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Your ST1300 stumbles above 4,000 rpm under load. One injector flows less than the other three, and the engine shakes the moment you ask it for power. The lasting fix is a matched set of four: this four-piece 16450-MCS-003 injector set for the ST1300PA 2006-2018, ST1300A 2003-2013 and ST1300 2003-2010 covers the whole bike.

What causes the shudder, in plain terms

One injector is wearing out faster than its three siblings. Here is why you cannot feel it until 4,000 rpm.

Injectors pick up varnish and wear with mileage. Each of your four sits in a different heat pocket of the V, so they age at different speeds. At idle the gap is too small to feel.

Above 4,000 rpm the engine demands more fuel, so the computer holds each injector open longer. The weak one hits its flow limit first, and that cylinder alone runs lean — lean means too little fuel for the air in the cylinder. That lean burn is the shudder, the fuel smell and the creeping gas use.

Why the correct repair is all four at once

Your four injectors have lived the same life: same mileage, same fuel, same filter history. The cylinder you feel is the loudest, not the only worn one.

A new part beside three tired ones just builds a new imbalance. And the labor is not trivial.

• All four injectors live under the throttle body.
• Manual section 5-64 charges you the same removal job for one unit as for four.
• Pay the labor once, replace the set, and the imbalance is gone.

Why the FI lamp stays dark while the engine stumbles

This is the part that confuses owners: the lamp stays dark and the bike still runs rough. Honda calls this fuel system PGM-FI, its name for programmed fuel injection. One shared command feeds all four cylinders, so there is no per-cylinder fuel trim to catch one weak injector.

Three sensors build that command:

• The MAP sensor 079800-5710 reads pressure in the intake, which is how the bike measures engine load.
• The intake air temperature sensor 37880-P05-A00 corrects that reading for air density.
• The road-speed sensor on the rear crankcase cover gates the decel fuel cut and feeds idle control.

So one injector flowing ten percent low sets no code while the four-cylinder average still reads fine. The lamp only lights for the sensor chain above.

Read the codes yourself first

Jump the DLC connector — the small service plug under the seat — and watch the FI lamp blink. A long blink counts ten, a short one counts one.

• Code 1 or 2: MAP sensor circuit or its vacuum hoses.
• Code 9: intake air temperature sensor.
• Code 11: road-speed sensor.
• No codes but a load stumble: unequal injector flow, the expected result.

One documented distractor: manual corrections note the printed labels for knock codes 25 and 26 are reversed. A 2005 owner meets a knock code near 4,200 rpm — a real code with a lamp, unlike the dark-lamp shudder.

The ten-minute pass before you order parts

• Run the MIL blink test with the DLC jumper and write down the codes.
• Check the speedometer reads steady. A frozen one points at the speed sensor.
• Aim an infrared thermometer at the four header runners at idle. The runner that lags marks the lean bore.

The numbers that matter for this repair

One part number, two listings — pick by year

16450-MCS-003 is the single Keihin reference for the whole ST1300 family. This store lists it twice only because two supply batches went up separately.

• The 4X-packaged set at $99.06 — four pieces, 1-year warranty.
• The 4PCS-packaged set at $93.62 — the same four pieces, same warranty.

Pick by model year: civilian ST1300 through 2010, ABS ST1300A through 2013, police ST1300PA through 2018. The packaging label is noise.

Swapping the set the way the manual expects

The whole job runs through the throttle body, and it is a bench job. Follow the manual order and nothing bites you.

1. Drain the coolant and remove the throttle body (manual section 5-64).
2. Lift the injectors out and seat the new set with dry o-rings.
3. Refresh the coolant hoses you can only reach at this stage.
4. Synchronize the throttle bodies. Do not skip this step.

Step 4 is the one that matters: the load signal comes from a single manifold reference. Unsynchronized bores bring the shudder straight back.

The verdict is a loaded third-gear pull from 3,000 to 6,000 rpm with even header temperatures.

Frequently asked questions