Mariner 50hp losing power WOT

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
I just bought a new to me boat. It has a 1985 mariner 50hp 4cyl outboard that I am having an issue and not sure what else to check.

Issue:
The outboard runs fine I but when I open it up the motor drops power like I just pulled back on the throttle. I have taken it out twice first time it did this about 20mins in. The last time it did this about 5 mins from leaving the dock.

It is running fine at all times, idles perfect, have no issues restarting the motor, and it never stalls out. The bulb is still hard and giving it an extra squeeze doesn't seem to make a difference. When I put boat back on trailer I felt all the plugs and they were all warm.

I have replaced the water pump(was bad when it got the motor), and had a mechanic go through the carbs as they were flooding out on idle when I bought it. He replaced the float, carb needles, and rectifier. He check compression which was around 125 on all cylinders and all had good spark. And that was about 3-4 weeks ago. I also replaced the hose and connectors on the fuel line ( Atwood kit w/fuel demand valve).

Anyone have any ideas what to check for? Right now my only ides are the fuel tank, the fuel demand valve, or maybe carbs out of sync? But I'm just guessing

Thanks
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
1985 50 says to me an original Merc 4 cyl block design with 2 carbs; no thermostat and no internal fuel filter (to clog) that I know of. Chris 1956 is pretty good on those engines of that vintage. Give him a shout.

You pretty much isolated your problem to loosing spark on at least a couple of cylinders. Other problem sources for this kind of write-up would be fuel pump not sucking properly, clogged fuel tank vent, or clogged fuel line all of which you bypassed when you said the bulb stays firm and squeezing it didn't help.

The fact that it immediately responds to starting commands really muddies the water. All plugs being hot could be conducted heat through the block, but maybe not. If all 4 are firing properly, it really muddies the water, for me anyway.
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
Thanks for your response. You are right that it is basically a Merc 4Cyl, 2 Carb. No thermostat that i can see on the motor or on the part diagrams I pulled based on the serial number. It does have a internal fuel filter which I have already replaced, old one was actually pretty good shape.

When i bought the boat i was able to take it for a test run, which is when I found out the carb issues which I had resolved (would flood out at idle or low throttle). But at that time when running WOT it worked flawlessly, probably pushing around 30mph. Right now i max out at 8mph

I do not know if the carbs had a sync done by the mechanic or not after he did my carb work, so wasn't sure if a link and sync could cause this issue or not. This is my first boat, so i am guessing and using these forums for a lot of help.

The fuel tank is still the one that came with the boat. It is a newer attwood EPA tank without the vents. Which is why i put in the Fuel Demand Value to avoid those issues. But was something I considered replacing as a possible cause.

You are correct on the plugs now that I think of it. I just remember reading someone mentioning that test in another forum. I will have to pick up a spark tester on my next run to rule that out. I know it all had good spark when i dropped it off at that mechanic as i watched him do the compression & spark test before i left.

The fact it runs and starts fine is what is giving me the trouble. I could be wrong (and probably am), but I would have thought if i was loosing spark or some issue with the ignition coils/strator that it would run rough.

One bit of information that maybe not be important and maybe normal. But it seems to have a lot of exhaust oil coming out the prop area after i finish running it. I actually freaked out the first time when i saw the black puddle on my garage floor. I know that is somewhat normal for a 2-stroke, but was more than i was expecting. But it could be normal as this is my first boat.

Thanks
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Couple of things:

1. Since everything ran fine before the trip to the mechanic to have the carbs done, possibly the mechanic didn't set the floats properly. If not set high enough the carb might not let in enough fuel to satisfy the demands of the engine. However, the 5 min vs 20 minute time variation is confusing. Have to ask what was the difference in your operating the engine, like WOT the whole time, or partly, if so how much on which trip, those sorts of things.

2. What else have you changed that you can't put back like it was before the problem till you resolve this issue? Just move one cup (shell game) at a time.
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
So there was a difference in operation between the 20 min and the 5 min runs. On the 20 min run, I was probably only running at 3/4 throttle. This was due to massive amounts of debris floating in the lake (Large branches & Logs). After about 15 mins I decided to call it and started to head back to the dock when the issue happened that first time. On the second run I went to a different lake (I sit about 10 mins between two different lakes). As soon as i got out of the no-wake zone, I punched it to full throttle and didn't let up.

Unfortunately there really isn't much I can undo without having other issues. I do have most of the parts that the mechanic did replace. But without that work done, the carbs were flooding out at idle and even at no-wake speeds. Made it very difficult to even run the boat for my short test drive. The one thing that was different about that run and now was I was using the shop fuel tank instead of the one with the boat. When I first used my tank and lines, engine would stall out as soon as I put it in gear. Replacing the line and connectors fixed that issue. Hose had some plastic motor connector and I think that was not getting a good seal.

My boat/motor doesn't have an RPM gauge so it is hard to tell what is happening there. But doesn't sound like it is over rev'ing.

I really don't want to take it back to the shop and pay those prices if it is something I can fix on my own. But of course figuring out what that is, is the million dollar question. I did get the seloc manual for the motor, but nothing is jumping out after reading it (yes sadly I actually read the whole thing).

​In order to check the floats do I need to remove the carbs or can I just drop the bowl? And if I have to remove the carbs do you know the best way to access the left side bolts? They are blocked by the starter motor.

