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Offline Sapper

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breaking up on low side.
« on: August 09, 2010, 03:02:31 PM »
03 kx125. It has V force reeds, Phathead with one size under stock chamber new top/bottom end, etc. We dropped 2 pilots and 3 mains/ It is breaking up at part throttle. When it hits the powerband it flies. Is this in the jetting or the cdi/coil? If jetting which way? I'm thinking if jetting, its in the pilot or needle clip. Stock pilot was #40. 35 in it now. main was 420, now 390. Clip at 3rd from top, screw out 2 1/4 turns.

Any ideas??? I'm not sure if I want to lean out the pilot any more or if it actually needs it. I can't figure out if the breaking up is a lean or rich or ignition.
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Offline Coop

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Re: breaking up on low side.
« Reply #1 on: August 09, 2010, 04:22:24 PM »
This is a good way to determine if you have the correct pilot in. It's from Spanky's Jetting Guide:

"Turn the air screw all the way in, then turn it out 1.5 turns to start. Start the engine, and turn the idle screw in until you get a slightly fast idle, or hold the throttle just barely cracked, to keep the engine idleing. Turn the airscrew slowly in, and then out, until you find the point where the idle is fastest. Stop there. Do not open the screw any farther, or your throttle response will be flat and mushy, and the bike may even bog. This is only the starting point, we will still have to tune the air screw for the best response.

Now is the time to determine if you have the correct pilot installed in your carb. The air screw position determines this for you, making it very simple. If your air screw is less than 1 turn from closed, you need a larger pilot jet. If it is more than 2.5 turns from closed, you need a smaller pilot jet."
- Mike - Don't take life so seriously, nobody gets out alive.

Offline Friar-Tuck

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Re: breaking up on low side.
« Reply #2 on: August 09, 2010, 07:12:09 PM »
 Hey Sapper,
  The bike should be running primarily off the needle at this point, (remember all the circuits do overlap).
Does the bike start well? and how about idle? (if you run it) That will be the pilots job.
 Now Coop makes a good point on the pilot, seeing as this is probably the hottest part of summer you just may leave the pilot alone and adjust to temps & elevation.
  As the weather cools down you will probably need to turn in that A.S. 
Ignition...If the bike runs good on top I would think the electronics would be working hardest on the pipe, but stranger things have happened..

 You might try pulling open the choke after you get moving, and see if any more fuel affects the midrange.
Now it will be tough to keep the bike running while you pull the choke, but you should see a difference as you wind the bike up.  Kind of like the eye doctor, Better? or worse?  Better? lower the clip (that raises the needle, = more fuel)
  Worse?  Raise the clip (lowering the needle = leaner midrange)

If after you have tried everything and can't shake the midrange stutter, there are tons of needles available with up to three tapers.
   Let us know how it went..
  Tuck\o/

Offline Sapper

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Re: breaking up on low side.
« Reply #3 on: August 10, 2010, 06:31:34 PM »
It's really wierd. As part throttle as you're just putting along or coming out of a turn and not on the pipe yet, it breaks up bad. Like a v8 running on 6 cylinders. Then it hits the pipe and rips. It starts 1 kick, idles perfect etc.
I gotta get this lined out, it's killing him in the tight turns vs the 250f's in schoolboy class. Otherwise he'll use the mean, green 250 in the lites.
If you want cheap & fast, it won't be good.
If you want cheap & good, it won't be fast.
If you want good & fast, it won't be CHEAP

Offline Super Trucker

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Re: breaking up on low side.
« Reply #4 on: August 11, 2010, 10:55:35 AM »
Vforce reeds don,t last very long, my mod 125,s carb is bored to 40mm, 15-20 hrs. and the reeds are chipped on the corners and the reed petal don,t seal. When my reeds are in that condition the bike still starts on the 1st kick. A 40 to 35 pilot is a small change, a  properply tuned 125 does not idle, pro,s set there 125,s to almost idle, but don,t. So you can use the engine braking for additional braking. I never ran a phat head, but if you have the higher compression head, you have to use race gas.

Offline TxTechRedRider

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Re: breaking up on low side.
« Reply #5 on: August 11, 2010, 09:53:10 PM »
St are the Vforce the best or have you gone to another brand, since the Vforce dont seem to last that long?
07 Honda Cr125.   
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Offline Super Trucker

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Re: breaking up on low side.
« Reply #6 on: August 12, 2010, 12:20:07 PM »
The carb on the mod bike is bored to 40mm, that could be the reason they wearout quick, very little performance increase with the v force,engine tuners say it shows up on the dyno. I put a rad valve on  my 06 practice bike, took it apart polished the inside smoother, rough casting stock. The rad v. has a better low to mid, little more mid,top end revs out good. I would get the rad valve, don,t know how the reeds hold up though, just 1 ride with the rad valve.

