KLX300 Secondary Throttle Plate: What It Does & Why Removing It Adds Power

What is the secondary throttle plate?

The KLX300 throttle body has two throttle plates in series:

• The main throttle — the one connected to your twist grip. You control it.
• The secondary throttle plate (STP) — also called the subthrottle — a second plate on its own little servo motor. The ECU controls it, not you.

Air has to pass through both plates to reach the engine. So the airflow the engine actually receives is capped by whichever plate is more closed. You can have the main throttle pinned wide open — if the ECU holds the secondary plate half-shut, the engine only ever sees half the air.

This isn't a KLX quirk. The setup is common on modern fuel-injected motorcycles and cars (often called "drive-by-wire" secondary throttles or intake runners). On the KLX it's a mechanical linkage for the main plate and a servo for the secondary.

If you want the big-picture view of every restriction the KLX ECU imposes, see the engine guide.

Why Kawasaki fit it (it's not just to annoy you)

Before we show you what the maps do, it's only fair to steelman the factory. The plate exists for real engineering reasons:

1. Intake air velocity and atomization at low RPM. With the plate partly closed at small throttle openings, air is forced through a narrower gap and speeds up. Faster air breaks fuel droplets into finer mist → better atomization → crisper, torquier response right off idle. This is the "tractor-like" low-end the KLX is known for.
2. Emissions and regulatory compliance. The maps we decoded include behavior that only makes sense as compliance / rev-control (a neutral-rev table that never fully opens, and a top-end re-clamp) — not performance tuning.
3. Rideability and traction. Limiting torque delivery where the bike is easiest to loop (1st/2nd gear launch) or where traction is marginal tames a snappy throttle.

So the plate earned its place. The rider's question is whether you value that low-end tractability more than the midrange power it costs. For more on why the ECU restricts power at all, see ECU limitations.

What the factory ECU maps show

Ross Olsen, an engineer in the PlanetKLX2 community, dumped the entire calibration region from the KLX300 Keihin ECU — fuel maps, ignition maps, and the four secondary-throttle tables. The raw secondary-throttle tables are available to download at the bottom of this page.

This is not a guess from a dyno. This is what the factory programmed.

One valve, four different jobs

Here's the least-obvious finding: the STP map is gear-aware. There are four separate tables — Neutral, gears 1–2, gears 3–4, and gears 5–6 — and the fuel tables are split the same way. They do genuinely different things. This is the plate at wide-open throttle in every gear — the green line is your main throttle (what you're asking for); every line below it is the STP holding back:

• Neutral — rev control. The plate is purely a function of RPM and never fully opens (it tops out around 95% at redline). Free-revving the bike is choked all the way up — a compliance / anti-abuse signature.
• Gears 1–2 — anti-spin clamp. The plate is pegged around 50% right through ~7000 RPM, then snaps fully open by ~8000. This is launch and wheelspin management: choke torque exactly when the bike is easiest to loop, then release it once you're moving. This is the most dramatic restriction on the bike, so every chart below uses the gears 1–2 map.
• Gears 3–4 — progressive ramp + a top-end re-clamp. The plate ramps from 50% to fully open across 4500–8000 RPM, then drops back to 50% above 10 000 RPM — a soft rev-limiter that lines up exactly with the ignition-retard power cliff.
• Gears 5–6 — progressive ramp, holds open. Same ramp as 3–4 but stays fully open to redline. Highway gears are the least restricted.

Side note for tuners: an old TunerPro definition comments that "gear 5–6 maps to the same table as gear 1–2." The decoded data contradicts that — gears 5–6 follow the 3–4 ramp, not the 1–2 clamp. Trust the data.

How much throttle is actually withheld

Because the two plates sit in series, what the engine receives is the more-closed of the two. At WOT your main throttle is pinned wide open (100%), so the engine is capped by whatever the STP is doing.

This is the headline chart — your main throttle (green) versus the STP plate (red), in 1st/2nd gear at full throttle:

A note on the numbers: the ECU's throttle-position sensor maxes at ~94% at the grip stop — that's why the WOT column in the raw tables is labelled 94.12. We plot your grip as 100% so the chart reads the way a rider thinks.

In the clamp band (3000–7000 RPM) the engine receives about 50% when you're asking for 100%. The ECU withholds nearly half of wide-open throttle right through the midrange. Hover the points to read the exact values.

