Gremlin Hunting Season Begins

[Bud Kennedy]

The gremlins that have been infecting my 1991 Johnson 150 hp Fast Strike will once again be the subject of my efforts to try to find out what is causing my problems.  While there have been many things tried and a whole bunch of parts swapped the problem still exists.

Here is a list of the things that have been examined, service or replaced.

All 6 carbs have been rebuilt and re jetted with the correct jets.

The entire fuel system has been replaced from the fuel tanks up to the motor including all new connectors including primer bulb (Also Twice)

Fuel filters replaced including the fuel/water separator.

All new spark plugs (twice)

New Power Head

New encoder and optical sensor

The bottom line is I still have the problem.  I will try my best to describe what I am experiencing.  The motor starts and idles great.  The problem comes when the motor has not been run for a week or so.  When throttling up to go on plane, I cannot get above 3100 rpms If I stay throttled up the motor flounders like it is not receiving fuel then all of a sudden after a few minutes of trying, something happens and the motor takes off like a stripped a$$ed ape and performs normal the rest of the day.  It jumps out of the hole like it should and attains all the rpms it needs to perform.  Basic high rev is just short of 5800 rpms max.  The problem comes the next time out and I have to go through all this bs again.  When the motor finally takes off it almost feels like a supercharger kicking in.  Additional investigation reveals some carbon build up on one plug.  I am suspecting at this time that the relating carb has a problem and might be causing this 

The only things not yet replaced on this motor are electrical related, such as the power pack and the coils and plug wires.  Although they both test good, they too are considered a suspect although I still believe it is in a one of the carbs.

The boat is going into the shop about the middle of January to begin the investigation.  At that time, it will also get routine spring service for the lower unit.  The water pump is still performing as it should so I will not be replaced this season unless a problem develops.  I am running a 27P 4blade shooter prop that is a bit tall for this motor and the search is on for a 24P but I have not found one as yet.  What I have learned over the past couple years is that the 27P was not uncommon for this motor originally but my new power head is not as strong as the original.  The originals were hot, hot, hot.  Wish me well on my search for the gremlin.

[Bassinkorea]

Does the motor have a fuel pump, what kind and have you replaced it?
Sounds like fuel starvation just as you suspect. Maybe adding a more modern type of fuel pump might help get the fuel to the right places faster.

[Bud Kennedy]

The fuel pump on this motor is nothing more than one of those old membrane deals and I too have questioned the mechanic about this and he insists it is ok

[merc1997]

bud

here are some of my thoughts.  i would borrow a smaller pitch prop and try.  you need to check your thermostats.  your engine may be staying cold too long because the thermostats are stuck open. this would cause a flooding issue, which it sounds like you could have.  if a carb, it could have a needle sticking and letting the fuel bowl over fill.  check all the connections on the power packs.  could be a loose wire.

there you have some quick simple checks that would cause the issue you are having.

bo

[Bud Kennedy]

It is now time to re engage the gremlin hunt.  Over the past year a lot of work had been done with the fuel system.  The problem still remains in one cylinder.  I have one that when you pull the plugs it tends to be wet with fuel and the plug looks to have a slight carbon build up even though the plugs are new.  The other 5 plugs look great.  I have decided to take the next steps to get this motor right.  Here is what I am planning.

Replace the power pack and all six coils

Check the carb that is associated with the non performing cylinder to ensure the needle valve is working correctly and to see if the float may be sticking.

The electrical changes I am requesting most likely has a problem or it might be as simple as one coil. With the age of this motor is is reasonable to just go ahead and replace the power pack and all coils so that I don't waste weeks and weeks of in the service shop by shotgunning one piece at a time.  From my research this is not all that radical idea.  With these changes I will have a completely new fuel system from the gas tank to the motor complete with all new fittings.  The electrical changes will also mean a totally new electrical system.  Just not all that much more stuff to do.  I have read where others have had the same problem on either their fast strike or intruder motors.  Their solutions are variable but on the bottom line the eventually swapped out everything as well.