Mercury 50 HP bogging down
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Posted on Tuesday May 7, 2013 at 9:00 AM  
My dad's boat has been doing this for a while now. I was hoping the problem would go away when I replaced the float and stopped the leaking issue.

It starts and idles great now. You can rev it up in neutral with no problem. When you put the throttle down in gear to try to get up on plane it bogs down and will die. My dad discovered that if he taps the choke when it starts to bog down that it will pick up rpms and get up on plane if you time it just right.

When I took it out for a test spin on Sunday evening I had to engage the choke and hold it closed before it would get up on plane. After it was up on plane, I could let off on the choke and it wouldn't lose rpm's. I only tried it a couple of times before I put it back on the trailer. After running on plane for a few seconds and pulling back on the throttle to come off plane it would die again...

Here's a short video that I took of it getting on plane. If you listen closely as the beginning of the video, you can hear the engine start to rev and then bog. Then you hear the click of the choke engaging and it jumps right up on plane...

http://smg.photobucket.com/user/bassser/media/2013-05-05201842_zps3332186e.mp4.html

My first thought is a vacuum leak. If it was a fuel supply problem, I can't imagine that it would have as much power as it does after you choke it. Could I spray some carb cleaner around the carb flange like you would on a 4-stroke to see if it's sucking air there without damaging the motor. I guess it would be better to use deep creep that has some upper cylinder lubricant in it...

I'm pretty sure the idle adjusting screws are adjusted right, but I might try to play with those some. I guess maybe back them off 1/4 of a turn at a time.

Any other suggestions?
Posted on Tuesday May 7, 2013 at 11:05 AM  
A vacuum leak should cause an idle problem. Sounds more like a main jet circuit restriction, low float level, weak fuel pump, air in fuel from air leaking into the fuel supply line, restricted fuel supply filter.
Posted on Tuesday May 7, 2013 at 11:06 AM  
Sean obviously its a fuel problem - at idle ranges you are basically sipping vacuumed gas from the carb bowls thru low speed jets to the carb throats to combustion chamber - on call for throttle your vacuum increases to pull more gas into high speed jet orifices which are bigger than low speed - by throwing the choke plate on you are then pulling more gas when getting on plane -

It sounds like your high speed jets aren't getting enough gas

Your float in the bowls could be sticking - so at idle there isn't much gas demand, you are putting along flat - you shower down and your boat angle of attack changes, which also changes float attitude - if the floats are adjusted too low you starve for gas when you need it most

I'd probably pull the cars and start all over with kits and gaskets - OR you can buy complete carbs for that engine pretty cheap on e-bay -- since you have fixed seats , that would be my biggest suspect at this point -- when you pull the carbs soak em in BG-44 a couple of days pull any jets you can and really blast all orifices with compressed air --

Not what you wanna hear I know --

I'd try having someone squeeze the siphon bulb at the same time you are trying to get on plane to see if you can force more gas - if boat picks up you will know its in the carbs --
Those who would give up constitutional freedoms to elected officials in exchange for a false sense of security, deserve neither freedom or security.
Posted on Tuesday May 7, 2013 at 12:09 PM  
Okay, you guys talked me in to it...

My dad replaced everything outside the motor within the last couple of years trying to fix the problem, but I think I'll go back through the whole system starting at the fuel tank and make sure everything is good there. I found some info online yesterday that gave a detailed outline on how to check for air in the system (using a temporary piece of clear fuel-resistant line) and also how to see if your fuel pump is creating enough suction using a vacuum gauge in-line.

If all that checks out okay, then I'll pull those stinkin' carbs back off again. I'm getting way too good at that...

