1991 Merc 150 power loss

[ganno595]

Alright guys here is a real doozy of a problem that hopefully someone can help me with. The engine runs great on the hose revs fine idles smooth. In the water its the same thing runs great idles smooth and when you take off it runs real strong but when you reach 5000 rpm give or take a hundred the motor falls on its face every time. It does not shut off it just feels like it is loosing a couple of cylinders. Now you can bring the boat back to an idle and immediately do it all over again. its like clock work. Now for the doozy part of it. I have rebuilt the carbs, rebuilt the fuel pump, changed all filters, new switch boxes, new stator, and new trigger, also disconnected the reg/rectifier and it didnt fix the problem. hitting the key does nothing nor does pumping the bubble. compression is fine across the board. about a month ago one of the switch boxes went bad and i changed both of them. then just the other day changed the stator. my only other line of thinking is that the stator wasnt great and took out the new switch boxes i put on it....PLEASE SOMEONE HELP before i turn this whole rig into a reef...haha. as i said its a 1991 merc 150 carburated v-6.

 

[Unbalanced]

Re: 1991 Merc 150 power loss

What's your max timing set at? And is your advance module still hooked up?

 

[ganno595]

Re: 1991 Merc 150 power loss

not sure on the max timing, and by advance module do you mean the idle stabilizer?.

 

[j_martin]

Re: 1991 Merc 150 power loss

not sure on the max timing, and by advance module do you mean the idle stabilizer?.

Yep, that's the one. Both functions are in one box. It should run normally with it disconnected, just timed a bit slow at WOT. If you leave it off, reset your WOT timing, which is at 19 degrees, to 23 degrees. If the engine is in good condition, idle stabilization is not needed.

What you are describing is fuel starvation. Rig a 10 or 15 lb pressure gauge on enough hose to read while you're running to the last (unused) fuel line hole in the carbs. (usually it's starboard side on the bottom carb.) It should read over 2 lbs and under 7 lbs at all times when running. I'm willing to bet it'll fall off when the engine balks. That engine needs 15 GPH of fuel at WOT.

I had that problem on the XR4. All of these items improved it.
Fuel pump kit
Replaced aftermarket fuel line/primer with mercury
cleaned fuel tank and pickup
removed stock filter and installed a 60 gph fuel filter/water seperator
and last, but not least, replaced the aftermarket plastic bayonet connector with.........nothing. Bypassed it and ran the fuel line right to the oil mixer.

That brought it up to 5 psi at WOT. That was 2 years ago and it's been screamin' since.

BTW, it's a little late on this one, but proper diagnostics can really put a damper on the parts expenses.

hope it helps
John

 

[ganno595]

Re: 1991 Merc 150 power loss

thanks for the reply, however it no longer has the idle stabilizer on it. so cant call that a problem. secondly if it were a fuel starvation problem wouldnt hitting the key to engage the choke solenoid multiple times or pumping the bubble rapidly at least cause some change. maybe not make it run right but i would think i would at least hear it try and gather itself back up.

 

[j_martin]

Re: 1991 Merc 150 power loss

thanks for the reply, however it no longer has the idle stabilizer on it. so cant call that a problem. secondly if it were a fuel starvation problem wouldnt hitting the key to engage the choke solenoid multiple times or pumping the bubble rapidly at least cause some change. maybe not make it run right but i would think i would at least hear it try and gather itself back up.

If it's raw fuel delivery, the enricher won't help much, because it robs fuel from the one most likely to be starved to start with.

Squeezing the primer bulb should help, or you should notice the sponge in your hand.

I know it was a real test to find all the bugs in mine, and until I put in a fuel pressure gauge, it seemed to be a moving target. That's because there were multiple faults.

This engine is so hungry at WOT that everything in the fuel system has to be perfect.

You're describing the symptoms of fuel delivery failure. You have replaced the entire ignition system, except for coils with no change.

All I can do is tell you what I would do.

hope it helps
John

 

[j_martin]

Re: 1991 Merc 150 power loss

Just out of curiosity, what plugs do you have in it?

 

[ganno595]

Re: 1991 Merc 150 power loss

the plugs that are in it are NGK BU8H. and they are brand new also.

 

[sschefer]

Re: 1991 Merc 150 power loss

John is right, if you've taken the strongly given advice of installing a fuel/water separator your fuel pump can't pull enough fuel through it at WOT. Mine's rated at 90GPH but my Mariner's pulse operated fuel pump (identical to yours) couldn't pull enough volume through the filters 10 micron filter. I bypassed it and the problem cleared so I put a cheap electric fuel pump in line with it and that almost cured it. Changing jets in the carbs to .002 larger and timining it under load at speed finished the job.

One of the contributors is also the fact that most fuel lines have been replaced with E-10 rated lines and the 3/8 lines that are called for don't fit around some of the tight spot. The cure was to drop to a 5/16 fuel line and without adequate pressure you can't deliver enough volume and your lower carbs will starve out at max throttle.

 

[j_martin]

Re: 1991 Merc 150 power loss

20 micron works fine in the fuel/water separator. Heck, the original "filter" is just a 100 micron screen.

The one that really surprised me is the atwater plastic bayonet fitting would not pass enough fuel. It was installed by a "mercury mechanic".

John

 

[ganno595]

Re: 1991 Merc 150 power loss

this boat has a mercury fuel water separator/ filter on it that the guy has run for a while now with no problems and when he changes it he always uses the same cannister or refill. it was recently changed also. Now the filter that is under the cowling the is right in front of the fuel pump, has anyone had any problems with those? i changed this one when i did the carb job but maybe there is a flow problem there?

 

[j_martin]

Re: 1991 Merc 150 power loss

When I was getting chit out of the gas tank, that thumb sized filter would run about a half hour, then plug up.

Yer chasing your tail without a pressure gauge.

over and out
John

 

[ganno595]

Re: 1991 Merc 150 power loss

I just want to thank you for all the help and ideas. I am going to give the fuel thing a shot and hope that it helps the situation.

 

[sschefer]

Re: 1991 Merc 150 power loss

Check to make sure that it's a 3/8 filter and get a Merc filter not the Sierra. I don't know who's making them for Sierra but they don't flow well. Also make sure the flow arrow is going in the right direction, this is a common mistake because if you install the Sierra so that it looks exactly like the Merc was installed, the Sierra will be backwards. I don't know what you have so I'm just throwing some things out there.

 

[ganno595]

Re: 1991 Merc 150 power loss

yeah copy that, it actually has a bombardier/omc filter on it right now but it is 3/8. So before i pressure test ill go and get a merc filter since those things are cheap anyway. thanks. but after brainstorming while lying in bed last night i think im gonna find the problem in the same place as i was with my johnson 70hp. i had a mercury fuel line on it that had the liner on the inside of the fuel line and the liner broke loose about mid way down the line and folded. why i havent thought about that until now i dont know. im gonna check here shortly but also as i said pressure test just to make sure thats up to par anyway.

 

[ganno595]

Re: 1991 Merc 150 power loss

well its fixed... and it turned out to be that gray mercury fuel line with the plastic liner on the inside of it. the liner completely broke down and the small pieces went through to the inline filter and three big chunks were stacked up at the T fitting where the oil mixes. im thinking these were the major culprits. replaced fuel line and filter ran great....thanks to all for help.

 

[j_martin]

Re: 1991 Merc 150 power loss

Classic.
Too bad you didn't ask before you sunk all that money into electrical stuff.

John