75 hp mercury bogging

[topwaterjunky]

Hi guys, I'm new to the site. I have a 1995 75 hp mercury oil injected motor. It ran perfect for about 9 months after I bought it. The guy said he cleaned the carbs before he sold it to me. It started bogging down and I noticed the oil was low so I put some in it and it ran fine for about a week. It started bogging down again and oil was fine. It would idle fine in neutral and idle fine in gear but under load it wouldn't get over 2200 rpm. I rebuilt the fuel pump, and pulled the carbs which they were spotless and I put a new fuel filter on it. The gas tank was about empty with ethanol fuel so I filled it up with non ethanol fuel and ran seafoam through it. I took it back out and it bogged down again but eventually took off and ran good. It ran good for about 3 or 4 days and one day I was running WOT and it started losing power, it was like I'd have power and then it go away and then come back. I shut it down and it started bogging down again but this time I'm able to get up on plane but it takes a while and I'm only pushing around 3300 RPM and I lost about 15 mph. I'm no marine mechanic by no means and i dont try to act like I am but I've checked my spark plugs and they were good but I replaced them anyways. I've checked all my gas lines and they all feel tight. I need some advice from yall experts. Thanks in advance for any help or advice.

 

[Texasmark]

Well I was going to suggest a Seafoam application but you have already done that. Next time you are out, after running for awhile, shut her down and quickly pop the cowl and put your hand on the top rear of the engine block. How hot is it? Can you leave your hand on it or do you have to remove it....how fast? Could have pistons swelling in the cylinders caused by a bad impeller, thermostat, or cooling popoff valve....or crud in your cooling system. How strong is your tell tale at idle?

 

[topwaterjunky]

my tell tale and I'm assuming your talking about it peeing?? It is a pretty strong stream but before all this happened, I did get some crud up in the intake and it stopped up the pee hole. I eventually got it cleared out with a coat hanger and it's been peeing good since. I took it to the lake a couple days ago to see how it would run after changing out the primer bulb and it's lines and still the same thing. it seems like it's in a safety mode or something but from a few things I've read, mercury doesn't have a safety mode but to answer your question, the motor was hot afterwards but it wasn't so hot you couldn't put your hand on it. I just don't understand what I'm missing because the boat runs fine, it just won't reach full RPM and full speed. I did put a new prop on it a while back and I lost roughly 400 rpm after doing that but it ran fine for a while after I did that and it ran fine for a few days after the first bogging down incident. Thanks for any help

 

[bwhalerman]

my tell tale and I'm assuming your talking about it peeing?? It is a pretty strong stream but before all this happened, I did get some crud up in the intake and it stopped up the pee hole. I eventually got it cleared out with a coat hanger and it's been peeing good since. I took it to the lake a couple days ago to see how it would run after changing out the primer bulb and it's lines and still the same thing. it seems like it's in a safety mode or something but from a few things I've read, mercury doesn't have a safety mode but to answer your question, the motor was hot afterwards but it wasn't so hot you couldn't put your hand on it. I just don't understand what I'm missing because the boat runs fine, it just won't reach full RPM and full speed. I did put a new prop on it a while back and I lost roughly 400 rpm after doing that but it ran fine for a while after I did that and it ran fine for a few days after the first bogging down incident. Thanks for any help

I just finished dealing with the exact same issue with my 1988 70hp Mercury. The party I purchased my boat/motor and trailer from disabled the oil injection system. I was out a couple weeks ago and the horn sounded. Obviously an overheat problem. Since then I rebuilt all 3 carburetors. I learned that it is difficult to find out exactly what size fuel lines to use. They may be tight but could still be leaking. The mechanic said to use fuel lines that are equal to the port opening on carburetors. I now use 5/32nd lines between carbs. Also, the float on one of my carburetors was 4/16" they are supposed to be 7/16" I adjusted it to proper spec.engine fired up on first turn of the key.

Here is what I've done to date.
1. checked all fuel connections. Ended up installing all new lines. If you have a fuel enrichment valve don't forget to check those lines as well.
2. rebuilt the fuel pump.
3. completely disassembled all three carburetors soaked them overnight in carburetor cleaner, installed new idle screws and needle valves on floats, adjusted floats to specification
4. Installed new primer bulb and oriented it vertical to boat floor
5. Reduced the length of the supply line on the motor side of the primer bulb and then lightly secured the line to throttle cables to remove stress on the clip connection
6. Dropped the lower unit. I purchased my boat/motor/trailer in December 2014. I was told that water pump service was done that year, WRONG. I pulled the lower unit it was caked with mud so badly that I couldn't clean the cover plate and had to replace it for $85.00 remarkably my local boat yard had just one left. When I fired up the motor the stream was double what it was prior. Just because it's peeing doesn't mean the motor is being cooled properly.

