'87 Mariner 45hp Loss of power past 1/2 throttle.....completely stumped

Hey everyone,

I've got an '87 Mariner 45hp 2-stroke 4cyl (2 carb) that has no power past 1/2 throttle. She'll crank super easy, idles great, runs great from 1/2 throttle and below. When you try to go any faster she hits a wall and just stays there. If you back off to 1/2 throttle it picks back up and will hold there. I've look and the timing IS advancing with the carbs. Throttle plates are synced and fully open at WOT. It doesn't sound like it's missing any. I pulled all the plugs and they are all the same color, so that tells me they are all firing. She ran great up until I installed the floats in the carbs too low, so I thought that was starving it for fuel, but I went back and adjusted them to OEM spec (5/16") and took it on the lake again....same issue, but not quite as bad.
The first time it started surging as soon as I put it in REV coming off the trailer and it ran WOT for a couple of minutes, then gradually kept dropping rpm's until we limped back to the ramp via basically idling.
After taking the carbs apart and getting the float heights to spec, we took it out again. She didn't surge at all backing off the trailer, idled great, but had no power or increase in rpm's after 1/2 throttle. It ran somewhat better, but still super slow.
This seems like an easy fix and I know there are tons of reasons why it has no power at WOT (I've scoured the internet looking at other posts, but no one seems to post how their issue was resolved.)
nIt ran great up until I over-adjusted the floats, so I'm thinking I did something in advertently and just can't seem to discover what that is. I haven't messed with the flywheel or timing, so I don't think that's in consideration.

Here's what I have done so far:

1) Rebuilt both carbs and thoroughly cleaned (several time)
2) Replaced ALL fuel lines from the tank to the motor
3) New fuel filter
4) Cleaned spark plugs
5) New high-end stator and trigger installed
6) Cleaned fuel tank
7) New primer bulb from Wal-Mart
8) Rebuilt fuel pump
9) Only using non-ethanol gas
10) New switchbox

Things I'm thinking it could be:

1) Water in gas tank from rain (I leave the cap vent screw open at all times to keep it from swelling due to heat/sun)
2) Carbs need cleaning again? (high-speed jet, etc)
3) Fuel filter caught some debris and is clogged
4) Spark plugs need replacing
5) Linkage between carbs/timing arm are slightly off?

Any help is greatly appreciated fellas. Thanks so much!

Since it happened after you worked on the carbs that is a likely place for the problem to be but just to verify that take the engine covers off and have someone else squirt some throttle body cleaner or brake cleaner into the carb while your running with the throttle wide open (on the water please!) to make sure that it is fuel related. If that pans out look for a fuel restriction, blow backward through the fuel hose at the tank and the fuel filter. If nothing there consider the fuel pump being the issue. While "the last thing that was worked on" is many times the problem don't forget that it could be a totally unrelated thing doing your lake trips in.

Are the " boost venturis " in place ?-----Have you checked exhaust tuner for blockage ?

racerone said:

Are the " boost venturis " in place ?-----Have you checked exhaust tuner for blockage ?

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Yes sir, the venturis are in place. I had the lower unit off to completely change the water pump system and didn't notice any blockage.

I took the carbs off AGAIN and completely cleaned them out super good (like crystal clean). She still acts the same way when we went on the lake yesterday. It even died while at idle when we stopped to swim. Then it took about 10 minutes to fire back up when we finished swimming. This has me super frustrated now. Just wanna get this back to normal operations

Removed lower unit and looked UP into the exhaust housing to check for blockage ??

racerone said:

Removed lower unit and looked UP into the exhaust housing to check for blockage ??

Click to expand...

No sir, I haven't check the exhaust with the lower unit off. I've had the adjust the carbs some because they seemed off a little. Does this sound ok at idle?

