2-stroke tuning problem

[UNICRONICUS]

Not sure if there's any 2-stroke engine enthusiasts in here but I am having a little bit of a problem with the running of my NSR125. Low end revs and high end revs seem fine with the throttle wide open, but if I hold the throttle on halfway and the revs get to 5k rpm it gets all jumpy and hesitates. It will do this until it reaches 7k (when the exhaust valve opens) and then if I open the throttle it breaks through this barrier. Would this be a case of a lean or rich mixture?

Any help is much appreciated!

[R5GTTRaider]

sounds like a sparg plug air filter or mixture screw

to be honest all 2 stroke hesitate on part throttle, depends how bad it is,

get a vid and ill take a look/listen as im in to 2 strokes and owned a old ns125r few years back, got a aprilia af1 and husqvarna sm atm!!

[UNICRONICUS]

Nice, mine's the older 1992 NSR, but they are similar, if not the same, engines. The difference being with mine is that I have just fitted a 34mm Dellorto carb on it that was fitted to an RS125. I left the jets standard (155 main and 60 idle) and I put the needle clip in the middle instead of right at the top. It isn't that bad, it's just annoying when the bike hits mid range revs at part throttle, but WOT is fine... I will try and get a video of it, hopefully my mobile should pick up the problem ok.

[rs250nut]

Nice, mine's the older 1992 NSR, but they are similar, if not the same, engines. The difference being with mine is that I have just fitted a 34mm Dellorto carb on it that was fitted to an RS125. I left the jets standard (155 main and 60 idle) and I put the needle clip in the middle instead of right at the top. It isn't that bad, it's just annoying when the bike hits mid range revs at part throttle, but WOT is fine... I will try and get a video of it, hopefully my mobile should pick up the problem ok.

I dident think that prilla 125's had 34mm carbs on, how clean is the power valve, did you have this problem before you put this carb on?

[UNICRONICUS]

They aren't but it's a very common mod, makes a world of difference especially at low rpm compared to the standard 28mm carb. I never had problems before, I was just sick of the poor low end on it so I started fiddling with things like I do... I think the carb just needs a tune but maybe I'm expecting too much? It is loads better performance wise though.

[R5GTTRaider]

sounds like you have the wrong jets for the engine , yer the later rs125's had 32 carb. and the eairlier ones having mituki 28mm carb

there not the same engine but i have got a 190cc bore kit with piston/ring, head cylinder and gasket kit

[UNICRONICUS]

I was (still am) tempted to get the Polini 154cc big bore kit for my bike but I think I will stick to a 125 for now until I get my bike license. I've ordered a couple of jets from Dellorto so I'll have a fiddle, do some plug chops and see what the best combination is, I do want a performance exhaust as well but they're like £280!

[UNICRONICUS]

Just got some info from the sales guy at Dellorto carbs and he says that the atomizer is probably causing the problem, that or the needle setting, or a combination of the two. Just ordered a new atomizer that is two sizes up from standard, hopefully this should sort it out!

[R5GTTRaider]

never heard of them beeing a problem, the main jet sits underneth that in the same thread so not sure, tho give it a try, 2 stroke always have hesitation just like the gtt does, its carbed fuel ratio is never perfect unless wot, so half throttle or any were between dosnt go down to well with the engine

[UNICRONICUS]

Well that's what rings a bell! I had a very similar problem with my old GTT and as soon as I changed the air corrector for a different the problem was solved! And you're right, it does sit beneath the main jet which I didn't realize before. Our KX250 is a 'bit' like this on part throttle but no way as bad, that again is brilliant on WOT, damn ring-a-ding-ding-dings!

[R5GTTRaider]

its all down to afr mate dont for get
1.theres one piston to regulate rpm
2.it fires every 2 strokes

so if the afr is wrong it becomes unstable firing and causes the jerking/hesitation.

have you got a k/n stlye filter of stock box?

also try going down one notch on the needle, if this dosnt solve any thing go 2 up (1up from original setting) and see what that does, takes 5 mins to take carb top off.

also try and determine which way made a better result if any and get back to me,

and im not talking a blast up the road and judge im talking a few miles, wot, pottering around messing about in traffic

but also it could be your air corecter screw, invest in a colour tune

[UNICRONICUS]

I've been playing with it today and I've come to the conclusion that it is fine and anything else I do to it will make little to no affect. I have the stock air box in situe as the K&N type filters throw off the AFR's big time and can cause a lean mixture, so what I've done is drilled holes in the airbox to get a little bit more air in there but not too much.

It is mainly hesitating during the first 5 minutes of the bike warming up, once it's got up to temp it is fine, or better anyway! I think I'm being too pedantic, especially as these engines are so simple and hard to tune perfectly, plus I've fiddled with all settings on the needle but to no avail. Once thing I do need to invest in is a proper sports exhaust, I hear this coupled with the 34mm carb makes a world of difference, it's just that they cost circa £250... One day...Once day...

Cheers for your help anyway dude!

Dan