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Carb and throttle cam questions; generating speed.

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I'm kinda sick and tired of my 07 125sx and generating speed...

Two issues to discuss... 1) carb related, 2) throttle actuation.

1) When I'm exiting a rut or corner where the RPM's might be low, when I go to yank the throttle it wants to stall, rather then go anywhere. Even if I'm in gear that might be too low, IE higher RPM's... But at the right gear, forget it, the only way to generate power is to use the clutch.

- You can steadily open the throttle and its fine, but usually thats the last thing you want to do on a 2 stroke, its all about go, now, go!!!

- You can slip the clutch, but all that does is slide the rear around and make you do more work as a rider.

Resolution: I have tried to change out the pilot jet with the OEM one (48) and its just made things worse. The previous owner put in a 42, which restricted the fuel enough so when you yank open the throttle, the mixture isn't so rich. Perhaps going one step even lower, like a 40 might resolve it, what do you guys think?

2) My biggest complaint about this bike in terms of getting to the power is the stupid throttle tube.

- You have to re-grip to get the throttle FULLY open and the only way to get the bike to move out of its own way is to, get it fully open.

- When you do re-grip, you have to un-regrip, but if you're entering a corner and you're still gripped at the position where you were, umm, you have tuns of throttle coming in to the corner, making bad things happen.

Resolution: I'm thinking about buying an aftermarket throttle tube with adjustable cam's so I can find the one which will work for me. Any suggestions, comments, has anyone tried this?

These are my issues, let me know what you think.

thanks

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Jetting, low compression or both. A G2 throttle cam would be a good idea. A JD Jet Kit to make the jetting easier and compression needs to be over 130.

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I agree, something is off. Jetting, worn out top end, PV adjustment, PV put together wrong in the cyl. KTMs have great power so its not right per your description.

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Its a new motor build, thing has 210psi of compression.

It works very well and it had this "issue" before I rebuilt the motor.

It also has new reeds...

Now someone else had mentioned the power valve. I would assume its in operating condition, but I was told how to check it, so I will check it tomorrow when I strip the bike down, perhaps its related to the issue.

I personally think its a jetting issue, as it has gobs of top end for miles... but gaining access to it can be somewhat of a hindrance.

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Has it always been this way? Have you owned other 125's? I am wondering if it's just the nature of owning a 125. If not that, I would experiment with the jetting. It is more involved than changing a pilot jet. +1 on the rear sprocket may get you into the power a little sooner making it more responsive.

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Has it always been this way? Have you owned other 125's? I am wondering if it's just the nature of owning a 125. If not that, I would experiment with the jetting. It is more involved than changing a pilot jet. +1 on the rear sprocket may get you into the power a little sooner making it more responsive.

I have a KX125 as well, it doesn't have this problem. But then again, its nowhere near the speed of the KTM. These are the only 2 strokes I have intimate experience with. I have ridden a lot of 4 strokes, but they don't have these issues at all.

It has always been this way since I got the bike and the previous owner who was a mechanic said his kid won two championships on it, so just live with the problems. I mean, how arrogant is that!

The bike has plenty of power, way more power then needed at the top end. But getting out of slow corners, ruts and such, it just wants to land flat on its head. You either go in a gear too high and need to shift when exiting, or you go in the right gear and deal with the bog. Brining the bike to life with the clutch is also not a wise solution as it winds up brining the front tire out of the rut sometimes. I've also had issues with sliding the bike into a corner and yanking on the throttle out of the corner and even with the clutch, due to the nature of the low RPM's coming in, the bike doesn't want to respond.

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Pv setting may help, but i would say jetting is likely where your solution lies, IF everything else is in order.

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You either go in a gear too high and need to shift when exiting, or you go in the right gear and deal with the bog.

I think its your riding style that needs adjusting, 125's need a lot of shifting and you need to carry speed in corners. It might help you to gear slightly taller so that each gear will feel "longer" and you will not have to shift as much.