I removed the lower unit yesterday to check the exhaust housing....found water on the walls of the mid-section all over the drive shaft and on top of the water pump, gear shift lever, etc. Grabbed my hose and gently sprayed water up until it came out of the tell tale. After a couple of seconds water started trickling down the walls of mid-section compartment where the driveshaft and water pump pickup pipe is located. There wasn't any rust buildup yet, so I think this is a recent development. Does this sound like the powerhead needs to be removed and the gasket replaced?

racerone said:

Removed lower unit and looked UP into the exhaust housing to check for blockage ??

Click to expand...

@racerone I removed the lower unit yesterday to check the exhaust housing....found water on the walls of the mid-section all over the drive shaft and on top of the water pump, gear shift lever, etc. Grabbed my hose and gently sprayed water up until it came out of the tell tale. After a couple of seconds water started trickling down the walls of mid-section compartment where the driveshaft and water pump pickup pipe is located. There wasn't any rust buildup yet, so I think this is a recent development. Does this sound like the powerhead needs to be removed and the gasket replaced?

??----Water is pumped up to the block for cooling.-----That water is returned to the lake.-----It comes out of the block and into the exhaust for cooling the exhaust.----The water that you are seeing is normal !!

racerone said:

??----Water is pumped up to the block for cooling.-----That water is returned to the lake.-----It comes out of the block and into the exhaust for cooling the exhaust.----The water that you are seeing is normal !!

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Thank you racerone. I appreciate the help. I'm still learning about OB engines. I did noticed on my parts diagram that I am missing the little connector fitting that seals & attaches the water pickup tube to the water pump. Will get that installed when it arrives.

I did look up into my exhaust housing and noticed some buildup, but no complete obstructions. Any other ideas as to what could make my engine be bogging down so badly? Timing? Dropping a cylinder while under way?

Any ideas are greatly appreciated

Did you ever get your problem solved?
I am experiencing the same kind of problem. Classic 50/45hp Seems to run great, to 1/2 throttle, then I don't get any more. I can see that the carb linkage is opening all the way, the trigger is advancing, I have spark in all 4 cyl, good compression...but when I go past 1/2, I can hear the carb throats open more, but no more power than 1/2.

This is under load testing on the boat in a lake. It the barrel is sure seemed to rev up to max rpm in neutral, but not under full load.

That motor will rev right up in neutral on 2 of 4 cylinders.----So that test means nothing at all.----What trouble shooting has been done if any ??

Displaced in the Desert said:

Did you ever get your problem solved?
I am experiencing the same kind of problem. Classic 50/45hp Seems to run great, to 1/2 throttle, then I don't get any more. I can see that the carb linkage is opening all the way, the trigger is advancing, I have spark in all 4 cyl, good compression...but when I go past 1/2, I can hear the carb throats open more, but no more power than 1/2.

This is under load testing on the boat in a lake. It the barrel is sure seemed to rev up to max rpm in neutral, but not under full load.

Click to expand...

I haven't solved it yet. Still troubleshooting. I've got a manual now and I'm working through it step by step. Just haven't had time to get it on the water to run tests under load.

I did find out that my rectifier was shot, so I just have replaced it. If that hasn't resolved my issue, then I'm gonna look at timing through WOT, stator/trigger testing under load, and putting a clear fuel line on before & after fuel filter to see if air is leaking into the system.

Compression test first.----Post the numbers.----Do you have spark that jumps a gap of 3/8" on all 4 leads, yes or no ?-----Changed out the water pump impeller every 5 years ?

Thank you Racer One for the reply. You seem very knowlegeable about these motors. Thanks for being willing to help.
New impeller. Pumps great, shoots water 8'.
I will check spark strength to see if it can jump 3/8" out of all 4 coils. I can use any spark plug to check this, right? The plugs for this motor are the type with an internal electrode and no exterior ground electrode. I can just take a plug, open the gap to 3/8 and look for strong blue spark?
Compression, top to bottom, 135, 132, 135, 133.

I own a '85 Merc, 60hp. It has a stator with a high and a low speed winding. The switch box "switches" between the windings around 2000 rpms. I don't know your specific motor but it is possible due to being about the same age that it may be a possibility. You will need a DVA to thurough test your stator..

