SJ23 Tech Tip D15, (Updated 2026-01-07) Bob Schimmel. 

Index

Outboard Repairs - 1976 Merc 75 Running Rough.
INDEX - Carburetor, Lower Bearing Check Valve, Reed Valves, Power Head Seals. 
 
My trusty Merc 75 was idling a little rough at the end of this season and not quite as smooth at cruising speed.  It starts OK, which is normal, since this engine has electric start.  Electric start is a very nice feature by the way.  The rough running has me wondering about the carburetor and the reed valves, power head seals, and check valve given the age of the outboard.  I might be able to do both jobs at the same time since they are mounted adjacent to each other, sort of.
 

CLEAN CARBURETOR  (2025) - Before anything else, check the carburetor body for signs of leakage.  Don't destroy the evidence by handling it and take photos as you may question your memory!  With the carburetor removed, its wise to plug off the air intake to keep debris out of the engine.  Now THAT is guaranteed to tick you off!

  • Release the blue fuel line cap from the side of the carburetor.  Keep the seal and screen clean.
  • Release the choke cable from the top of the carburetor.  Don't drop the screw into the bottom of the outboard.
  • The body of the carburetor is mounted to the power head by 2 nuts on either side.  Remove them and pull the carburetor off.
  • Dissemble the fuel pump on the side taking note of the condition and order of the 4 seals.  The fuel pump operates with pulsing air pressure from the crankcase so the seals cannot have holes, must be pliant and are installed in the correct order.  Keep them clean.
  • Remove the float bowl to see if debris has accumulated inside.  The type of debris and amount of it is a telltale sign of the problem.
  • Check the float and needle valve for free movement and determine if the float is buoyant.  Directly affects engine response and performance. 
  • Remove the float and needle valve to confirm the needle is free to move and the valve seat is clean (not worn).  Use a punch against the protruding float pin.  The needle point must be sharp with a smooth face to stop fuel flow when the bowl is full.
  • Are the idle and high speed jets (.045) clean?  Use the exact size screw driver to remove brass jet.  Spray brake cleaner through jets.  Do not enlarge the side holes with a poker.  The steel wire of a twisty tie can usually fit.  Set idle jet screw 1-2 turns out.
  • Replace any worn seals and parts.  Clean all parts with brake cleaner before assembly.
  • Pay close attention to the float bowl seal when installing it.  The seal must make contact with the entire rim of the bowl.
  • Tighten the carburetor screws by hand.  No need to be a deck ape about this.
  • Finally, tighten the carburetor mounting nuts with an open end wrench.  Tedious but at least easy to reach.
 Fig 1 - Carburetor front view prior to removal.

I found three problems.

  1. Some extremely fine sediment in the float bowl (See Fig 6) and sediment imbedded in the fuel pump screen. (See Fig 2).

  2. There was also the body of a no see-um stuck to the end of the high speed fuel nozzle in the carburetor throat.

  3. A bit of grunge was past the needle valve.  This surprised me as I have always filtered the gas.  I cleaned the float bowl 15 years ago and 8 years ago I installed an external fuel filter.  The filter is translucent so it can reveal flow which is helpful out on the water.  It just shows that you can't be too careful.  Go figure.

Given these minor fuel restrictions and the fact that the engine starts on the first push of the start switch, I've decided not to check the reed valves and power head seals at this time.  Removing and splitting the power head open to access these is a major undertaking.  I may check them at a later date if the engine continues to run rough.  
 

  Fig 2 - The fuel pump is behind the blue cup on the side of carburetor.


 
 Fig 3 - Fuel pump outside casting where the screen resides.


 

  Fig 4 - Inside fuel pump.  All whistle clean.


 
 Fig 5 - Float bowl & idle mixture screw, 1-2 turns out.

 Fig 6 - The really fine debris in float bowl appears to be metal.


 

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POWER HEAD LOWER BEARING CHECK VALVE (2026) - There is a tiny brass check valve screwed into the bottom left outside of the power head.  Its purpose is to suck accumulated gas and oil from the bottom bearing via an external vacuum tube connected to the top of the intake port.  A defective check valve can adversely affect the running of a Merc 75. 

