Mid Range issue - CR125

[Troy V Smith]

Hey Folks - I found an issue yesterday at the track - chased the issue all day long with no success in resolving it. What I have is a mid range studder in the motor. I found the issue when trying to feather the throttle a bit through a mid speed corner in 4th gear. I was unable to hold the throttle about three quarter without it wanting to stall on me. The problem starts at 8500 and seems to clear up at around 9500.

To get the rpms, I got on the longest straight and tried running the motor at 9K in all gears - same issue - If I left the throttle in that particular postion, the motor would die on me. It pulls nice and steady all the way through 6th reaching the 12K nicely as long as I am Wide Open. The problem only presents itself in a mid range application. Low speed seems to be fine as well - exiting from the pits and such it pulls nice and clean up to 8500.

Made a couple runs with new plugs - shutting the motor down directly after a full speed run, and coasting into the pits - plug looks great! Air Density was at a 95 for the day beginning and 93 at days end. I ran 175/55 jets all day. EGT was maxed at 1180 at the end of straight, water temp was consistant at 126. Running pump gas 95oct mixed with 927 at 20:1. Air temp was about 98.

Carb is running a #6 slide / CCJ needle, clip in middle postion. Non-pump around system. Motor has been checked with no air leaks, piston and head look good (no det). I have not checked the timing, but am assuming I didn't have a shift or anything.

Any ideas as to what I can look for of how to narrow my issue? I hear many run a DGJ needle, is that right? Can anyone explain exaclty what the gains or loss is when comparing the two needles? That was going to be my next change. Thanks...
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TVS Motorsports -
"We may not be the fastest on the track...
but we're having the most FUN!"

[Venkat Reddy]

Troy

Check the ground wires by the coil, this was the cause of mid-range shudder on my mod moto.
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Stock Moto Zanardi KZ2
Stock Moto CRG RoadRebel
ICC TMK9 CRG RoadRebel
Rotax MAX Tony Kart EVR

[jim P lewis]

Troy,

Try the DGJ in the middle position, should work. Or just move the needle clip down on the CCJ. The CCJ is a bit leaner at the mid range 1/4 throttle to 3/4 throttle according to the Keihin needle chart.

Definitely sounds like a lean condition that should be needle related since it's a mid range deal.

jim
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Jim Lewis

Vista CA

[Troy V Smith]

Thanks guys very much - appreciate the input. Will give these two checks a try tomorrow and let you know the result.
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TVS Motorsports -
"We may not be the fastest on the track...
but we're having the most FUN!"

[Noah Stark]

With a non pump around set up, you arent far off. Stay with the CCJ needle, and raise the clip, esentially dropping the needle. Also try experimenting with a 52 pilot. Float carbs need jetted a bit differently, and with the air density you were running at, I'll bet great money that you are way fat on the needle. Also try a CGJ or CGK. DGJ will not help your situation, but in fact make it worse. Feel free to try richening it up first, but Ill guarantee you that the condition will not get better until you start dropping the needle.
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Noah Stark
RIP my friend:Paul Dana you will be missed by many

[Venkat Reddy]

Noah

You mean lower the clip(which raises the needle) to richen, right?

My understanding is to move the clip up the needle lean, lower to richen since the needle is tapered down from top to bottom.
_________________
Stock Moto Zanardi KZ2
Stock Moto CRG RoadRebel
ICC TMK9 CRG RoadRebel
Rotax MAX Tony Kart EVR

[Steve Buckner]

Yep, Down=up, Up=Down, Or raise the needle, more flow. Lower the needle, less flow.

[Jimmy McNeil]

Venkat Wrote

[Troy V Smith]

Uuhhuumm....what? Thanks Guys...

I'm in line with you guys with the needle and clip position, but looks like we have a difference in opinion as to whether is is a lean condition or rich condition through mid range. I haven't tried anything yet - still laying flooring in the trailer - but it's on the "to-do" list. I'll try them both - a bit richer and a bit leaner to see where the changes are!

Should we start a pool? Who's in for a "square"?

