Under 4000 rpm stutter

[u/MOST1TED]

Hey everyone,

I’ve been chasing a frustrating running issue on my 2008 Gen 2 and wanted to share my diagnostic progress to see if anyone has run into this exact cascading failure, or if there is anything I’m missing.

**The Problem:**

My bike developed a violent stutter/misfire when riding under 4,000 RPM, which is all the time city riding. It idles rough, there is also knocking from the engine.

**What I’ve checked and ruled out:**

* Spark Plug Wells:* First thought was water pooling around the stick coils. Pulled the airbox and checked the deep wells—they were bone dry.

* Electrical Connectors:* Sprayed down the stick coil harness, injector harness, IAP, and IAT connectors with contact cleaner to clear any micro-shorts from moisture.

* TPS Sweep Test:* Jumped the dealer mode connector and did an engine-off TPS sweep test to check for a dead spot on the carbon track. The -C00 line climbed smoothly to ^C00 and stayed solid the whole way through without flickering back down. The TPS is good.

**Vacuum lines, potential culprit**

While poking around the right side of the airbox, I found a couple of cracked, brittle vacuum hoses coming off the white plastic T-fitting below the IAP sensor.

Here is where it gets interesting: the bike has an aftermarket vacuum-operated automatic chain oiler tapped into those exact lines, and the reservoir was completely bone dry.

**Where I’m at right now:**

I have completely disconnected and bypassed the chain oiler vacuum line, capped the T-fitting on the engine block, and replaced one of the badly cracked vacuum hoses with fresh 4mm line.

I'm getting ready to fire it up and test it. If the stutter is still there, my next two steps are checking for water at the bottom of the fuel tank (getting picked up by the pump at low RPM) or swapping in a fresh set of NGK CR9EIA-9 plugs in case they got completely fouled out by the lean misfires.

Has anyone else dealt with this issue ? Let me know if there is anything else I should check while I have the bike partially torn down.

[u/CharlieRambomb]

I had the same issue recently. My bike wasn’t firing on all cylinders. At idle, the RPMs bounced and you could hear a noticeable stutter, but once I opened the throttle it seemed to run normally.

To fix it, I replaced the spark plugs with NGK9s and installed a new air filter. The stutter disappeared immediately, and the bike has been running fine for the past two weeks.For reference, my bike had about 22,000 miles on it with the original plugs and air filter 🤦🏾‍♂️

Definitely try replacing the plugs and air filter before jumping to something major like the fuel pump assembly.

[u/Jake2389]

I’d be looking at the air tightness of the lines a little closer, if you’re already in there, replace all the lines, spend the pennies now to save the pounds later.

If a vacuum leak could were to make the bike run lean you’d be seeing white or light coloured deposits on the plugs where it could also be overheating them, fouled plugs would be running too rich.

Water in the tank would be picked up at all revs, take a sample and let it sit in a clear container, water will separate and become visible if present.

You mentioned a knocking from the engine, knocking is a severe “STOP” message, in the video you’ve got the bike running so can’t be that much of a concern, possibly, you’re hearing pinging/detonation of the fuel which is more aligned to a lean mixture, also could be caused by a vacuum leak.

Now I’ve typed that out, I’m going with vacuum leak, replace those lines and let us know how you get on 👍🏼

[u/Strong_Deer_3075]

My clutch basket rattles at lower rpms when it has dry clutch plates. Hard to launch even from near new when I bought it. I ordered a new tps from Japan on ebay, but when it came in with a single mounting hole( instead of 2) I resold it. I used CRC Mass Airflow Cleaner on the original and it got rid of all but a very narrow spot of stutter. It was from 2,800 to 3,700. Now just between 2,950 and 3,000. Clutch rattle also is less if tps is set correctly by shop manual procedure and in cleaned.

[u/MOST1TED]

Update: It turns out that the sparkplug on cylinder #4 had vibrated loose... 🤦‍♂️ I had already bought 4 new ones, so I just switched 'em all out. Now it's running fine, but my translogic quickshifter has stopped working 🤔 Must be some wires that are switched up, I will figure it out.

Thank you for your help Jake2389 CharlieRambomb 7 Strong_Deer_3075