I have the same engine and year range that has a similar problem as stated . Idles and runs to aprox. 2000 rpm's but wont take more throttle .
I ordered a manual and waiting for its arrival mid-week .
Watching closely for your fix .

lowder----Describe the trouble shooting done so far on your motor if any?

IF its the same as mine, top carb has a hose coming off the main jet ( this also holds the bowl on) that goes to the enriching valve. Look closely with a magnifying glass to check for trash in the jet. Took me pulling the carbs 3 times to find it. Good luck and let everyone know what the solution was.

I notice a possible problem . I have a mercury midi manual for the engine and on their schematic for the stator they show 6 wires , 4 going to the cdi . 2 red , 2 blue , 2 yellow to rectifier .
My stator does not have the red wires and left off the cdi (nothing their)…. This I noticed as I was going to ohm test the stator as per the manual .
My stator also has the long rectangular black box with wires on both ends that goes between the stator and cdi shown in parts book as going along with the electric start stator , so maybe my mercury has the wrong one ???
I’m no master mechanic but I stumped with why ?
also I recently purchased this boat for a project . It started on the muffs so I assumed it was ok … only to learn after using it it would not take rpm’s . Idles and start fine though .
Are the stators interchangeable or is mine wrong for this engine … since I purchased it untested ?
The motors on a 1988 Tracker 170 pro that I’ve redone all the rotten wood and carpet . Looking forward to finishing it up and using soon -hopefully .

Its had a red stator kit installed

Yes there is a brand new looking red stator on it . Is this the correct stator and if so why does it not have the red wires which there is 2 places for on the cdi . When I look up parts to fit this engine I noticed 2 different stators coming up ,the red plastic coated unit without the two red wires like mine and another stator kit with the 6 correct wires . Is there a chance the last fellow installed the wrong stator ?
By the way I am also in Ky. and noticed your KDBA fazbullet .
Back in the mid 1990's I ran a 24 arronow catamaran with 2 2.4 merc bridgeports around n.ky on the river before going to i/o's .

My manual came , if it’s the right manual . I’m s a select outboards 1965-115hp 3 and 4 cylinder 2 stroke models and the Mercury MiDi manual and I really see nothing in the other that shows my particular engine ?

Displaced in the Desert said:

Did you ever get your problem solved?
I am experiencing the same kind of problem. Classic 50/45hp Seems to run great, to 1/2 throttle, then I don't get any more. I can see that the carb linkage is opening all the way, the trigger is advancing, I have spark in all 4 cyl, good compression...but when I go past 1/2, I can hear the carb throats open more, but no more power than 1/2.

This is under load testing on the boat in a lake. It the barrel is sure seemed to rev up to max rpm in neutral, but not under full load.

Click to expand...

I replaced my rectifier, but still have the loss of power issue. I did discover that my prop hub is spun so will be replacing it. Hoping this resolves my power loss and gets my top speed back up.

Replacing the rectifier didn't solve the loss of power. I did discover that my prop hub is spun and was slipping so I've got one ordered and hoping that helps my top speed return. Will post after I take it out on the water.

I just wanted to post a follow-up to this thread b/c I know that step can sometimes be forgotten. Long story short, I bit the bullet and bought a used carb body for my lower one and cleaned it really good. I also adjusted the max timing advance since it was way off. Not quite sure how that happened. I replaced the prop that I have with a lower pitch (11.8x10) which helped my acceleration out of a hole shot tremendously. The motor has never run better! No misses, surging, or loss of rpm. I actually gained 4 mph in speed and my rpm also went up to within the acceptable range as it should be. I really appreciate all the help from everyone on here and sometimes I suppose we just have to go step by step until we get our issue resolved. Slow down...think it through...and use your gut. That helped me out tons. Can't wait to get it out on the water in the spring. Happy & safe boating everyone!