150 Merc Slows down after WOT....ideas?

[saltfisher]

I just got a boat and it has a 1988 merc 150 black max. I took it out for the 1st time today, and anything under 50-75% throttle it runs fine, plans nice, ect. But when i smack WOT it will stay WOT for about 20 seconds, and then it feels like someone lifting up on the throttle and the boat will slow down and almost come to a stop unless i lift the throttle back to 75%. So I can only run bursts of WOT for about 20 seconds, and then back off throttle a little and its fine this way. But I cant stay at WOT or this "backing off" issue occurs. Any ideas?

 

[MikDee]

Re: 150 Merc Slows down after WOT....ideas?

Sounds like a fuel delivery issue, you have a lot of items to check with this.

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

What is usually the most common problem? Sorta sounded like every once in awhile the rpm's would go slightly up or down while running but nothing major. It almost seems if i keep on WOT while this backing off issue occurs, and almost let the boat slow down, and then back off into neutral the motor will shut off and be hard to start for a few tries. Almost like its getting flooded. It almost seems like its getting to much gas, or not enough..... I dont know. Its almost like the motor itself is not accepting all the fuel that its trying to get put into the engine at WOT and backs itself off.

 

[hkeiner]

Re: 150 Merc Slows down after WOT....ideas?

When troubleshooting, I like to use test procedures to check the proper operation of system components whenever possible, rather than merely replacing parts randomly based upon guesses (informed decisions?) as to which component may be bad. The below linked Mercury Service Bulletin describes a good way to test the fuel delivery system.

http://www.marinepartsman.com/Mercur...2001/EN_17.PDF

Another thing you can do to test the fuel supply is to temporarily connect a portable fuel tank directly to the fuel pump to see if the symptoms go away. If the symptoms do not go away, you might then try connecting a pressure gauge between the fuel pump and the carbs (using a t-connector similar to the vacuum test method described in the service bulletin above) to see if the pressure falls below spec when the motor is bogging.

If these tests do not indicate a problem with fuel, you might then check for proper spark when the engine bogs. I find that using a SmarTach meter to check secondary voltage for troubleshooting purposes is the easiest way. It connects to the spark plug wire with an inductive lead similar to an inductive timing light and can be moved from one wire to another while the motor is underway. Another way to test for ignition failure during motor bog is to use a DVA meter to test primary voltage at the coils, but I find this method awkward to do while the motor is under way at higher speeds.

Others on this forum may have additional or better advice than this.

 

[Gevans17]

Re: 150 Merc Slows down after WOT....ideas?

My 135 Hp Black MAx was doing same thing. couldnt take full throttle without bogging down. The engine had sat for 2 years without use due to hurricane Katrina.
Found several things contributing to poor performance.
clogged high speed jet #3 cyl
no spark to #4 cylinder- corrected by changing switchbox
broken spark plug wire terminal at coil #6
replaced spark plugs and in line fuel filter under cowl
cleaned carbs with Deep Creep (Seafoam spray.

Runs great now. Hope this helps you.

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

Maybe i will try a few of those things. My motor does go WOT for a bit, then boggs down untill i pull back the throttle to compensate. It also runs a little rough, and has a up and down idle. Even at crusing at like 30mph you can hear the motor go up in rpms a bit, and then down. I want to run some seafoam through and see how that works. i also am going to replace spark plugs, wires, fuel filter, and a few small things. How do you check the jets, and clean them? Also someone said about turning the high speed idle on the carb. I have 3 carbs, so would I turn 3 screws? Like 1/8th of a turn CCW, but which screws? I seen a fat bronce looking screw on all 3 carbs, with a flat head slot in it. On the right side of the carb at the bottom left. Would this be it? Also when i was crusing at like 30mph i pushed in the key to choke, and it bogged, so i guess that works? Also it likes to stall sometimes when comming to a stop and going into neutral, then it takes a few times to start back up, and sometimes it tried to start, starts sputtering, then shuts back off. It i raise the fast idle when it starts it seems to help.

 

[gss036]

Re: 150 Merc Slows down after WOT....ideas?

You need to change the fuel/water seperating filter and check for water by dumping the contents in a clear glass jar.
Look FAQ and do a decarb on the engine, then chage the spark plugs and then try again w/running. If those efferts don't do the trick you will need to dig deeper as suggested above. Maybe fuel pump? If it has never been replaced, probably time.
Let us know what you come up with.

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

If I take it to a local marina, can they test the fuel pump to see if its ok? I know its working, but say its only pumping 75% of what it should, could they test that?

Also I dont have a fuel/water seperator installed yet. I only ran the boat 2 times, and only put 10 gallons of gas in at a time and pretty much ran all of it out so there was no gas left in the tank.

Ok, so here is what i am going to do for the next trip out....

