RX125 Tuning - Jetting

[Muzzab]

Any 2-smoke experts have any tips on Jetting a RX125.

The main jet my bucket came with did not appear to have a number on it.
I changed it to the one from my parts bike, which I assumed would be the standard size. To see if it made any difference. I don't think it did.
The one in it now is a 125. It starts and runs fine and gets up to about 130kmph on a good day, which I think is not too bad for nearly 30 year old bike with a 100kg rider. But am curious to know if a 125 jet is considered big or small. The bike hits its powerband at about 6000 rpm and pulls to about 8500 rpm.

If anyone has a selection of jets for a RX125 surplus to their requirements I'd be keen to get some to experiment with.

Cheers
Muzza

[all4A50s]

have you looked at adding an expansion chamber. I'm running an Rx125 engine and increased the power band so it kicks in just above 4 thou and tops out at 9 thou.

Have you looked art lifting the needle?

[Muzzab]

have you looked at adding an expansion chamber. I'm running an Rx125 engine and increased the power band so it kicks in just above 4 thou and tops out at 9 thou.

Have you looked art lifting the needle?

I should have said it has a chamber. The clip on the needle is one from the bottom, if I move it down and lift the needle what should I notice? I'd be pleased if I could get the power starting a bit sooner.
What size jet are you running?

[SHELRACING]

Two strokes are fussy at the best of times, depending on the carb, pipe you are running, petrol, the two stroke oil, mixture ratio... Blah blah. Taking my RX from Taumarunui to Auckland Requires a jetting change.

Idle mixture is adjusted by the AIR (not Idle) screw on the front or back of the carb

Mid range mixture is adjusted by moving the the needle up and down on the clip

The top end mixture, maybe the last quarter of the throttle is controlled by the main jet.

Set your mixtures in order Idle, Mid range, Main. Changing anything suggested previously may effect your jetting

125 is probably a good start for the main. Keep an eye on your plug it should be a brown colour for correct mixture. Careful not to run too lean (grey Plug)

I'd say you were doing very well to be getting 130kmh. That would be faster than you could ride on a kart track.....

Good luck

[Muzzab]

Two strokes are fussy at the best of times, depending on the carb, pipe you are running, petrol, the two stroke oil, mixture ratio... Blah blah. Taking my RX from Taumarunui to Auckland Requires a jetting change.

Idle mixture is adjusted by the AIR (not Idle) screw on the front or back of the carb

Mid range mixture is adjusted by moving the the needle up and down on the clip

The top end mixture, maybe the last quarter of the throttle is controlled by the main jet.

Set your mixtures in order Idle, Mid range, Main. Changing anything suggested previously may effect your jetting

125 is probably a good start for the main. Keep an eye on your plug it should be a brown colour for correct mixture. Careful not to run too lean (grey Plug)

I'd say you were doing very well to be getting 130kmh. That would be faster than you could ride on a kart track.....

Good luck

Thanks for that, The bike starts and idles fine, so I will adjust the needle to see what happens in the mid range. I'm not entirely sure how accurate the speedo is, most of my racing has been this year over at Ruapuna and a few days at the local Kart Track. The bike is to high geared for the Kart track and gets to 80-90kmph by the end of the straight and just into 4th gear.

[Undertow]

Sounds like your RX is doing pretty well, I'm only 70kg and mine occassionally hits 135km/h.

Sorry, if I hijack the thread but does anyone happen to have a full exhaust for an RX?

[F5 Dave]

ok I'm going to piss all over this. No one on earth will be able to tell you what jets to run in your modified bike. There is no shortcut. You have to experiment.

ok 2nd; write this down & refer to everytime you think of 2 stroke carb tuning;

-The jets are in relation to throttle position.

Do not use phrases like 'midrange' that implies revs. These are not CV carbs.

Full open is where you run the bike on a long track & mainjet will be how you experiment using jets from same manufacturer (ie genuine mikuni or Keihin jets have logos on them). One step at a time. Then once that is set (at least on the day) consider how the bike works on less than open throttle.

[Muzzab]

ok I'm going to piss all over this. No one on earth will be able to tell you what jets to run in your modified bike. There is no shortcut. You have to experiment.

ok 2nd; write this down & refer to everytime you think of 2 stroke carb tuning;

-The jets are in relation to throttle position.

Do not use phrases like 'midrange' that implies revs. These are not CV carbs.

Full open is where you run the bike on a long track & mainjet will be how you experiment using jets from same manufacturer (ie genuine mikuni or Keihin jets have logos on them). One step at a time. Then once that is set (at least on the day) consider how the bike works on less than open throttle.