Offline TxTechRedRider

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Re: breaking up on low side.
« Reply #7 on: August 15, 2010, 06:47:29 PM »
Thanks St for explaining that.
07 Honda Cr125.   
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Offline Friar-Tuck

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Re: breaking up on low side.
« Reply #8 on: August 20, 2010, 11:20:19 PM »
Sapper,
Quote
At part throttle as you're just putting along or coming out of a turn and not on the pipe yet, it breaks up bad.
   If the bike starts easily and Idles well I would have a difficult time believing the reeds are bad, but stranger things have happened.   Coop is right on about the pilot. 
   If the only time the bike stumbles is coming out of a corner it would be awful hard to blame the ignition system. 
    It should only take a few minutes to drop the needle and see if the bike responds any better.
( Once the rider figures out what rich and lean feel like it will make your job much easier.)
  Have you tried anything else since the main and pilot change? 
  Tuck\o/   

Offline ford832

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Re: breaking up on low side.
« Reply #9 on: August 22, 2010, 04:13:49 AM »
It's really wierd. As part throttle as you're just putting along or coming out of a turn and not on the pipe yet, it breaks up bad. Like a v8 running on 6 cylinders. Then it hits the pipe and rips. It starts 1 kick, idles perfect etc.
I gotta get this lined out, it's killing him in the tight turns vs the 250f's in schoolboy class. Otherwise he'll use the mean, green 250 in the lites.

Not trying to sound like an idiot(oddly enough)but this sounds alot like how a 125 runs.A buddy had one of these from new and it was all upper mid and little else.Did you get the bike this way(with reeds/head) or is it a recent development?
Obviously,I don't know your experience level with 125's-say,if it's your 50th or 1st.
If you bought it with the add ons I'd replace the head,reeds with stock and rejet to stock specs and see what you've got.
Certain bikes don't react well to certain parts or combinations thereof.
In tight turns on a 125 you need to keep the revs way up and control power to the ground by slipping the clutch-otherwise it's like throwing out a boat anchor.
If you're 125 experienced and a new issue has surfaced my apologies.I only mention it as I've seen this same complaint many times from people new to 125's.
I'd rather a full bottle in front of me than a full frontal lobotomy.

Offline Friar-Tuck

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Re: breaking up on low side.
« Reply #10 on: August 22, 2010, 10:22:21 AM »
 Thank you Ford,
 I didn't know how to put that statement down without sounding ..condescending...
  If I remember correctly, the pilot of the bike has recently moved up from a smaller machine, and the Team Mechanic has a great reputation on setting up a competitive program.   That being said, Riding a 125 is a blast, however it is work keeping the bike on the pipe.   
 Ford also makes an excellent point on a baseline or stock set up.  Modding a bike to be competitive will usually narrow the power band, albeit making more HP.  I don't necessarily mean to De-Tune the bike, but moving the powerband to the low-mid should help this rider be more competitive.  Moving the power back up to the mid-top after the team has a bit more time with the bike.

  Moving the power down to the low-mid ( kind of like Ford suggests) should be as easy as going back to stock.
The stock parts should also be less expensive!   
  I hope this helps Sapp, Please do not take this in the wrong context,  I have a lot of respect for yourself and team!
     I'm sure the rider has been pretty excited about "moving up" and then seem to get the short end of the stick on race day.  I would encourage him to hang in there!  Rome was not built in a day!  And I'm also pretty sure when you guys started the program there was a learning curve, and so it would only be natural to "start all over".
    You guys get to spend alot of time together when most kids want nothing to do with their parents. 
  Hang in there!  Moving up to the 250's too soon may be even worse on the lad, those guys might find pleasure in hammering a "New Guy"  :o
     Tuck\o/

Offline Sapper

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Re: breaking up on low side.
« Reply #11 on: August 23, 2010, 04:54:47 PM »
Well update on the issue. I leaned the clip all the way to the top notch and it run excellent. No sputter, no miss and rips out of the turns. It's like a whole new bike.

Tuck,Your last comment really had me thinking last night. He's racing open C class on the 250 and was battling for first in his 2nd, heat, last lap, one turn and 3 jumps from the finish. He was bashing bars with a good friend of ours and as they were heading out of a turn coming up on a 100' down hill triple, 4th gear pinned and they got together. The other guy weighs 330lbs, my boy is 120lbs. Well, this led into an EMS ride. But no broken bones, just alot of road rash and bruises.
But he flipped out when I mentioned taking him out of the class for the remainder of the season. He's in the run for the points. He took 1st in the first heat.

The good thing is they really don't hammer on him cause the BIG guy has his back all the time plus he gets along with all the riders. He's usually the highlight of the race.
If you want cheap & fast, it won't be good.
If you want cheap & good, it won't be fast.
If you want good & fast, it won't be CHEAP

Offline Friar-Tuck

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Re: breaking up on low side.
« Reply #12 on: August 24, 2010, 06:30:08 AM »
Well Allrighty Then!   Good News 8)
  Glad to hear you got the bug out,  I had misunderstood the rider was getting left at the back of the pack. 
Sounds to me like your program is on the fast track  :D   You guys definitely have the HP to Weight advantage..
   Mixing it up with the big guys like that takes guts!   
Take Care and Best of Luck...
   Tuck\o/

Offline Sapper

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Re: breaking up on low side.
« Reply #13 on: August 24, 2010, 08:36:33 AM »
   You guys definitely have the HP to Weight advantage..
   Mixing it up with the big guys like that takes guts!   
Take Care and Best of Luck...
   Tuck\o/

Haha, yep you should see him on the holeshots. :o
It's hilarious. The bike doesn't even know he's on there.
If you want cheap & fast, it won't be good.
If you want cheap & good, it won't be fast.
If you want good & fast, it won't be CHEAP