And it's a bit worse than the percentage suggests. Servo travel isn't open area: by the standard throttle-area model (1 − cos(θ)), 50% plate travel is only about 29% open area — the plate is physically ~70% shut. The ~4 hp midrange gain riders see on the dyno is the honest arbiter; the maps just explain why.

The whole map, not just the redline

That last chart is one slice — wide-open throttle across RPM. The real picture is two-dimensional: every combination of RPM and throttle has its own result. Here's the entire gears 1–2 map, color-coded by how much throttle the STP withholds — green means the plate is open at least as wide as you're asking (it's not the bottleneck); red means it's choking you:

Pick your throttle position on the left (what you're asking for), go across to your RPM, and hover any cell to see how much throttle actually reaches the engine. The map is almost entirely green — because the STP only costs you at large throttle. Ask for ~50% or less and the plate is already more open than that, so it's not the bottleneck. The cost lands in the top-right corner: pin the grip past ~50% below 7000 RPM and the STP starts choking you — at wide-open it caps the engine at ~42–50% throttle, about half what your wrist asked for. Above ~8000 RPM the plate opens fully and the whole top of the map goes green.

The other three maps: higher gears open up

Gears 1–2 are the outlier — the hard launch clamp. The other three tables are progressively more permissive, on the same axes and the same green-to-red scale (worst case ~45% delivered, best 100%):

Gears 3–4 — the clamp eases off much sooner. At wide-open the plate ramps from ~50% at 4000 up to fully open by ~8000. But watch the far-right edge: at 10499 RPM wide-open drops back to ~50%. That's a top-speed re-clamp.

Gears 5–6 — the same mid-range ramp, but no re-clamp at the top. Once the plate opens it stays open all the way to the limiter. This is the highway map: you're already moving, so the ECU stops fighting you.

Neutral — no clamp and no throttle-mapping at all. The plate follows a single RPM ramp (40→95%) regardless of grip, opening smoothly as revs climb. You never ride here, but it confirms the lower-gear clamps are a deliberate choice, not a default.

The dead throttle band you can feel

The single most rider-tangible effect is in 1st and 2nd gear. The STP plate is pinned near 50% regardless of what your wrist does — so asking for more than ~50% throttle changes nothing:

Here's what actually reaches the engine at 5000 RPM in 1st/2nd — the lesser of your grip and what the STP plate allows. Up to about half-grip you get exactly what you ask for (the red line rides the green). Past that the STP plate has hit its ~50% clamp and the effective throttle goes flat: twist from half to the stop and the engine sees no more air. That whole upper half of the grip is dead weight — until the revs climb past ~7000 and the clamp releases, the plate snapping fully open by ~8000.

That's the surge riders describe — the bike feeling gutless, then "coming on the pipe" around 7000–8000. It's not a powerband; it's the ECU finally getting out of the way.

KLX250 vs KLX300: in 1st/2nd the 300 is clamped longer

Counter-intuitive but clear in the data. At WOT in 1st/2nd, the KLX300 holds the plate clamped at ~50% all the way to 7000 RPM — about 2000 RPM longer than the KLX250, which starts releasing the plate at ~5000:

Both bikes clamp around 50% in the low-mid range, but the KLX250 lets go first and is fully open by ~8000 RPM, while the KLX300 stays pinned until 7000. So in the gears where you launch and accelerate hardest, the 300's plate is actually the more restrictive of the two. (Switch to the gears 3–4 maps and the story inverts — the 250 runs more closed through the midrange — which is exactly why it's worth looking at one gear set at a time.)

The two models also choke power in different RPM windows: the KLX300 pulls ignition timing at the top end (a high-RPM cliff), while the KLX250 pulls timing in the midrange but stays advanced up top. Different calibrations chasing the same emissions targets.

Does the ECU add fuel to match? No — that's the catch

This is the part that decides whether you need a fuel tuner.

At large throttle openings the ECU fuels from an alpha-N map: it looks up RPM and main throttle position and commands a fuel value. The secondary throttle position is not an input to that calculation — the ECU never asks "how open is the second plate?"