The only time you should be playing with the pilot jet is when the bike does not idle properly. Do a search for how to jet your bike properly to make sure that is not your problem

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well I would take the carb apart and tell us all of the parts in it, needle, slide too. Also look at the owners manual and see if it is stock parts or been changed a bunch. Also to check n see if the PV is working does not mean it is timed and installed corerctly either, it has to come apart for that. But start with jetting cuz thats much easier.

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well I would take the carb apart and tell us all of the parts in it, needle, slide too.

Pilot Jet 42 ( tried the stock 48, but it exacerbated the problem)

Main Jet 175 ( have not bought the 165 proper one yet, but the top end is perfect)

Needle 2nd ring from the top (have not tried adjusting this yet, but will tomorrow)

I assume when you mean slide, you're referring to the throttle plate?

Sorry new to carb's, but this is a good learning experience, the more I read and get hands-on experience, the more I understand!

I think its your riding style that needs adjusting, 125's need a lot of shifting and you need to carry speed in corners. It might help you to gear slightly taller so that each gear will feel "longer" and you will not have to shift as much.

Yes, 125's do require a lot of shifting and I agree that they need to carry more speed then your average big bore bike. But honestly, if I can't go through slow 180 degree ruts with a 125, then what's the point of having a 125?

The issues I'm having are for sure accentuated by my stop and go riding style, very much a 4 stroke way to ride, but the issues are unique to this bike as my other KX125 doesn't have the same issues at all. They both have the same gearing and pretty much the same transmission ratio's, so the only difference is the KX probably has no carb issues.

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Hmmm...this is an interesting issue.

Could it be a lean bog? When you snap the throttle open down low and don't have fuel to feed the engine it can bog/hesitate on you. The same malady plagues 4T motors as well...grab a bunch of throttle with low revs and she bogs from fuel starvation...hence "quick shots" and other mechanisms to squirt fuel into a snapped open throttle on a 4t to prevent a lean bog.

On a 2-stroke motor, needle position becomes important with throttle positions open beyond 50%. Raise the needle one clip position and see if the hesitation/bog improves. Also...double check float operation and check float level to be sure you have plenty of fuel in the bowl.

That's all I got. Hope it helps.

-Jay

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Update,

Rode for a few hours today with the revised carb settings (air screw) and the "issues" arn't as pronounced as they were before I started working on it. I focused on carrying more corner speed and that of course helps a great deal.

Now it still bogs if you whack it open, but I tried to be more smooth on the throttle opening and it helped a great deal.

I'm also ditching the idea of changing throttle tubes. It turns out the carb settings were the reason why the bike felt like it was struggling for power at the top end.

I'm for sure going to buy the right main jet and toss it in there as a "test" but I'm fairly sure the reason why the bike was setup the way it is right now, by the previous owner has some merit. The stock pilot jet made the throttle opening more snappy then it needs to be, so just putting back the smaller pilot jet made a considerable difference in the bike's reaction to simple inputs. This helps because you're always giving the throttle input, even if you don't know it and the subtlest of input was causing the thing to rev up quickly. Maybe thats good for some riders, but for me I like it to "ramp up" with some smoothness.

I wish the track I rode today was in better shape, or I'd post the video I shot, but it was such a train wreck, I had a hard time getting into the groove. But at least I tested things, which is more important.

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Update,

Rode for a few hours today with the revised carb settings (air screw) and the "issues" arn't as pronounced as they were before I started working on it. I focused on carrying more corner speed and that of course helps a great deal.

Now it still bogs if you whack it open, but I tried to be more smooth on the throttle opening and it helped a great deal.

I'm also ditching the idea of changing throttle tubes. It turns out the carb settings were the reason why the bike felt like it was struggling for power at the top end.

I'm for sure going to buy the right main jet and toss it in there as a "test" but I'm fairly sure the reason why the bike was setup the way it is right now, by the previous owner has some merit. The stock pilot jet made the throttle opening more snappy then it needs to be, so just putting back the smaller pilot jet made a considerable difference in the bike's reaction to simple inputs. This helps because you're always giving the throttle input, even if you don't know it and the subtlest of input was causing the thing to rev up quickly. Maybe thats good for some riders, but for me I like it to "ramp up" with some smoothness.