I did download my Mycrhon data and took a look. In the lap I used to narrow the issue - I used the long straight in 4th gear and tried to hold a pedal and throttle positon to maintain a steady 8500, my rpms fluctuated from 8200-9700 regularly and almost violently over a period of about 15 sec.? Above or below that rpm - motor shows great response and steady acceleration thoughlout.
_________________
TVS Motorsports -
"We may not be the fastest on the track...
but we're having the most FUN!"

[jim P lewis]

Ok some pretty sharp guys have tuned in on this one, and one really fast one, so here is my question. Does the pilot effect mid range at the 8500 rpm level as described? I thought the pilot had more of an effect right when you went to throttle?

Either way I would say a 55 is probably a little rich at the air density levels being indicated. My chart shows a 52 would be the correct main at the air density of 95.

Still sounds like a mid range lean situation. DGJ with the clip in the middle position would be a good starting point.
I would only use the CCJ when the temps were right at 100 and air density was 90 and below.
_________________
Jim Lewis

Vista CA

[Steve Buckner]

www.keihincarbs.com

Zero throttle operation - (idle circuit) idle screw: depending on your type of riding adjust the minimum idle speed to desired rpm making sure the engine is up to operating temperature
If you do not desire any idle make sure you turn in the adjusting screw just enough so the engine will not idle. This is especially important on Keihin PJ series carbs in that the idle adjust knob (#4 in illus.) cannot be completely closed.
Such an adjustment will result in a sluggish response off idle.
Air/gasoline adjustment screw: the carburetor pictured in the exploded view uses an air adjustment screw (#5 in illus.) in that it is located upstream of the throttle valve (slide) and meters air; turning it counter-clockwise leans the mixture off idle. Some carburetors have this screw located downstream of the throttle valve, in this case the screw meters fuel and opening this screw results in a richer mixture. The idle mixture screw usually has a range of one to two turns out from fully closed. If you need to adjust above or below this range then the slow jet will probably need to be replaced with one richer or leaner as required, consult your owners manual for the standard setting.
1/8 to 1/4 throttle slow jet and throttle valve cutaway: keeping in mind that the idle adjust screw (air/fuel screw) gives a good indication of a properly sized slow jet (#6 in illus.).The slow jet calibrates the mixture from both the idle bypass and idle orifice in the jet block. If the idle screw is properly adjusted but the engine does not have good response when the throttle is wicked open it is usually a sign of a lean mixture and the slow jet will need to be replaced with one larger size and the air/fuel screw readjusted. consequently, if the throttle is only partially opened such as in a trailing throttle situation and the bike tends to load up and emits a deep tone when the throttle is returned to full open it is usually sign of a rich slow jet. If the slow jet does not clean up this part of the circuit the slide can be substituted for one with a different cutaway. The higher the number the larger the cutaway will be, admitting more air to the jet block/nozzle screen leaning the mixture and consequently a smaller cutaway will richen the mixture with its greater effect up to 1/4 throttle.

1/4 to 3/4 throttle jet needle: the jet needle (#2 in illus.) is comprised of five major elements.
1. straight diameter section: in keihin carbs either the last two digit or the last letter denote the diameter of the needle. The higher the last two numbers the leaner the needle the lower the letter the richer the needle. By going to a thinner needle there is a larger area between the jet needle and needle jet supplying a richer mixture.

2. length of the straight section: this determines at which point the needle taper will start relative to the clip position. If you have to run your clip in the highest position a needle with a longer straight section will need to be used.

3. needle clip position: this works in conjunction with the length of the straight section. If the engine is too rich above a quarter throttle raising the needle clip (#1 in illus.) will lean the mixture.
4. needle taper: a larger taper will result in a leaner mixture in the first half of the taper and a richer mixture in the last half of the needle. For example, a 1.34 taper will be richer in the first half and leaner in the second half of the taper than a 1.45 taper needle.

5. number of tapers: the needle can have one or more tapers. The number of tapers is not usually changed.

