1994 100 hp mercury - misfire @ Low RPMS

njm_fish

Recruit
Joined
Jul 7, 2010
Messages
5
I have a 1994 2+2 100 hp mercury outboard.
Took the boat out - ran fine for an hour or longer on the way in engine started running really rough at low rpms below 1200rpms

Engine runs fine at higher RPMS - Idle is misfiring at low RPMS, #1 cylinder plug is carboned up others are even and wet. Cleaned carb and replaced all inlet needles, new gaskets. Carbs looked clean and not defects to needles.

#1 and #2 cylinders run perfect while only running one cylinder at a time - when both cylinder are running together there's a misfire.

Next step is to check timing. Anyone have any thought on this?. Have not checked compression but engine preforms great and runs great at high RPMS
 

njm_fish

Recruit
Joined
Jul 7, 2010
Messages
5
Re: 1994 100 hp mercury - misfire @ Low RPMS

Think I figured it out after running numerous tests. The simple test worked well.

Put my hand on the carb in question and problem fixed itself. Air/Fuel - reed sounds like it's bad.
 

Tregaskis

Seaman Apprentice
Joined
Aug 23, 2005
Messages
42
Re: 1994 100 hp mercury - misfire @ Low RPMS

I just picked up a 1994 2/4 Mercury 115...it is having the same issues as your 100 did. Was it a bad reed valve that caused your problem? I have done everything else, carbs, sync, compression...all look good. I have been leaning toward a bad reed. Did you try the hand on carb test with all plugs hooked up? Are there 2 reed blocks only on this motor or 4? I hope you see this...it is a while since you posted yours.
Thanks
Greg

Think I figured it out after running numerous tests. The simple test worked well.

Put my hand on the carb in question and problem fixed itself. Air/Fuel - reed sounds like it's bad.
 

Laddies

Banned
Joined
Sep 10, 2004
Messages
12,218
Re: 1994 100 hp mercury - misfire @ Low RPMS

Adjust the low speeds (richen) in the water in gear that should take care of the problem
 

Tregaskis

Seaman Apprentice
Joined
Aug 23, 2005
Messages
42
Re: 1994 100 hp mercury - misfire @ Low RPMS

I have adjusted the low speed needles till I am blue in the face (only top 2 carbs are adjustable on this 2/4 motor). I have sped up the idle to 1000rpm (800 rpm in gear) just to be able to keep it from dieing when I put it in gear...it still shakes and loads up until 1200 rpm where it cleans up and sounds great. Then transition to 4 cylinders around 1800 rpm goes smoothly too all way to 5200 rpm at full throttle. It has been raining here for the last week so I am going over to the marina tomorrow to try to see if I can isolate the problem some more. If I can smooth it out by choking off a carb, might be able to verify it is reed valve. All sparks, mixtures and linkages have been set and then professionally checked and verified again.
 
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