74 Mercury 500 WOT bog problem. too rich?

burner67

Cadet
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Jun 11, 2013
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9
I have a 74 mercury 500 50hp long shaft electric start. SN is 4074870

I have had a few problems with this motor which started out mostly electrical which are now solved. I am sparking on all cylinders correctly and timing is set at max and pickup.

Now the fuel problem I am having is holding the motor back from reaching full RPMs at wide open throttle. I am pretty sure it is getting to much fuel because when i am holding it wide open and pump the bulb it bogs down worse. Also when im holding it wide open and give it a little choke it bogs down worse. So when i pull the fuel tank line from the motor and run it wide open it will bog as usual then i will start running out of fuel and go lean for a few seconds where it shows its true power and rpms, it seems proper until of course it runs out of fuel.

So here is everthing I have done:

rebuilt carbs. twice. soaked 10hrs and sprayed with compressed air.
fuel pump rebuild kit.
new timing belt.
new plugs.
new switch box which solved ignition problems.
tried new gas and different fuel tank: no difference.
i also rebuild the lower unit.

Here are some videos to help see what i see.

running on the river full throttle bog: Outboard motor trouble - YouTube

video of bog in neutral in barrel at home Mercury 500 bog - YouTube

I dialed in new floats on the carbs according to my service manual.
This model carb has no high speed adjustment.

It has ran proper before. And the motor will run all day anything below 70% throttle. idles and accelerates great even under load. idle adjustments are dialed in.



If any pictures, videos, or further explanations would help diagnose this problem let me know. i am now 2 months into this motor and it's not yet 100%
 
Joined
Jun 4, 2013
Messages
17
Re: 74 Mercury 500 WOT bog problem. too rich?

I noticed in your videos that your tell tale stream is extremely weak. Replace the water pump impeller before you fry the motor.

Before going too much further, I would check the compression in all of the cylinders to make sure they have life left and/or weren't damaged by things such as overheating.

Next, what do you mean that you have the timing set "at max"? There are factory specs for where the timing should be at idle. Make sure you aren't above that or else you might be causing damage to the pistons and cylinders (and causing running issues).

I know you said that you rebuilt the carbs twice now. Did you replace the float needle and seat? Does the primer bulb get and stay firm when the motor isn't running?

Motor bogging is normal if you apply the choke when it is already warm and running. The choke adds fuel to the air/fuel mixture to aid in cold starting. Adding the choke when the motor is already warm will definitely cause the motor to bog (and most likely die) due too much fuel.

If you are positive that you are getting too much fuel, your floats are either sinking (which is unlikely because you have new floats), your float needles and seats are leaky (which happens), your floats are set incorrectly (allowing too much fuel into the bowl), you have an air intake restriction (which really doesn't happen on outboards because there isn't an air filter to clog up), and/or your choke system is staying partially closed. You could always try playing with the float height setting and see what happens.

Finally, I'm half thinking that you still have an ignition system. Use the timing light to see if the ignition advances correctly as the motor rpm increases. Edited to Add: You can usually set the ignition by pulling all of the spark plugs except cylinder 1. You can then have someone crank the motor with the electric start while you check the idle ignition timing and the WOT ignition timing. Just get the starter a rest for a bit during this process so you don't over heat it.
 

bgc

Ensign
Joined
Jul 13, 2011
Messages
980
Re: 74 Mercury 500 WOT bog problem. too rich?

She not hittin on all 4, check compression and replace that impeller.
 

burner67

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Jun 11, 2013
Messages
9
Re: 74 Mercury 500 WOT bog problem. too rich?

ok sorry i guess i left a couple things out. I was tired it was late.

The carbs do have brand new needles and seats.

The impeller is perfect and the water pump housing has all new seals. i rebuilt the lower unit so i replaced that stuff and it seems to me the stream could not get much stronger i think its doing well.

I have checked compression it was 140 or so across the board all within 5psi

by max i mean i have the distributor set to stop at 35 degrees. carb pickup is at 7-9 degrees. idle is a bit negative to achieve low idle. i was told that was normal. link and sync per manual.