A quick check on a warm winter day confirmed that Panache's check valve functions hunky-dory.  However, once warm in Spring it will be removed, washed out and reinstalled.
- If this valve is plugged so it can't suck, the engine can't start due to having too rich fuel mixture. 
- If this valve is stuck open so it can pass air down to the bearing, the check valve is defective.

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POWER HEAD - The power head holds the crankshaft with pistons and end seals, the reed valves plus a tiny external check valve.  The power head must be air tight for the outboard to function.  Some tell tale signs of leaky power head are; rough starting, erratic idling, and poor high speed performance. 

It may be possible to test reed valves and end seals while still installed.  Gently pump 3-7 PSI air through the carburetor throat.  The reeds will open slightly to let air pass into the crankcase and then they should close to hold pressure in the power head when the pump is removed.  If you hear air leaking out the carburetor, or if you can't build any pressure in the power head, the reeds or power head end seals are leaking.

REPLACE REED VALVES (future) - Reed valves in a 2 cycle outboard are installed directly behind the carburetor.  They are essentially a one way air valve to let combustion air in, but NOT out.  It is absolutely necessary they close air tight when the piston compresses air on the downward stroke against the closed reeds.  As the piston reaches the bottom of its stroke it opens the intake port and all that compressed air in the crankcase rushes into the cylinder for the power stroke.  This guy gives an excellent description of the operation of reed valves.  Pay attention to his description of the operation at the end.  With time reed valves fatigue (leak) so they no longer close to seal tight.  A worse case scenario would be gas droplets expelled from the carburetor, hence the reason why the throat is wet.  A reed valve can also stick open, crack or break with similar symptoms.  Good reed valves provide quick starting, strong acceleration, smooth power throughout the RPM range with optimum fuel economy for the long distance user.

The reed valve cage on many outboards is located right behind the carburetor, making it fairly simple to inspect or replace.  However, the reed valves on a 1975 Merc 7.5 HP are installed around the middle of the vertical crankshaft (Part 1), and (Part 2) splitting the crankcase into upper and lower chambers.  Each chamber has a set of 4 reeds to control air flow.  The combustion air is sucked through the carburetor, the front of the power head, then through the side of the round reed valve cage and finally up or down through the reed valves into a cylinder crankcase.  To access the reed cage requires splitting the power head in half (4 Phillips screws down each side of the power head).  This exposes the crankshaft with reed cage (8 SS reeds).  Splitting the power head also exposes the crankshaft upper (O-ring & spring loaded seal) and the lower (spring loaded seal) that may need to be replaced to seal a leaky power head.  Once you have the old reeds out, hold it to the light and you should NOT see an air gap between the tips and the cage.  Any sliver of light is a sign of a weak valve and warrants replacing. 

Carbon fibre or fibreglass reeds generally improve the performance over stainless steel reeds.  If one breaks, the loose part shouldn't damage the internal workings of the engine as a part of a SS reed might.  Tighten the reed stop screw by hand to prevent over torquing against the cage (.75 ft lbs).  Doing so could distort the reeds and the air flow, especially if a reed extends over the edge of the cage.  Use a dab of Locktite (blue) on the thread to ensure it doesn't loosen to damage internal engine works.

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REPLACE END SEALS (future) - Prior to ordering parts for this job take note of the top and bottom seals that also make the power head air tight.  An air leak through any seal in the power head will create a lean mixture, resulting in difficult starting, erratic idling and poor acceleration. 

- The upper O-ring.
- The upper & lower spring loaded seals, (1 1/8" OD, 3/4" ID).
- Boyesan Reeds, 2CCSM-08SSF (for the round reed cage).
- Check valve.
- Permatex Permashield Gasket Dressing & Sealant for power head. (made for 2 cycle engine, stands up to gasoline).
- Permatex Form a Gasket for the water chamber.
- 2" ball hone if you replace the rings.

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ASSEMBLY - Be careful when cleaning the power head seal surfaces.  If you scratch or gouge a surface it creates a potential leak.  Apply the Permashield sealant and tighten the power head bolts uniformly when assembling.  Torque is light, 20-25 in lbs..
 

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