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Discussion Starter · #1 ·
Hi everyone! I'm new here and I was hoping to get some expertise. A bit of background on this, I've bought a 2008 ZZR600 14 months ago and I have been having issues ever since. I've looked around the forums for solutions as these all seem to be common problems (Stumble at launch, hanging idle, sloppy acceleration), but I have never seen someone have all three problems at the same time! I have tried many potential solutions to no avail.

1) Bike Stumbles at launch from 0 - When Cold. Holding the clutch, revving to anywhere between 1500 and 4000 and then releasing the clutch somewhat rapidly will cause the bike to stumble (i.e. RPM suddenly drops, engine makes an "eek" sound and accelerates sloppily). No stumbling occurs when the bike is hot (>175F) and no stumbling ever happens if the bike is above 10mph. The stumbling also does not occur if the bike is accelerated very slowly (i.e. keeping the RPM to <1200)
2) Idle hangs - I have to set the idle at ~700-800 RPM for it stop hanging, and even then, when the bike gets hot, the idle starts to hang (slightly) around 1300-1500 RPM. The owner manual calls for the user to set the idle @ 1350 RPM if I recall. Setting the idle anywhere above 1000 RPM will cause the RPM to hang at 3000-3500 RPM. The hanging gets worse as the engine gets hotter. This leads me to believe that the bike may actually be running rich at idle - I took out the spark plugs and they were wet! To add more to the craziness, I adjusted the A/F mixture screw to ALL THE WAY OUT and the bike ran much better which is making me believe that it's running lean(?!)
3) Sloppy Acceleration - When Cold. When the bike is cold, the acceleration feels a bit sloppy. The acceleration gets much better when the bike is really hot (>200F), but as the bike cools down when riding on the highway, the sloppy acceleration comes back. This leads me to believe that the bike is running lean

Other Symptoms:
a. The spark plus were wet when replaced
b. Fuel was found in the air box duct, on the air filter and the top of the carburetors - it seems like fuel is being pushed back up through the carbs leading me to believe that there may be an air leak/vacuum leak

Full Yoshimura Exhaust
KLEEN System Removed
K&N Air Filter
Dyno Jet Kit

Attempted Solutions:
  • Cleaned Carbs --> No Impact
  • Resynched Carbs --> No Impact
  • Replaced several vacuum lines with new ones and checked the correct routing --> Slightly better acceleration at full throttle (>10000 RPM)
  • New Air filter (also a K&N) --> No Impact
  • Replaced Spark Plugs --> No Impact
  • Installed DynoJet Kit --> No impact
  • Adjusted mixture screws to all the way out (I think that's 5 or 6 turns out - the manual calls for 3) --> Hanging idle improved, but it still would hang when the bike was really hot.

Potential/Upcoming Solutions:
  • Replace fuel filter
  • Valve clearance check/adjustment + replace valve cover gasket
  • Replace float needles
  • Check Float Height
  • Replace all vacuum lines
  • Check for vacuum leaks (with some good 'ol Chemtool B12)
  • Replace fuel lines
  • Resynch Carbs
- Drive bike off of a cliff
- Sell it and never buy a carbureted bike ever again

I apologize for the long wall of text, I am shooting in the dark with my potential/upcoming solutions, but I am hoping that someone might have had this problem in the past and might have had a solution at the ready. I really appreciate any help that you can give me!

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