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Hi, my name is Sam
Ive always researched on the internet and used these style threads to gain knowledge on what I’m trying to do. For the last year I’ve been tuning a Honda NSR 125 F 1991 JC20. I’ve done a fair bit of work on it and spared no expense, now with that Ive had a load of headaches and “what if I did this” moments. For example a 34mm carb, jolly moto exhaust and a VHM head. So I thought I’d give my 2 pence and if anyone else had any curious thoughts like me they can hopefully have those questions answered by reading this. First I’ll give an overall state of the bike as it sits right now and then a list of all the modifications I’ve done to it.
so right now the bike is in what will most likely be it’s final state, I’ve invested around £3300 into it and I think I’m near my limit. Right now I’ve got the fairings off and am restoring them, going to have a professional job on them to finish off the bike. Ive completely stripped it. Every single bearing and bush and seal in the whole bike I’ve replaced. Every rubber seal in the brake assembly, the headstock bearings, the wheel bearings and seals, the forks etc etc it’s all been done. It rides lovely and like a brand new bike. The whole frame and all metal parts have been powder coated, some bits better than others. I’ll never go back to the place I went to get them done. SP blasting in Melton did a hideous job of the bike and charged me 600£.Took little over 3 weeks which one would assume is a pretty good turn around, however things went back to him 3 times and still were patchy or not covered properly. Nether the less I’ve learned something doing business with the company and will not be going back, woukd advise no one to go there. Now, I’ve restored the Honda the way I like it and I completely get it’s not going to be to everyone’s taste, I’ll try include some pictures, I’m new to this and have no idea how too. As I say it’s not done yet because of the fairings. But I hope everyone can appreciate me bringing an old Italian 2 stroke back to life.
THE FRAME
  • I’ve got Bridgestone battle axe wets on it front and rear.
  • brand new OEM rear brake disk.
  • stuck a good condition one on the front that I had spare in a box because I couldn’t find a new one online.
  • brembo brake pads front and rear.
  • both brake calipers have been rebuilt. and with new seals, with the front getting a 90% brake efficiency test on it’s MOT.
-it’s got a 33 tooth rear sprocket from pjme, and 14 tooth from them too, this is all little different to the standard 13,35 setup from stock as I wanted to reach 100mph, a little goal of mine.
  • it has a new snell 520 chain
  • the original forks were in terrible condition and it was more cost effective to buy JC22 raiden forks to replace them which are a straight fit into the original yolks.
  • A tyga carbon fibre front fender which fits the jc22 forks, I think it looks Awsome, was about 130£
  • has new veinhell red brake lines front and rear
  • it has aftermarket clip ones
  • it has CNC black brake and clutch levers ( yes the ones from eBay)
  • finally, I rewired the switch gear from a Sachs Xtc 125 on both sides as the original nsr switch gear was just terrible and very worn. The Xtc had standard domino throttle setup so it was better in my opinion than standard.
THE ENGINE AND CARB
The engine and carb set up has been the biggest head ache I can assure you. I wanted it to remain a 125 as to keep some of its originality. I’ll be more than happy to help anyone who wants to do these mods to their bike if anyone is so inclined:)
  • VHM cylinder head with a standard insert.
  • NGK iridium spark plug, BR9EIX
  • full JOLLY MOTO exhaust system
  • full engine rebuild (top and bottom)
  • brand new piston and rings
  • Re-plated nickel bore
  • new engine and gearbox seals
  • new crank bearings
  • New clutch plates and springs
  • running on premium motul oils
  • new starter motor
  • new battery
  • the plastic air flow restricter from the Reed cage was removed
  • it has carbon fibre reeds in the standard Reed cage.
  • new Samco Red coolant hoses
  • It has a brand new genuine 34mm LD DELLORTO carburettor
  • a custom 3D printed inlet manifold
  • a ram air pod filter
Now it’s a fairly long list I realise that aha, I can go into detail as everything as a step by step install and power gain of each thing, but I’ll be here all day. I did it all myself except from actually rebuilding the forks and rebuilding the bottom end of the engine. Just because sometimes it’s better to get a professionals experience and point of view. I’ll cover the big mods now though just to save anyone time.
CARB AND REEDS
The reeds gave it a more crisp response and gave it a nicer tone. When I let go of the throttle it didn’t hesitate as it did with the old reeds. Now the carb cost me 250£ and I woukd say it’s 100% worth it. I’ve got it running rich with a 172main jet, no idle jet just the emulsion tube, I think?! I can’t really remember it was some months ago I did it, And a 60 choke jet. It has a load more power. It’s a 100mph plus bike and the 34mm carb really shines with the jolly moto exhaust. It is such a nice carb, it’s easier to work with and does fit nicely in the bike. If anyone needs the contact I have to make any manifolds just get in contact and I’ll point you his way. He’s very good and very quick and kind. He’s located in Europe somewhere I cannot remember where, but within contacting him only 2 weeks later it was in my bike. He solved my impossible idea of putting a 34 on the Honda. Only charged me 40£ Too?!