Thanks for that, This is kind of where I started, what range of jets should I buy? I'm assuming I'd only want to go bigger, but wanted to get an idea of whether the 125 was nearing the maximum practicle size. Any suggestions?
130? 140? 150? Cheers Muzza

[Muzzab]

Sounds like your RX is doing pretty well, I'm only 70kg and mine occassionally hits 135km/h.

Sorry, if I hijack the thread but does anyone happen to have a full exhaust for an RX?

Are you after a standard one? or a chamber? I have a standard RX125 chrome
exhaust and a rusty header. I do not know if the exhaust is any good as the bike it is off was seized when I brought it. It would cost more to freight to Auckland from here than what it is worth.

Cheers
Muzza

[SHELRACING]

You're quite correct Dave and i didn't quite word my response correctly. I would conceed you and a few others on this site are more experienced than me. No need to be rude tho.

I have seen standard RX's running on a 120 jet so suggested a 125 a good place to start if the bike appeared to be running ok which it does. As I suggested there are many cosiderations to two stroke jetting. My RX is actuall running on quite a bit higher jetting than 125, and it took me quite a while and a bit of Help from TZ350 to get things right

What I meant by my poorly worded response was throttle position.
Idle screw = stop to 1/8-1/4 throttle position
Needle = 1/4 to 3/4 throttle position
Main jet - 3/4 - full throttle position

Please correct me if I am wrong, You should check your mixture at each of these throttle positions, I used this is the method to solve several mixture related problems on my RX.

One thing I am sure of is to regularly check the Plug. Especially if you change anything, even your 2 stroke oil.

[Muzzab]

So...quite a bit higher jetting than 125 means ????? This is the bit of info I'm after. The plan was to push it out till it went overly rich and then step it back.
I just wanted to get an idea of what would be a large jet for a Rx125.

[bucketracer]

.

Some pictures that might help explain things.

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[SHELRACING]

So...quite a bit higher jetting than 125 means ????? This is the bit of info I'm after. The plan was to push it out till it went overly rich and then step it back.
I just wanted to get an idea of what would be a large jet for a Rx125.

Well as F5 commented I couldn't presume to tell you the correct jet for your bike. the jet sizes in my bike would not work in yours because of the various modifications in my bike. You have to experiment. I had my bike running. To a similar point to yours then brought 3 jet sizes either side of that and experimented from there.

Remember the main jet is affected by approx the last quarter of throttle movement. So that's the throttle position to run the bike and check the mixture.

[Buckets4Me]

So...quite a bit higher jetting than 125 means ????? This is the bit of info I'm after. The plan was to push it out till it went overly rich and then step it back.
I just wanted to get an idea of what would be a large jet for a Rx125.

haha just to throw a spanner in the works

and I know the gp's are a different beast

but you MAY need to go down in jets just as much as you may need to go up

TZ's bike ran better and 20% more HP after droping a few jet sizes (and advancing the ignition) about a 110 main I think and 20 hp

if you run to rich you controll detonation better but loose power
the leaner you run the closer to the right fuel ration and more HP (unless you run to lean then you start to loose HP again water cooled engines are easier to get closer. Better cooling)

BUT and a big BUT the hotter the engine runs and the more chance of detonation

saying all that the standard gp125 engine starts with a main jet around 85-90 ?? and your rx had a 120 in it

good luck and dont blow the engine up

so to answer you question it's big compared to my gp125 and small compared to shelracing ?

[all4A50s]

After talking this over with a couple of people, the main question I got asked was why are you starting with the jetting so start off with? The could see the attraction (of trying to simply get more in to the combustion chamber), but felt that an expansion chamber would make a much bigger and immediate improvement than changing the jets. I noticed a couple of good deals on TM for suitable donors using 1/2 a 250 2-stroke exhaust.

The next questions was has he cleaned up his ports to warrant changing the jets? As it is worth spending sometime with a demel type tool and simply cleaning up the casting first, trying it out with the needle set at different levels. Or in other words taking one step at a time, and saving your money for later when you have identified where to best to go next.

If its any help this is the process i am following with my bucket (an RX125 engine in a RG50 rolling chassis).

Identified engine and frame I wanted/could afford/had access to

The fun of fitting something into something that shouldn't, including riding position, fairing, etc

Sorted out suitable exhaust expansion chamber

Worked out gearing (sprocket size(s)) suitable for where I am racing.

Sorted out tyres i am happy with/have access to.

Got the bike to run reliable

Had fun with some paint.

and am now working on porting, needle height, etc

and then if I need to jetting.