I wish the track I rode today was in better shape, or I'd post the video I shot, but it was such a train wreck, I had a hard time getting into the groove. But at least I tested things, which is more important.

If you are running a stock triple taper needle, that is where a lot of your problem likely lies. You never specified which needle and clip position you have. A bigger/fat pilot gives a nice meaty pull, typically a leaner pilot makes a bike jerkier off idle. Main jet only affects mid to WOT. The needle controls how the power comes on, along with the PV setting.

There's likely a better needle for your application. Also, I rarely hear of anyone going down on a main jet unless they are riding at elevation or high temps. More fuel equals more power to a point.

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You never specified which needle and clip position you have.

Its above, but here it is again:

Pilot Jet 42 ( tried the stock 48, but it exacerbated the problem)

Main Jet 175 ( have not bought the 165 proper one yet, but the top end is perfect)

Needle 2nd ring from the top (have not tried adjusting this yet, but will tomorrow)

Its not the 3 ring needle, it has 5 or 6 positions. I will have to look at it more, but its for sure more then 3 positions.

A bigger/fat pilot gives a nice meaty pull, typically a leaner pilot makes a bike jerkier off idle. Main jet only affects mid to WOT. The needle controls how the power comes on, along with the PV setting.

Explain the PV setting... I thought I understood, I'm kinda lost now...

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Its above, but here it is again:

Pilot Jet 42 ( tried the stock 48, but it exacerbated the problem)

Main Jet 175 ( have not bought the 165 proper one yet, but the top end is perfect)

Needle 2nd ring from the top (have not tried adjusting this yet, but will tomorrow)

Its not the 3 ring needle, it has 5 or 6 positions. I will have to look at it more, but its for sure more then 3 positions.

Explain the PV setting... I thought I understood, I'm kinda lost now...

The needle will have a code stamped on it like NOZG or something similar, we would like to know WHICH needle is in there. :thumbsup:

Read thru the PV pictorial in the FAQ. It will help you understand what is going on. there is a set mark and inspection window on the shifter side, the location of that mark determines when the hit comes on.

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Ok so I took the carb apart again; here are the missing specs.

Pilot Jet 42

Main Jet 175

Needle R1469D

Slider 55

OK...

Go here and look at page 2: http://www.ktmshop.se/documents/16152_Teknisk_Data_Motor.pdf

Look at the box labled basic carb settings and then read the 125SX section on the left.

You are using the stock OEM needle but not in the OEM position.

Your main is too small...main jet selection with the OEM needle is 188-190.

IMO...you should return the carb setting to OEM spec and start adjusting from there. Your current carb hardware is quite a bit off from factory settings. IMO the oem settings are the best place to start...bear in mind you will need to also adjust your fuel/oil mix ration to oem as well.

-Jay

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OK...

Go here and look at page 2: http://www.ktmshop.se/documents/16152_Teknisk_Data_Motor.pdf

Look at the box labled basic carb settings and then read the 125SX section on the left.

You are using the stock OEM needle but not in the OEM position.

Your main is too small...main jet selection with the OEM needle is 188-190.

IMO...you should return the carb setting to OEM spec and start adjusting from there. Your current carb hardware is quite a bit off from factory settings. IMO the oem settings are the best place to start...bear in mind you will need to also adjust your fuel/oil mix ration to oem as well.

That data you linked me to is for a 2000 year, mine is an 07. Now they might not have changed very much, but according to my 07 manual, my main jet should be a 165 and my needle should be on the 2nd setting from the top...

So yea, it is kinda confusing on this end!

Thanks for the document though, thats a kool piece to have for reference (if it is accurate)

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That data you linked me to is for a 2000 year, mine is an 07. Now they might not have changed very much, but according to my 07 manual, my main jet should be a 165 and my needle should be on the 2nd setting from the top...

So yea, it is kinda confusing on this end!

Thanks for the document though, thats a kool piece to have for reference (if it is accurate)

Sorry...here's the 2007 model year link. http://www.ktmshop.se/documents/7671_Teknisk_Data_Motor.pdf

Virtually the same. Yes...it's accurate.

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