I'm no mechanic either but I think if you attend to these 6 steps you may solve the problem. And as a final check, evaluate the condition of the thermostat. Most references I've seen indicate thermostat is an unlikely failure but it does happen. Given the age of my boat that is my next replacement regardless of condition. In my estimation it probably is as old as the motor.

As a final note, you will find a lot of good information when you post on this site. One word of advice. If something doesn't seem right, STOP ask for help. Making a mistake without confident solutions could be very costly

 

[topwaterjunky]

Thanks for all the advice bwhalerman! I have spent a couple months reading posts on here with problems similar to mine but nothing has worked or I guess I can say it worked but only last 3 or 4 days then it went back to bogging down after running great but this time wasn't like the first. This time I can get on plane but it takes forever to do it and I lost 15 mph and 400-500 rpm at WOT but the boat runs perfect during all this. No missing, sputtering or dying, it just doesnt have the power. I will try these things you have said and see what happens. some of them I've already done but I will double check them to make sure I didn't miss something. I will definitely make a post if this fixes my problem because I've been fighting this for 2 or 3 months now.

 

[Texasmark]

my tell tale and I'm assuming your talking about it peeing?? It is a pretty strong stream but before all this happened, I did get some crud up in the intake and it stopped up the pee hole. I eventually got it cleared out with a coat hanger and it's been peeing good since. I took it to the lake a couple days ago to see how it would run after changing out the primer bulb and it's lines and still the same thing. it seems like it's in a safety mode or something but from a few things I've read, mercury doesn't have a safety mode but to answer your question, the motor was hot afterwards but it wasn't so hot you couldn't put your hand on it. I just don't understand what I'm missing because the boat runs fine, it just won't reach full RPM and full speed. I did put a new prop on it a while back and I lost roughly 400 rpm after doing that but it ran fine for a while after I did that and it ran fine for a few days after the first bogging down incident. Thanks for any help

Follow the hose. The Pee comes off the exhaust manifold water jacket cover on my similar 90 hp. That is a different cooling loop from the engine block and is uninterrupted by the stat or popoff. Dirt dobbers and such love to clog the tube. Mr. Whaler covered a lot of bases you might choose to consider. At your engine's age, engine hoses should be deteriorating (my 2002 did it in 10 years) which will clog your internal engine fuel filter if enough get in there. I don't think you have a cooling problem because if you did, you wouldn't have passed the "is the block too hot" test.

Before you dig in, one thing to try is when you get up to your best rpm/speed and all, squeeze the bulb a few times and see if the engine responds. If so, you have a bad fuel pump, pieces of fuel line in your gas flow loop, or cracks in your engine's fuel lines letting air in thus reducing your fuel pump's ability to supply the engine....which includes punctured diaphragm in the FP and/or leaking valves.

 

[topwaterjunky]

Thanks Texas Mark for replying. I have already tried the squeezing of the bulb thing and it doesn't help at all. I will try some of the things Mr. whaler mentioned and see if any of it helps. None of it makes any sense to me. Do you know if there are any safety things on this motor that will shut it down to half power or is it true mercury doesn't have any??

 

[Texasmark]

Thanks Texas Mark for replying. I have already tried the squeezing of the bulb thing and it doesn't help at all. I will try some of the things Mr. whaler mentioned and see if any of it helps. None of it makes any sense to me. Do you know if there are any safety things on this motor that will shut it down to half power or is it true mercury doesn't have any??

No. The computer controlled larger engines probably are the only ones with that feature. I read about it but forgot which engines were being represented. I know you don't have it because all you would have would be an over rev module but that has to sense rpms up above 5500 rpms or so to kick in. The 4 cylinder versions (115-125 hp) of your (and what was my) engine series may have had it. I may have run across it in my service manual which covered 75 thru 125, 3 and 4 cyl engines. But where and what is just speculation. I know I didn't have it in my 2002 90 triple, but I did have the module that modulates the low oil alarm which was optional on those engines.

Years ago Merc had a switch that shut down the engine if the lower unit kicked up, like if you were running along at a high throttle setting and hit a log causing the prop to clear the water allowing the engine to overrev. I guess this feature was just a modern idea of a protection feature that has been on the books for a long time.

You can look in your engine for one however. The sense line would have to be the grey wire from your rectifier/regulator module. Normally it goes to your 9 pin engine harness to remote control harness connector so that the voltage regulator input pulses from the rectifier can be sent to and counted by the dash mounted tachometer. If that line Tees or Wyes, then follow it to the module. The module would then have wiring that would route to the ignition trigger circuits. I'm speaking for the 2002 engines which have separate modules for the electrical functions. Your engine may have a lot of the electronics grouped in a common switch box. Following the wire color coding is how I would search for an answer.