Thanks again for your help.
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
You can just drop the bowl. When you do, the float will fall to about a 30 degree down position. Get something to catch fuel that will come out of the carb, cup, rag, whatever will fit. With the float down, squeeze the fuel primer bulb and you should notice fuel coming out of the upper casting. As you are squeezing, with your finger lift the end of the float till it is parallel with the upper casting mating flange and when you get there, while keeping the bulb somewhat firm, the fuel flow should stop. If it stops before you get parallel you man be onto your problem.

There is a little tab on the float arm that bends somewhat easily and is used for setting this function. Personally, I'd push up on the float with that finger until you force it to become parallel. Have done it numerous times and you aren't going to hurt anything. The tab bends before you tear up anything.

On fuel tanks, getting the one you used before would surely help. That way you are able to concentrate on an engine problem, not wondering where to look, just incase the problem is fuel supply related. Personally, I'd get the tank and try that before I messed with the carbs. Easiest solution if it works. Would expect that the mechanic that worked on your engine knows how to set the float! Even if you have to buy one with a hose and primer bulb, would be cheaper than another trip to the shop and you would have a spare if you needed it.
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
Unfortunetlly I can't get that same tank as I bought the boat about 5 hours away.But i am thinking of getting a new tank this week to try that. Current one doesn't have a gauage so it will be a nice upgrade anyways.

Thanks for the info on the bowl/float. Assuming the tank doesn't fix it I will give that a try this weekend as well. and if that doesn't work i might be forced to take it back to the mechanic. I did give him a call and he mentioned a possible issue with the electronics. But i am still questioning that as it runs fine in the other cases outside of full throttle. I would think if I was having a spark or a electronic issue it would run rough.

I'll keep you posted, thanks for you help.

Ryan
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Why would an electronics issue pop up after a trip to the mechanic unless he accidentally changed something which can happen in any maintenance function. That's always been a concern of mine: Fix what's broken and don't break anything else in the process. Anytime you touch something you have that to consider. I guess I have had bad luck. Of the few times I have had someone else do things for me, especially around the house, they wind up breaking something non-related in the process. Makes me do my own work. But that is not a reflection on the repair, remodel, whatever business as a whole.......I guess. Don't know as I only know what's effected me.

The portable tank that plugs directly into the engine will eliminate several potential issues and I think is the right way to go.....money well spent. If that works then you know where to look for your smoking gun!
 

ehenry

Commander
Joined
Jan 6, 2002
Messages
2,393
I had the same engine only it was a Merc with pretty much the same issue. What I found the problem to be was the internal fuel line from the fuel pump to the carbs was sucking air where it Ts at the bottom carb to the top carb. Bought a brand new Mercury fuel line that still leaked. What I did was go to one of those old time hardware stores that had full wall of every brass fitting you can think of and replaced the all original crimped on line fittings with hose barbs and screw clamps which solved my problem. How I found the problem was.....with cowling off, squeeze the primer bulb and fuel was leaking at those connections
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
Ok got an update and pretty sure this would be the issue. Was about to go to the store to pick up the new tank and decided to quickly try and test the tee connection. When I squeeze the bulb I actually saw a small drip of fuel but not at the tee. Looks like there is a very small fuel leak on the fuel pump on the bottom carb. It is a small drip from behind that blue circle the fuel line plugs into.

It is small which is probably why it only happens at full throttle and never lost bulb pressure.

So the question is .. Do I just remove it and rebuild the whole pump? And how would I remove it just the pump or the entire carb? Being on the bottom there isn't a lot of access because of the starter motor and the front is blocked as well (will try and post picture).

Thanks
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
After looking at the part diagram it is leaking between the fuel pump cover and the fuel pump on lower carb. So would you rebuild the whole pump or just try and replace the gasket and screen under that cover and maybe the cover too?
 

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
I haven't been into an engine of that vintage for years. You have me confused on "fuel pump for the lower carb". Most fuel pumps supply the carburetion system. The pump is made up of a diaphragm that operates off engine crankcase pressures (suck and blow) and 2 check valves, one for sucking and one for blowing. Whatever it is, no you don't want fuel dripping, but a single drop now and then isn't going to be your smoking gun.
 

Rgetter

Cadet
Joined
Apr 5, 2015
Messages
17
So there are two carbs mounted vertically one on top of the other. And they have basically a fuel pump/Diaphragm mounted to the side of each carb. And the fuel line goes from the fuel filter to a tee and feeds each pump. There is more than a single drop. When i squeeze the bulb it is a consistent bubble coming out of the bottom. Enough that I would think under heavy load it would pull in some air. I included a picture of the carb diagram and a picture of where it is leaking. On the first picture it is leaking where the arrow is at between the blue cover and where it joins to the pump. And on the blown out diagram it would be parts 31/30/29. 27-22 is the fuel pump.

Carb.jpg Boat_Carb.gif
 

Attachments

  • Carb.jpg
    Carb.jpg
    59.2 KB · Views: 0

Texasmark

Supreme Mariner
Joined
Dec 20, 2005
Messages
14,544
Ok, agree with you on potential smoking gun. Might as well do your fuel lines and the other pump while you are at it and also check the fuel supply line for debris.......when you are replacing your lines in the engine, open one that is first in line from the external fuel supply and point it into a clean container. Squeeze the bulb several times and check the fuel to ensure that there are no pieces of fuel line or any other contaminants indicating a supply problem due to age deterioration.
 

quicktach

Petty Officer 1st Class
Joined
Aug 24, 2012
Messages
394
You mentioned the oil dripping out the exhaust. What gas/oil ratio are you running? After a run when it has bogged down, have you pulled plugs shortly afterward to see if they're fouled? Running the right heat range plug?
 
Top