The primer bulb is always hard it is brand new as well as the fuel pump.

I have used a timing light to verify that all wires are firing. as well as a spark tester to make sure the spark is getting all the way thru the wire. And like i said new spark plugs. this is even the second set i put in there to be sure.

Also the choke flaps are out of the way and not causing the problem i have made sure by removing them completely to test.

this basically leaves the fact the floats may still be set incorrectly even tho they are set per manual specs


thanks for your input.
 
Joined
Jun 4, 2013
Messages
17
Re: 74 Mercury 500 WOT bog problem. too rich?

Sounds like you have most things covered. I would add 1/16" or 1/8" to the float height and see what happens. I still think you have a very weak tell tale. I personally wouldn't run it without figuring out why the tell tale isn't stronger.
 

burner67

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Jun 11, 2013
Messages
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Re: 74 Mercury 500 WOT bog problem. too rich?

I will take another video to show the tell tale.
By adjusting it do you mean how far the needle drops to the seat or to make the float close the valve earlier with less fuel in it? The settings on this float are someone difficult for me to understand adjustment from spec. Its the side bowl carb
 

burner67

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Jun 11, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

does anyone have any information about adjusting the carb to get less fuel at wot
 
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Re: 74 Mercury 500 WOT bog problem. too rich?

The high speed jet is a fixed sized so there is no adjustment on that. The only thing you can do is trying to adjust the float height.
 

Aqualift

Petty Officer 2nd Class
Joined
May 20, 2009
Messages
145
Re: 74 Mercury 500 WOT bog problem. too rich?

Tell-tale stream is just fine. The way the No. 2 spark plug looks concerns me. I have to agree with BGC that it's running on only three cylinders. The way it sounds in the video also points to that.
 

burner67

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Jun 11, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

ok so even tho im getting good spark on all cylinders i performed the test where i removed a spark wire one at a time from each cylinder while it's running. when i removed a wire from either of the bottom 2 cylinders the thing bogged down noticeably and continued to run like crap.

when i pulled the wire from cylinder 1 it bogged down a slight bit but not near as much as when i removed either of the bottom 2.

And when i pulled the wire from 2 i could hardly tell if there was any difference at all in operation. and it didn't rev any higher when i put the plug wire back on.

so im wondering if maybe im not running rich at all but im just not getting any fuel to the second cylinder? like i said it had good compression like the others (130) but it doesn't get as much oil on the plug. would this be a reed problem not opening to let fuel in? is that what the reeds do? im very lost on where the reeds are and what they do. if there is separate ones per cylinder or its one big rack?
 
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Jun 4, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

If #1 is getting a good air/fuel mix, #2 will as well, since they run off the same carb.

You have an ignition problem. Start simple and test the spark plug wire with an ohmmeter and make sure it is still good. If you don't have an ohmmeter, swap the spark plug wire with another cylinder's and see if the missing problem moves (remember to swap the cable at both ends).
 

burner67

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Jun 11, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

I don't think its getting water. Its just like its not getting any fuel to that cylinder idk if the reed is not opening or what.

I don't understand how it will be an ignition problem. All the wires can jump a half inch gap easy. And constantly
 

burner67

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Jun 11, 2013
Messages
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Re: 74 Mercury 500 WOT bog problem. too rich?

ok well we are solved here. After this last weekend of SLOW boating around the river i was fed up and went to my marine shop and picked up a used block. when i finally got the old one off i looked at the exhaust housing that went down through the leg and.. it was closed shut blocked off with hard chunks of charcoaly terrrddss.

thanks for the input you guys
 
Joined
Jun 4, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

So it turns out that all of us were wrong. On the plus side, you should have a spare block now if the root problem was plugged exhaust.
 

burner67

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Jun 11, 2013
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Re: 74 Mercury 500 WOT bog problem. too rich?

haha actually the shop returned the block for me and i bought an anchor since wallowa lake ate mine. the mounting points for the distributor were a bit different then mine as well so glad i wouldn't have to deal with rigging that
 
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