Decarb (seafoam in gas, and spray deep creek in carbs while running?)
Change plugs
Change fuel filter
maybe plug wires also

I will repost back and let you guys know if it worked or not. Any other simple items i can replace that could be the cause, or maybe good to do anyways? I heard about some people saying about replacing some check valves, or something? Is that the "T" that connects the oil line with the fuel line?

 

[fishingdan]

Re: 150 Merc Slows down after WOT....ideas?

Slow down my friend. It is too costly to replace stuff just in case it is bad. Take it step by step. Let's assume you have spark on all cylinders for now.

Follow the advise above for cleaning fuel related filters and inlet screens.

Without the engine running, pump the bulp and look for leaks. Bulb should firm up and you shouldn't see any leaks. Do this with the engine cover off so that you can view the interal fuel lines as well. If anything leaks, replace that part. If there is a small leak, the engine could be sucking in some air at higher rpms.

Get an assistant and go run the boat. Take it up to full throttle and observe the bulb. If it suddenly looks like it is magically getting squeezed, you may have a blockage at the fuel pickup in your tank. If it looks normal, have your assistant gently squeeze the bulb as the engine starts the problem starts to occur. If the engine picks up, you probably need to rebuild your fuel pump. Easy to do.

The carbs on these engines are very simple. If the gaskets look to be bad for any reason. Get a gasket kit and replace the gaskets.

If all that checks out, you may find that you are loosing spark on a cylinder at WOT. It could be happening, but it doesn't sound like it at this time.

 

[dhsbuckley]

Re: 150 Merc Slows down after WOT....ideas?

this may sound a little simple but this is the exact problem i had with a 200hp yamaha brand new i found that the fuel tank could suck enough air through the breather a idle and half throttle but when you ran it flat out it would slow down and stop some times, didnt relise until i lifted the floor to check the fuel filter that the tank had actual been sucked in, i then open the fuel flap and suddenly the tank let a wosh of air in, it was just a bad breather on the tank.

 

[j_martin]

Re: 150 Merc Slows down after WOT....ideas?

It took me half a summer to shoot this bug outa my black max XR4. Fuel pump was iffy, filter change (the little finger thing) seemed to help for awhile.

I put in a pressure gauge (15 psi, of'n an old pressure cooker) hooked to the last carb in the string by removing the plug and putting in a hose barb. Meter in my lap, I could see that when she bogged, pressure was diving. It ran about 5-6 lbs when it was good.

It turned out to be a little sludge in the fuel (from water and alcohol) and also an aftermarket plastic bayonet connector that a dealer had put in. It just could not deliver enough fuel through the bitty hole in it to satisfy this gas hog.

Final fix
Fuel pump rebuilt (now an annual maintenence item)

Big water seperator/filter

Replaced aftermarket bulb and hose with genuine mercury

Removed the bayonet connector and ran the line directly to the oil mixing

Tee.

Be sure you fix it soon, and good. Leaning out this engine is the quickest way to burn it up. When you hear it go, it's gone, too late.

hope it helps
John

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

When you say the bayonet connector, where is that? Sounds like a easy fix there to run a straight line. I could even do that tonight.

I dont have a fuel/water seperator, so think that may be the problem?

 

[j_martin]

Re: 150 Merc Slows down after WOT....ideas?

The connector between the squeeze bulb line and the motor that goes in with a slight twist.

The original merc parts are all metal. The aftermarket plastic parts aren't as strong so the hole through the middle has to be smaller, which is the problem. It's ok on a 115, but a 150 is too thirsty.

It's well worth the trouble to rig up a fuel pressure gauge, even if it's temporary, and be reassured that it's getting enough fuel, especially at wot. If it goes totally flat, it dies, but if it's just too lean, it'll burn down in a heartbeat.

hope it helps
John

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

Ok I found my problem. Today It was hard to turn over and wouldnt start. I squeezed the primer bulb and seen a squirt of gas come from it. I was like.... hmmmm..... i think i found the problem. Squeezed it again, and found a tiny pinhole in the bulb. I temperary installed a wire tie around it so it wouldnt be able to suck in any air, and it started right up. I seen when It was starting before that the bulb would like, suck itself like there was a air pocket. It ran like a charm all day long, started on 1st crank every time. Now i have a new problem.... my motor is not charging the battery when the motor is running. But Im glad i got this problem fixed.

Thanks guys for all your help.

 

[saltfisher]

Re: 150 Merc Slows down after WOT....ideas?

Well looks like it didnt help......... My motor ran great the last 2 times i was out. I changed spark plugs last night, and installed a new primer bulb. Well the problem started back up last night. Did the same thing as before. Shut down at WOT, and doesnt stay running for long at idle before it wants to idle at lower then normal RPM and then shut off. Not sure what else to do. Why did it run great the last 2 times? Thats what I dont understand.