Needle jet: the needle jet/nozzle, varies the fuel/air mixture up to 3/4 throttle. How it overlaps with the jet needle depends on the needle jet orifice i.d., air bleed holes, and type of nozzle screen. Most modern japanese carburetors use a fixed needle jet/nozzle assembly which cannot be removed. If your carburetor has a removable needle jet/nozzle please contact the manufacturer in order to decipher the nozzle code. It is also not advisable to calculate how rich or lean a needle jet is using exclusively the nozzle inside diameter to needle outside diameter discharge area.

Wide open throttle (w.o.t.) main jet: the best trackside method to determine the size of the main jet (#7 in illus.) is to fully load the engine on a long straightaway or hill. At the end of the stretch chop the throttle and hit the kill button simultaneously. The plug is then pulled for a "reading". The parts of the plug you should be looking are: the positive electrode and the last 1/4 of the ceramic insulator . Best power will usually result in a very light tan colored insulator tip and dark colored ring around the tip of the electrode. The electrode itself should have fairly sharp edges. For example, if the ceramic insulator has a nice tan coloring but the electrode has a white ring around the tip and the plug is of the correct heat range then you can easily run a size larger main jet.

Please keep in mind that different types of pre mix oil & ratios, along with the gasoline, will give different readings. Also, that race or av gas is more prone to oxidization and storage deterioration, along with the fact that a multitude of types are used worldwide. When jetting your main jet try to remember to jet for the best power for a given track. An example of this is endurance where you would want to run a main large enough to keep the engine cool, this means that you may be on the rich side but the engine will fade less towards the end of the race. Another situation could be a stadium sx track where you spend much more time on the low to mid circuits. In this case you will probably be running a main jet that is much smaller than your usual "outdoor" jetting along with a hotter/extended electrode plug. As you gain experience and knowledge, you will be able to use other methods to determine your jetting. A good tuner can "feel" most of the circuits by slowly revving a parked bike, or just by looking at the color of the unpainted pipe and silencer.

But in the mean time always remember to change only one calibration component at a time ![url][/url]

[Jimmy McNeil]

I would try the DGj needle first. Im not saying Noah is wrong but If your pilot is already to lean and you lean it out even more you may stick it.

Richin up the pilot first, if it makes it worse go the other direction.

If I knew what the adjusted altitude was I may be able to help more. We dont use air density.

[Tony (Toe-Knee!) Morrison]

- The symptoms seem to suggest that the problem is vibration related. I would look at Ignition first. It would seem that the fuel circuits are close enough, if it runs well, below, and above, the indicated RPM's.
- Look for an ignition wire broken inside the insulation, or a loose connector, or something that would be sympathetic/unsympathetic, depending on engine RPM.
- Swap the entire ignition system (Rotor/Windings/Harness/Coil/Ign Module) with someone, if this gets painful. If the TOTAL ignition swap cures the issue, change the parts back (one at a time) until the failure reappears. A bit laborius, but no electronic skills required.
Good luck
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-Toe-Knee!

[Matt Stephenson]

Sounds like everyone is on the right track with changes to the needle, assuming thats what it is.... Heres my thought:

Thats the bottom of where your pipe comes in correct? holding those RPMs at part throttle can cause some funny shudders as things power up and down jumping in and out of the pipe. Just an off the wall idea.
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Matt Stephenson
S2 Shifter
www.StephensonRaceTech.com

[Troy V Smith]

In regards to needles alone, is there a direct impact in chages to a specific portion of the needle? Wow - does that make any sence to anyone but me?

Basically, I'm asking...as the needle is divided into taper, length and diameter - could that correlate to a low/mid/high change? For example, would a change in just the diameter (say a change from CCJ to CCK) have a '"richening" impact on just the higher end of mid range? Or a similar change in the just the taper (say a change from CCJ to DCJ) have a "richening" impact on just the low end of mid range?

Will be trying out a few clip arrangements and needles at the next outing - how many spare clips do you guys carry on hand. I could make a real habit of loosing these buggers!
_________________
TVS Motorsports -
"We may not be the fastest on the track...
but we're having the most FUN!"