So the consequence is mechanical, not a tuning opinion. At any given (RPM, wide-open throttle) the commanded fuel is fixed. With the plate in, the clamp band starves the engine of air while the fuel map sits ready for it; pull the plate and that same fuel cell suddenly ingests far more air — more air, same fuel, so the mixture goes lean, worst in the 3000–5000 RPM clamp zone.

What these tables can and can't tell us: we can prove the direction (leaner) and the mechanism (fuel is a function of RPM and throttle only) directly from the maps, and bound the size from the plate geometry. But the firmware is encrypted and there's no injector-flow scalar in the unencrypted calibration, so there's no way to compute an exact air-fuel ratio from the tables alone. Corroborate the shift with a dyno or AFR log.

What removing the plate actually does

Stacking up the effects, all straight from the maps:

So it's not free. You trade a little low-end tractability and stall resistance for a meaningful chunk of midrange — and you push the mixture leaner exactly where you just gained power. Both matter for the next section.

Do you need a fuel tuner?

Honestly: you can run the delete on a stock ECU. Plenty of people do. But "runs" and "right" aren't the same thing.

The saving grace is the ECU's closed-loop path. Below about 75% throttle the bike fuels from a speed-density (MAP-sensor) table and trims with the oxygen sensor. That path partly self-corrects when you remove the plate — more air raises manifold pressure, the MAP sensor sees it, the O₂ sensor trims. So around town and on part-throttle, the bike largely sorts itself out.

Above ~75% throttle it switches to the open-loop (alpha-N) table we just showed you — no MAP feedback, no O₂ trim. This is exactly the WOT midrange that goes lean. A fuel tuner (or a richer injector, or a reflash) does two things here: it recovers the power the extra airflow makes possible, and it protects the engine from running lean and hot under load.

So: not strictly required, but if you're going to use the midrange you just unlocked — especially with an open airbox or exhaust that flows even more — adding fuel is the difference between realizing the gain and leaving power (and reliability) on the table. See ECU tuning for fuel-management options, or our writeup on why EFI bikes bog on throttle snaps for how these systems actually behave.

Removing it: the mechanical side

This page is about why, not the wrench-turning. The short version:

1. Get at the throttle body (some bodywork may need to come off).
2. With a JIS driver, remove the two screws holding the secondary plate and lift it out.
3. Idle can go unstable afterward — the ECU manages idle via a little rocker arm on the throttle plate, and removing the secondary changes the air path. The standard fix is to remove a small spring inside the throttle body.

The full photo walkthrough and the idle fix are in Intake Modifications. Most riders leave the servo motor connected (the plate is passive; the servo is what the ECU may monitor), so a code isn't typically set — but if you disconnect the actuator you may need a servo-eliminator plug to satisfy the ECU.

FAQ

Will removing the plate throw a code? No

KLX250 or KLX300 — bigger difference? Depends on the gears. In 1st/2nd the KLX300 actually holds its clamp ~2000 RPM longer than the KLX250 (to 7000 vs ~5000), so the delete wakes up the 300's launch gears more. In gears 3–4 it inverts and the 250 runs more closed midrange. Both benefit; where you feel it most depends on which gears you ride in.

Is it reversible? Yes. Screw the plate back in and stock behavior returns.

Do I have to remove the snorkel / open the airbox too? No — they're independent. The delete helps on its own. But the intake restrictions compound, so most build paths pair it with a freer snorkel or airbox. See the intake guide.

Will it fix the throttle bog? Partly. The STP clamp is one cause of midrange softness;

The verdict

The secondary throttle plate is real engineering — atomization, compliance, traction control — not a mistake. Kawasaki got value from it.

But the maps show it holds the plate at ~50% through the midrange and renders the top half of the grip dead in 1st/2nd below 7000 RPM. Removing it reclaims roughly 4 hp where you actually ride, at the cost of some low-end stall resistance and a leaner WOT mixture. Whether that's a good trade depends on how and where you ride — but at least now you can make it knowing exactly what you're trading.

Raw data

All four secondary-throttle tables (KLX300 Neutral / 1–2 / 3–4 / 5–6), the KLX250 gears 1–2 table, and the WOT comparison data — decoded from the factory ECU binaries, in CSV.

Credits

The ECU dump and table decoding are the work of Ross Olsen from the PlanetKLX2 community, shared openly. The ~4 hp midrange figure and the 5000→7000 RPM dyno behavior are corroborated by the dyno data in our intake guide.