Honda NSR 125 performance upgrades

  • Does anyone have experience tuning these engines? The JC22 specifically.
    Today I run a Romeu pipe and everything derestricted. But it would be fun to see what power you could really get from these engines.

    I'm thinking about getting a bigger carb for starters, perhaps a 34mm.

    Then perhaps changing to a forged piston and lastly getting it ported. Opinions?

  • Anyone??


  • Yeah I'll definitely go for that! 36mm or bigger, open air filter, pin the powervalve closed and change sparkplug. Then I should beat a tuned RS.



  • @irongamer727 Massively busy at work, and with my losing my eyesight over the weekend I think I am a little preoccupied.

    You sound interested in tuning smokers, I'd highly recommend Graham Bells Two Stroke Performance Tuning.

    Bigger is not always better with carburettors. Since the P=F/A, a bigger area results in loss of pressure, which will adversely affect performance. There is a sweet spot that you must manage with carbs.

  • Delorto 34mm vhsa is supposed to give good gains, a Honda rs125 carb inlet will fit the carb and bolt straight on to nsr engine. But it mounts the carb at a different angle, so on mine I'm looking at making a angled adapter to use between.
    Make sure you have the full power cdi and power valve controller. Earlier jc20 barrels had more aggressive porting but are very rare now, and run slightly different power valves. Nsr150 engine is basically the same engine and top end will swap over onto the nsr engine.

  • @dtr-nsr yeah a 34mm seems like the way to go. Is the rs inlets hard to aquire? I'm thinking about buying reeds as well. Or is a complete cage the way to go?

  • I managed to buy a brand new rs125 inlet off ebay wasn't cheap though. The rs125 inlet fits perfectly into the nsr125 reed cage, so probably ran the same part. The rs125 barrel does look like it may fit. But not heard of one being fitted. I'm pretty sure a certain age of nsr250 barrel will fit, which may have better porting. The nsr250 was a v twin basically using two nsr125 barrels.

  • @dtr-nsr will it be a problem having the carb slightly tilted with the rs inlet? Why would you not want it that way? Guess you could adjust the float if necessary.

  • On my jc20 with the rs inlet the carb fouls on the regulator bracket. You might not have that problem on your jc22, but I think I worked the angel difference out to be 45* so it's quite a big difference.

  • Don't waste money on big carbs as not needed to get decent hp and waste lots more fuel. You would be far better off getting the cylinder ported with head done to match and get far bigger gains.

    The Aprilia RS 125 due to the 34mm carburetor has a very good midrange, it's one of the peculiarities of Rotax 123/122 engines. But if you look at the power curves of Aprilia RS 125 SP or Aprilia AF1 125 Sport Pro, the midrange is less but a lot higher revs. Why ? Their cylinder is slightly different (ports raised by 0,25mm - dedicated head lowered by 0,25mm leveling the top of cylinder ) and most important, they have a Dellorto 28mm oval section carburetor. This, together with a racing exhaust, gives more than 2HP extra power over the 34mm version! More than 35hp at rear wheel.

    The TZR 125 R SP used a Dellorto PHBH 28mm carburettor in the original SP racing championship. They went from 31Hp to 33Hp at rear wheel, depending on the special handcrafted setup they've received in the factory.

    So do not always need to go for a bigger carb but get the porting work done to suit the carb and exhaust you are running as it is a combination of all the parts perfectly matched to each other that gives you the horsepower.

  • @2stroketerry any chance you know someone able to do the porting?

  • Hi, ask Karl at kpm and he will do the porting for you and machine the head to match. He can also bore out your standard carb to suit and save money buying carbs, his prices are very reasonable and can supply forged pistons etc.

    If you don't mind waiting a while as i'm retired now and have a few nitro race buggy engines that need porting doing, then can do your porting for you but be a while.

  • @2stroketerry damn man! I have a small kyosho gx 12 at home. Never used it but my plan is to 3D print an RC car as a school project and put it in there.

  • @irongamer727 I have an e firestorm

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