBH 28 FD, CDI unit CI572, [there is also 06 221 KY4 marked on the CDI unit].
The power valve seems to operate ok, whether it’s fully opening I’m not sure, need to investigate.
Also any information on the possible restrictions on this bike, and how to de-restrict it, would be appreciated.

[alains]

something strange : your CDI is CI572 and KY4 means earlier model 1989-1992 but the carb as you said is a 28PHBH/FD which is the model fitted on 1993-2001 NSR's otherwise the carb would have been 28PHBH/FS . FS means screws on left , FD screws on right , this is because the frame has holes to operate adjusting screws and they are placed differently . the genuine jets on the FS were 145,45 or 50,70 and on the FD 132,38,55 . in fact the right jetting of both is 138,42,65 . also on the FS the needle has 4 notches , the FD has 5 , set the clips on the 2nd notch from top
adjust the pilot screw at 2 1/2 turns out and the idle screw at 1400 rpm +-100 after 5 mn warming
____________________
the worst day fishing is better than the best day working

[Adrian Phillips]

Thanks for the info Alains, I do have an FS carb the FD was the one fitted, thought it was awkward getting to the adjuster screws. Apart from adjustment on left or right side I assume they are both the same and I can use the better of the two [the FD]. Also can you tell me if the CDI is ok for using on a future derestricted bike or will I be better of finding another one. In fact any info you can gleam from the supplied numbers would be greatly appreciated. Also would running on the 110 main jet cause any problems or effect performance greatly. I'll try 135 40 65 this weekend [don't have a 138 yet] however the last time I had those jets in the engine seemed to bog down at around 30 to 40mph, any info appreciated.
Oh and thank you for the quick reply as well.

[alains]

this bike is not restricted and the genuine CDI is your's . the 110 main jet is in use on 26PHBH which was the carb fitted on 1987-1988 TC01 (NS125R) . running with underjetting is dangerous for 2 strokes . you may clean your air foam filter (wash , rince,dry, slightly oil and squeeze) set your needle as i said
____________________
the worst day fishing is better than the best day working

[Adrian Phillips]

Once again thanks Alains, one more question though. I have now replaced the jets to 138 42 65 and set needle as suggested, also replaced o rings etc in carb. Bike seems to perform better at low revs but starts to choke at around 5/6000 and wont rev higher. Tried it a few times and sometimes seemed to rev above 6k in 1st but then bogged down again after changing gear. The RC valve seems to be operating ok. Have seen a post suggesting dodgy electrical connections around the coil so will look at that. Just wondering if you have come across this and have any suggestions.

[alains]

just remember that this bike needs at least 5mn warming before feeling the power . if your bike bugs around 6-7000 rpm it's a question of your valves opening . the sequence should be :
- contact > initialize on LO
- start and idle > on HI
- acceleration under 7000 > on LO
- 7500 > on HI
so you should do that (let it warm few minutes) and watch if it's as i show you
____________________
the worst day fishing is better than the best day working

[Adrian Phillips]

Thanks again. I will try this later today, I assume high is valve open hence unrestricted exhaust port. Know what you mean about warm up, something I've managed to install into the boy as well, seen so many of his mates on various machines start up and rev the engine something wicked. Oh by the way- you suggest 2nd notch down on needle, is that for a four position needle and would it still be correct for a five position needle or should I go to 3rd notch. [Picked up from one of your other postings]. Thanks.

[alains]

yes valves open is HI , closed LO
the pulley has a little arrow and on the plastic tray (which hold the servo-motor) these marks are shown
when the controler is restricted the arrow does not reach the HI mark , it stay few mm before
about the needle , the notch to set is function of your jets and the jetting is function of restrictions and air quantity reaching the carb
____________________
the worst day fishing is better than the best day working

[Adrian Phillips]

Thanks Alains, that was quick. Will try checking RC valve tonight. Running on 138 42 65 jets with needle 2 notch down on FD carb. If running lean, by checking plug, move to 3rd notch on needle. Will let you know how it goes. Thanks for all the help.

[Adrian Phillips]

Okay, have checked the RC valve sequence and that seems ok, have also checked the adjustment. Valve moves to Hi around 8/9000. Bike is running a lot better with the new jets, thanks. The only thing that seems not right is that it seems to misfire/bog-down at around 6 to 7000 rpm. Under hard acceleration in 1st 2nd it goes through ok but when building up through the revs normally it has seems to choke at 6/7000rpm. I take it this is not normal, any ideas greatly appreciated. Have cleaned coil contacts, plug is only a couple of weeks old.??

[alains]

remember this : you cant get a smooth and powerful engine before at least 5mn warming
to control your mixture : after 50 miles with a new plug , take it off and look the color on this table
https://www.dansmc.com/Spark_Plugs/Spark_Plugs_catalog.html
____________________
the worst day fishing is better than the best day working

[Adrian Phillips]

Okay, I think the time has come to put everything back together and see how things go, it is certainly better than it was and the boys happy. Thank you for all your help and tips. I O U a beer next time I'm in Geneva. Happy biking and fishing.