Tuesday Tip: Hidden Power? Some Speed Secrets For The KTM 125SX

Mechanic: Andrew Langston
Text: Dan Worley
Photos: Garth Milan

Got some time on your ride? Even if you recently rebuilt the top end and your clutch is fresh, we’re willing to bet your bike can still go faster. Andrew Langston has heard numerous complaints from KTM privateers who are in the same boat as you. The usual grumble is that the 125s in question don’t have the same arm-wrenching bottom end like they used to.

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Sometimes factory mechanics aren’t the best guys to talk to about production bike problems. Factory bikes are so meticulously maintained that long-term problems aren’t normally even seen by the factory wrenches. Luckily, Andrew Langston has been working on KTMs for a long time, and in the process the likeable South African has helped out his share of under-privileged privateers when the occasions arose.

One thing that Langston has learned over the years is that KTMs feature one of the best power valve systems of any bike, but if not timed properly this technological marvel can cost valuable ponies. One key feature is that the system is adjustable and can be tuned for any rider’s preference, but on the flip side this also means that it has many parts, some which tend to wear out over time. This makes the preload on the valves improper, which makes the timing incorrect. If they open a few hundred rpms too soon or too late, the small 125cc motor is greatly affected.

The main culprit of the problem is a worn actuating arm (part number 503.37.057.000). Andrew swung by the shop to explain exactly how to check for worn parts, and at the same time he showed us how to set up a 125SX power valve to the exact same settings that Langston and Metcalfe use! Grab your toolbox and owner’s manual and follow these steps to get your KTM back on trackā€¦

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Step 1. Start with a clean bike, as many internal motor parts will be exposed; a dirty bike will only amount to a lot more work. First, remove the pipe, cylinder side covers, and power valve adjuster cap (30mm wrench or socket will be needed). Loosen the retainer bolt located at four o’clock on the governor housing of the right side case (10 mm bolt). The main sign of a worn arm is free play when turning the adjuster. Turn the adjuster on the right side cover and watch the slave actuating arm (black arm seen in right side cylinder opening). The arm should move instantly when the adjuster is turned. If there is any lag time, it’s a sure sign that the actuating arm is worn out. If everything looks tight you can skip to the last step for the proper timing procedure.

Step 2. It’s time to dig in! Start by draining the antifreeze and transmission oil. Next, remove the right side cover. This is easier when the rear brake pedal is removed. Take your time and make sure that the two dowel pins don’t fall into the motor.

Step 3. Once the cover is removed, use a workbench to lay out the parts. The water pump assembly and governor arm cover will also have to be removed.

Step 4. Remove the slave arm (the black arm that fits into the cylinder). This is done by taking a 10mm wrench and loosening until the bearing falls out of the right side case. Once the bearing is out, a T-handle can be used to speed things up.

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Step 5. The power valve adjuster/governor has to be removed to get the actuating arm out. The gear might have to be clamped if the bolt is too tight. Be careful not to damage any teeth.

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Step 6. Now that the governor is removed, the actuating arm will slide out. Check the lobe for wear. Here is an example of a worn arm and a new one. Once the lobe is worn, it will cause slop in the power valve linkage and improper timing will result.

Step 7. Everything is put back together the opposite way it came apart. Maake sure the actuating arms and ends of the governor are greased. This will also help keep the small washers from falling out of place. Two thin washers go on first, then the small thick one. Langston finds that black moly grease is usually best for motor parts. Once the governor is in place, use a little Blue Loc-Tite on the threads of the bolt that holds the governor assembly in place.

Replace the parts in this order:

(refer to owner’s manual for torque specs)

  1. Slave actuating arm (grease shaft with black moly)
  2. Push bearing back into case and put cover back on
  3. Water pump assembly (slightly lube O-ring with grease or oil)
  4. Right side cover (Placing dowel pins and gasket on the case side makes this a little easier. One key thing to watch for is placement of the slave actuating arm. It must fit into the “V located in the cylinder.)
  5. Brake pedal
  6. Transmission oil and antifreeze

Step 8. Now it’s time to set the preload on the governor. This can be a little tricky because the adjuster is on the right side of the bike, but the locating marks are on the left side of the cylinder. The sight hole on the left side of the cylinder has two bars with marks in them. The front bar has two marks and doesn’t move, but the back bar has a mark that the can be adjusted up or down. Start by turning the adjuster so the back bar’s mark moves to the highest position possible. This will ensure that power valve moves freely and doesn’t trick the setting. Next, turn the adjuster so the mark on the back bar moves just below the top mark on the front bar. The ideal spot is when the bottom of the front bar’s mark is aligned with the top of the back bar’s mark. This must be getting confusing by now, so look at the attached diagram to help straighten things out.

The setting just described is a little different from the factory specs. This is the preferred setting of team racers Grant Langston and Brett Metcalfe. This may not be the best setting for you, but that’s the advantage of KTM’s adjustable power valve system; it can be adjusted for what works best for you. Take the time to try different settings and tailor your motor to match your riding style.

Once you have achieved the proper position, tighten the retainer bolt on the right side case and replace the adjuster’s cover. Then install the cylinder covers and the pipe, and you’re ready to ride!

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