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2005 Carb'd Merc 75HP stumble/sputter

A

adudinsk

New member
Problem
I have a 2005 carb'ed merc 75HP outboard came with a boat I bought last year.
Did have a slight stumble would run fine for 30 sec or more.. then sputter ONCE.. then fine.. for 30+ seconds.

All of a sudden... it died one day would not start
Checked stator .. fine. (bought the special tool to do it..with your multimeter)

  • Checked all electrical/spark/coils/computer/ fine as per service manual
  • Changed fuel filter
  • Rebuilt lower end (just because)
  • Changed all fluids/filter
  • Changed all fuel lines
  • Changed all fuel
  • Compression fine.

I rebuilt carbs.. insane white powder in carbs from previous owner.. installed 4 carb kits..

Engine fired right up

Still has that occasional stumble..gets better when warm.. does not go away. No change from before.
Like a slight cough every 30-60 sec.. varies.

HAVE NOT changed fuel pump.. but it did "pump" when disconnected the hose. (Maybe a pinhole in diaphram?.. No rebuild kits avail .. ordered new pump)


  • No backfiles.. happens with cowling on or off
  • Replaced all filters INSIDE the carbs..
  • Check valve on carbs appeared ok.. (will double check #804198)
  • Replaced all plug wires.
  • Have good output from coils.
  • Replaced Throttle Sensor.. was bad.. potentiometer had NO reading.. new one is in spec as per manual.
  • Replaced Voltage Reg. (old one that came with boat was blown to bits.. big hole in it.. new one works fine and charges fine..prev. owner lied about engine)

INTERESTING..
When I put hand over intake runners (induction silencer) engine speeds right up
?? Running rich??
I did replace all o-rings in all the carb plates .. 100%

Its an EPA version of carbs.. so the pilot screw is capped.. and not adjustable (?remove cap.. clean? Adjust? or replace?)
(all else cleaned.. )

Would this problem typically be fuel pump.. or should I remove the seal on the pilot jets.. and replace them/clean them?

Why would it speed up when I hold 1/2 a hand over the intake runners? (not anywhere near blocking them off)
I have not yet synced the carbs.. (have the gages.. no done yet.. would this cause such a problem?)

Rings / Valves carboned up?
Could it be a head gasket leak.. but ...No fluid in oil... at all .. clean as a whistle... no oil in the water either..
?? Exhaust leak??

Suggestions Please?
Outboard SN 1E000385

Thanks in advance, and sorry for such a large post.

Andrew
 
Oh I would remove the mixture screw and clean for sure but
record the EXACT turns for the screw. Measure with micrometers
as well and put it back the same way you removed it.
EPA does not want you/me to touch that screw but you have
to clean it.
 
Thanks.. what I was thinking.. BUT.. needed someone to give me the "push" to do it..
Will try it this weekend.. if weather is nice..


Thanks
Andrew
 
Looked up the Service manual.. it states
1) Seat the screw lightly on all 4 carbs
2) unscrew 2 1/2 turns +/- 1/2 turn each
3) If engine runs smooth .. leave it (umm.. prob wont..)
4) If not.. adjust ONE carb at a time turn screw IN until rpm drops 40rpm
5) Then back out 3/4 turn
6) Repeat for all remaining carbs..

Does not sound too "precise" from the service manual.
Hope to pull the caps.. drill them out.. clean and try this weekend..
(FINGERS CROSSED!)

Thanks
Andrew
 
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