EXHAUST
The jolly moto exhaust is by far the most expensive exhaust I could find for 480£ delivered from Italy. Now yes arrow exhausts are great, I had one on my aprilia rx, however it reved so high it was like a grenade. My Honda still revs to 12000 if I wanna kill it. But the power band hits at 8-9k and it is a lovely thing. It’s got way more grunt than the arrow and tyga do. Low end power is certainly there to put about on. I can ride it below the power band if for some reason I left my balls at homer I can walk the throttle back and feel like I’m going through warp speed ahah. My only thing that I didn’t like about it was that it’s not treated with anything from the shop. Arrow treat their exhausts with a clear coat of some description, whereas jolly moto leave it bare, so within one small winter worth of riding it’s red rusty. Which is my own stupid fault for riding it in the winter I just couldn’t stop. I love the bike so much and it’s such a drug to ride. But yeah, if you buy the jolly moto which I extremely support you to do, be prepared you may have to take it upon yourself to treat it. It has an amazing tone that isn’t too load at idle, but when u hit the powerband man you hear it sing.
AIRBOX
Eveyone says stick with your stock air box, but I disagree. In regard to my gains with the Honda NSR JC20 engine it loves it. It has a HUGE difference in the exhaust note, finally sounded like a propper smoker, and it is able to suck in air easier. After fitting one I can hear it breath much easier. Definitely a worth while mod even without the 34mm carb. Plus it’s so much easier than fitting the stock air box, it’s very finicky and fiddly.
VHM CYLINDER HEAD
Now I know that these are worth about 250£ which is extremely expensive for me, but my old one had a crack in the thermostat holder at the top of the head and it began to leak so I thought why not go for the vhm head if I’m going to have to buy a new one anyway. I’ll start of by saying it looks amazing. Really shines up nicely, very easy to install and anyone can do it. I have not got it dynoed so I couldn’t tell you if those hp gains that you are promised are accurate, but i bet it does. It most likely burns better, with the iridium plug and working powervalve I bet it helps. It’s a much nicer head than the stock, and when I do go to dyno it, it may be that I need to get a slightly different insert inside the head and that’s no problem as they are easy to swap.

CONCLUSION
Now I really should dyno it to see what I’m pushing. If I wanted to do anything next I’d probably tune the carb a little better and put a racing piston and crank in. However with the bills stacking up for it I don’t think I will realistically. I’m not a rich man and I work 40-45 hours a week evey week. I’m only 18, started the Honda project when I was 17. I was riding an R125 2018 at the time but sadly I got hit by a car, bike was written off, other driver was at full fault, ( driving without due care and atterntion and over the limit on coke) and police arrested him, I went to the hospital on a stretcher. I have got an injured arm and probably will do for life, and had to take a break from riding. I love riding bikes and can’t get enough of them. I have a YAMAHA FZR 400 1988 right now which I will ride when I complete my A2. But I will always love 2 strokes. Hope everyone who reads my tuning story can appreciate the modifications I installed. I tired to build a bike that gave me chills yet was still a 125. I know 125’s have not got big bike power and some people will just say buy a bigger bike. But to me it’s nice to tune this bike. I learnt so much and it’s out a smile on my face.
So speed, right now I havnt been to the jewel carriageway to test for top speed. But on my country roads I’m going 85mph top of fifth on a wet and windy day(posting this in winter) . So I’m assuming it’s a 100mph bike. Hp wise I’m looking at 25 I hope, I would love to stick it on a dyno and check. The JC20 are not the best I know that, they are quite reliable though and I believe more robust than it’s fellow 125cc 2 strokes being a Honda ;). The rotax and mito engines rev higher, they probably have a shorter stroke, they are probably machined a lot better, but this is the bike I have so yeah. I have a friend with a cagiva mito six speed in stunning condition, fully derestricted with an arrow exhaust and only 7,000 miles. My Honda beats his bike up to about 85mph by a fair bit, and then his bike takes me from then on. He revs his engine to about 15-16k apparently which is a testament to the engineer who built and designed the mito engines. But where my Honda is Reving at 12 k his mito has to rev to 16k to keep up. That might have something to do with the exhaust setup but also the porting and engine quality. The nsr 125 isn’t a race bike like the mito is, compare the 2 and you can really visibly see the differences. One day I hope to own an NSR 250, hopefully an mc21 or an mc28. But who knows. If anyone could value my bike I’d also love to hear what you think it’s worth, as many of us know, what we put into them sometimes doesn’t change the price they are worth. Happy to chat to anyone who has any questions and any advice as to what I might be doing wrong. Always appreciate someone who knows more than me about 2 strokes.
Thank you, Sam
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