Running rough in lower revs

[Nashvegas]

I ran a high-resolution frequency, pulse, and timing analysis on the exhaust audio of my bike to figure out that dull, low-frequency "BUM" / popping sound I’ve been getting at 2,200–3,500 RPM (especially on slight decel).

I initially thought it was just characteristic afterfire, an SAI leak, or a minor exhaust leak since the bike felt smooth and I wasn't feeling any power loss. However, the data completely changed the diagnosis. Without boring you with complex charts, here are the three major findings. (Since we don't know the physical cylinder number yet, I'll call the problematic one "Cylinder A" and the other two "B and C").

1. It’s not a normal exhaust quirk; it’s a real misfire / partial combustion.Whenever that low-frequency pop occurs, the crankshaft actually slows down measurably (a sudden drop of ~13 RPM that recovers within 150 ms). On a massive 300+ kg bike, it's impossible to feel this single-cycle slowdown in the saddle, which is why the engine felt smooth to me. But the data doesn't lie: combustion is failing or extremely weak on that specific cycle, and the unburnt fuel is igniting in the exhaust.

2. The issue is isolated to a single cylinder (Cylinder A).These weak combustion events aren't spread across the engine. Almost all of the deep drops and weak pulses line up precisely with "Cylinder A's" firing order. Cylinders B and C are running almost perfectly. This rules out general issues like the fuel pump, standard ECU mapping, or a main exhaust leak. It proves it's a hardware issue specific to one cylinder.

3. The misfire rate doubles on throttle closure (Vacuum effect).The weak combustion events in Cylinder A happen more than twice as often when the RPMs are dropping (rolling off the throttle) compared to holding a steady throttle. Rolling off creates peak vacuum in the intake manifold. This strongly suggests a tiny, localized vacuum leak at that specific cylinder's intake tract (throttle body seal, injector O-ring, etc.). We've heard of 2023 models having similar issues due to warped intake manifold mating surfaces. The other strong possibility is an intermittent weakness in that specific cylinder's ignition coils/boots or injector.

Next Steps for Diagnosis:I'm done looking at the exhaust. My next moves will be:

• Do a cylinder drop test (or use a diagnostic tool/misfire counter) to physically identify which one is "Cylinder A".
• Swap the coils and spark plug boots from Cylinder A to a healthy cylinder to see if the misfire follows the parts.
• Perform a targeted smoke or spray test specifically around that cylinder's intake manifold boots to check for a vacuum leak.
• Check the injector on that cylinder.

A quick favor to ask:If anyone has a perfectly healthy running Rocket 3 GT, could you record a short audio clip for me? Engine fully warm, in neutral, hold your phone about half a meter away from the exhaust at a 45-degree angle. Hold it steady at 2,300 RPM for about 20 seconds, then slightly roll off the throttle to let it drop to 2,000 RPM. (Please use your phone's native voice recorder, not WhatsApp, to avoid compression). It would give me a perfect healthy baseline to compare my data against.

Seems like switching the coil/wire to a different cylinder might be the easiest way to see if those parts are the issue.

 

[Journeyman]

Any chance you hear same sound in this video with the stock exhaust?

 

[TURBO200R4]

my old ears is not real good but was wondering that if you pulled one of the two plugs on each the cylinders one at a time if that would identify the cylinder that is making the noise.
i know that is a long shot so if you do not want to try it i do not blame you.

 

[mo8414]

My bike after getting home from work

 

[ncrawler3]

My bike after getting home from work

Thank you so much for taking the time to record and share this! I really appreciate it.

After listening carefully, I can hear that the random “boom” pattern is exactly the same as on my bike. Your exhaust is different from mine, but the pattern itself is identical.

Thanks to your recording, I’m now pretty confident that this is simply part of the Rocket 3’s normal engine character.

My theory, which I’ve also seen discussed in similar threads on Rocket 3 forums, is that emissions-related fueling at low RPM and light throttle may contribute to these occasional weaker combustion events. That could explain why the random “boom” sounds are more noticeable at steady throttle or when gently rolling off. Of course, this is just a possible explanation rather than a confirmed diagnosis.

It’s a relief to finally hear the same thing on another Rocket!

I’d also like to thank everyone who told me from the beginning that this was normal Rocket behaviour. I guess a short answer would have been enough after all! But on the bright side, I’ve learned a lot about the Rocket 3 and how its systems work throughout this whole investigation, so I certainly don’t consider the time wasted.

Thanks again to everyone who contributed, especially for taking the time to make the recording.

If I make any further observations or discoveries, I’ll be sure to share the results here as well. Hopefully this thread will be useful to other Rocket owners who notice the same thing in the future!

 

[TURBO200R4]

I guess i like to play also
thermal gun on each pipe to see if there is a difference.

 

[3-D]

Sounds like a perfectly normal Rocket to me ! - get some better earplugs, and the noise will go away !

I totally agree

 

[3-D]

I week also say mine made plenty of odd sounds in the lower rpms when new. Virtually all stopped after getting the ECU tuned.

 

[Kiwicol]

TBH sounds just like mine & it's a smooth mo fo!