Thanks for the help!
Posted on Tuesday May 7, 2013 at 6:41 PM  
Sean one thing I forgot about is that model merc was notorious for getting pin holes in viton pump diaphram - that fuel pump has a Sierra kit that rebuilds it too --

I believe this is your carb

http://www.ebay.com/itm/Mercury-Carburetor-50-hp-1987-1379-9450-/251063516050?pt=Boat_Parts_Accessories_Gear&hash=item3a748d3b92&vxp=mtr

might be a good deal

Those who would give up constitutional freedoms to elected officials in exchange for a false sense of security, deserve neither freedom or security.
Posted on Thursday May 9, 2013 at 8:28 AM  
I got the boat out to the boat ramp yesterday evening to do some testing. I had installed a clear 12" section of vinyl tubing right at the fitting going in to the motor to watch for air bubbles in the line. No air bubbles were visible at idle or wide open throttle. (BTW, it still bogs down about half-throttle when on the trailer in gear. I didn't think to try squeezing the bulb when it bogged, but choking it still worked.)

I also had a vacuum gauge hooked up in-line between the bulb and the motor. If there was some kind of restriction or non-venting issue, it should have pulled excessive vacuum when at full throttle, but it didn't register any noticable vacuum at all.

The last test is the one that I want a second (or third or fourth) opinion on. This is to test that the fuel pump(s) are pulling adequate vacuum. You are supposed to pinch off the fuel line between the tank and the vaccum gauge and it should pull between 3-5 inches of vaccuum within 10 seconds. Mine showed 5 inches of vacuum in about 5 seconds and stayed there to a count of ten before I released the restriction. So, if that test is accurate, my fuel pumps are working good. Any thoughts?

If everything from the tank to the fuel pump is good, that leaves float adjustment or some kind of restriction inside the carbs... The next thing I'm going to do is pull the carbs back off and re-adjust the floats. (I know I set the lower carb float a hair low the last time I pulled it off because the needle didn't want to seat at the higher setting. I later figured out that I was squeezing the bulb too hard and it was probably seating fine at the higher level.)

I really think the carbs are clean. I've sprayed and re-sprayed every orifice several times and they soaked when we rebuilt them last year.
Posted on Tuesday May 14, 2013 at 12:32 PM  
I thought about the fuel connections at the motor and decided to bypass that just in case that was causing a restriction. That didn't change anything either.

I guess I'm down to rebuilding or replacing the carbs.

I did note while synching the carbs/timing that the motor bogs as soon as the secondary throttle actuator touches the throttle linkage and starts to open the carb up a little more after the ignition reaches max advance.
Posted on Monday May 20, 2013 at 9:01 AM  
I have a 7.5 Mercury that did the exact same thing on Saturday. It ran great for about 10 minutes then out of the blue it did that. We were in the spot we wanted to be so I didn't try to restart it for a few hours. The rest of the day it ran fine. I'm starting to dislike that motor.
Posted on Monday May 20, 2013 at 11:50 AM  
I would suspect maybe some trash that worked its way in to an opening and then moved back out when you shut it off.

That or maybe vacuum built up in the fuel tank due to a restriction in the tank vent.
Posted on Monday May 20, 2013 at 1:15 PM  
Thanks Basser,

I never really thought of the tank vent doing that but when I did pop the gas cap it was under major pressure.
Posted on Wednesday May 22, 2013 at 2:19 AM  
I had an old 115 Mariner years ago that would do something similar and found that the fuel pump was not strong enough to completely open up the spring operated ball valve fitting at the fuel tank. Just as the boat would get up on plane it would have the bulb drawn down completely to where the motor wouldn't get any gas and the motor would bog down and die. I replaced the fitting and didn't touch the fuel pump and it worked fine for the couple of years I had the boat. Before you do anything drastic I would go buy one of those fittings without the spring operated ball valve in it (they're only a few bucks) and see if that solves your problem. If it does then it's an indication that your fuel pump needs attention but it will get you running anyway. Good luck.
Posted on Wednesday May 22, 2013 at 9:04 AM  
I completely bypassed that fitting and it did the same thing...
Posted on Wednesday May 22, 2013 at 9:26 AM  
BassSER, can you run it with the intake silencer off and get it to bog down? If you can look into the carb and see if fuel is being pulled into the air stream when it bogs or does the air seem to stand still or have a fog of fuel in front of the carb.
Posted on Wednesday May 22, 2013 at 11:34 AM  
I'll see if I can find a way to clear the front of the carbs so I can see in there...
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