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Title: Mercury 3.5 HP 4-Stroke (2010) – Extremely High Idle, Runs Fine Initially, Then Revs Too High

W

wimdeleeuw

New member
Hi everyone,

I'm having a frustrating issue with my 2010 Mercury 3.5 HP 4-stroke outboard (86cc, Tohatsu-based), and I'm hoping someone here can help me figure out what's causing it.

The problem:

The engine starts easily, but the idle RPM becomes extremely high.

Recently, after having the carburetor professionally cleaned in an ultrasonic cleaner, I reinstalled it and started the engine.

  • Initially, it ran perfectly at a normal, smooth idle.
  • I started with the choke halfway on, then turned it completely off.
  • The engine continued running perfectly without the choke for approximately one minute.
  • I then gave it a little throttle.
  • After returning the throttle to idle, the engine continued revving extremely high.
  • I shut it off and restarted it, but it immediately ran at very high RPM again, even with the choke halfway on.
Previously, partially applying the choke would bring the RPM down to a normal idle.

What I've already checked or done:

  • Completely disassembled and cleaned the carburetor multiple times.
  • Removed and cleaned the pilot jet and idle passages.
  • Had the carburetor professionally ultrasonically cleaned.
  • The mechanic inspected the carburetor and couldn't find anything incorrectly assembled or visibly damaged.
  • Checked the float height according to the service manual.
  • Confirmed that the float bowl contains plenty of fuel.
  • Replaced the intake gaskets and intake spacer with new parts.
  • Confirmed that the throttle linkage returns fully to its idle stop, including after opening and closing the throttle.
  • Adjusting the idle mixture screw by approximately half a turn in either direction made very little difference.
Additional observations:

  • During the most recent test, I noticed a very small amount of smoke somewhere under the engine cover, but I couldn't identify its exact source.
  • The cooling-water telltale stream is strong.
  • Before i got the carb ultra cleaned it would idle normal with the choke halfway when i turn the choke off it would idle high. I put the utlra cleaned carb on and it would ran normal even without the choke for 1 minute like i explained so that is a little diffrence.
This is my first engine i work on so i dont have a lot off experience but i want to learn.

What could be the problem?


Advice would be greatly appreciated.

Thanks in advance!
 
Disconnect the linkage to the carb and see if it will idle. If it doesn't look down the bore of the carb and see if the butterfly has uniform contact all around when closed. If it's not uniform there should be screws holding it in place. Loosen the screws slightly and center the butterfly
 
Thanks for you replay! I have a photo off the butterfly. I am go disconnect the linkage as soon as possible when i have some time.
 

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