1995 75 hp Mariner losing power at WOT

[Bonvillian]

Hello Everyone. This is my first post to the forum. I have used this forum as a reference many times in my adventures of keeping my 1995 75 hp Mariner running. The knowledge I've found on this forum is more than most and I find the discussions quite knowledgeable and helpful. As we all know, New Outboards are darn expensive, and as a DIY person I feel better about gaining the knowledge and performing the repairs myself. Not only to gain knowledge, but to have that overwhelming sense of satisfaction when you change a part, and it works.


So....... to the problem at hand. About a year ago I developed a miss in the engine but only at WOT. I'd back off the accelerator and the engine would smooth out. As most people I know that most of these problems are due to fuel issues. Ethanol gumming up in carburetors and screwing up fuel lines and the sort.

So I pulled the three carburetors off the engine and gave them a good cleaning. I put fresh gas in the tank, and launched the boat to see if that was the problem. She ran like an animal for about 15-20 minutes. I mean ran better than ever. I WAS PUMPED UP and thought the issue was resolved. And then........she started missing again. You can imagine my face after that. I initially thought maybe i had trash in the fuel and some little piece got clogged in a jet of one of the carbs again. So with my head down in defeat I picked up the boat and hauled it home. The engine would crank fine in the driveway and rev up as if nothing was wrong. Still I figured it was a carb issue. So, I pulled the carbs again and cleaned them thoroughly again. This time I gently used a torch tip cleaner to ensure all the orifices and jets on each carb was clear and sprayed carb cleaner through each one confirming that I had good flow through everything. I changed all the fuel lines, even the lines on each side the fuel pump and the lines on the carburetors. I bought a new primer ball and rigged it to a portable gas tank that was completely cleaned with fresh "Ethanol Free" gas and the proper oil mixture. Put a brand new in-line fuel filter in between the gas tank and the fuel pump, as well as an inline filter between the fuel pump and the carbs (just in case the fuel pump was deteriorating and pushing tiny pieces of rubber to the carbs).

So its time to test again. Launched the boat and it ran good for about 5 minutes. Then started to miss but not as much as the last time. It would get to 4500 rpm and bog down then pick up again. The longer I rode the more the bogging down continued. So again I picked up the boat with my head down.

The next day I went to the auto parts store and picked up three new spark plugs. The ones specifically for the motor, and a spark tester. Went home changed the plugs and placed the spark tester on each plug to ensure I was getting fire to all three cylinders. Each cylinder had spark at idle as well as when I would rev the motor.

Test time yet again. I launched the boat, idled out of the no wake zone, and she ran like a champ pulling upwards of 4700 rpms and running smoothly for about 20 minutes again. And then the dreaded bogging and missing continued only allowing me to turn 4000 rpms. So......I pulled off the motor cowl (cover) as well as the air breather to give me access to the front of the carbs. I had went equipped with a spray bottle with gas in it to eliminate the fuel issue. I took off in the boat holding it at WOT as it was missing and sprayed one shot of gas into each carburetor. If it was a carb problem my thinking was when I sprayed into the carb that was clogged, the motor would pick up and re-gain the lost rpms. This was not the case. Each spray into each carburetor bogged the engine down as if it was flooding. So at least that eliminates the fuel issue. The carbs are well supplied with fuel and seem to be working normally.

I started heading back to the launch and had to slow down to get off of step a couple times due to other boats fishing on the banks. When I gave it throttle the last time it missed and bogged worst than it ever has and I could barely get on step. Like I lost a cylinder. So this has me really thinking that its a coil pack that goes out once the engine reaches temperature. I would have loved to ohm out the coil pack when it did this but didn't have my volt meter with me. I tried doing so when I got home but all three coils read normally as the engine had cooled down.

So....since all three coils are of the same age, I ordered three new coil packs to replace on the engine. And this is where my saga concludes. They will be in friday and I plan on changing them all out then giving it a try again on Saturday Morning.

Now I am no mechanic. But with the internet and forums like this I can find and learn alot of information quickly and learn to troubleshoot an issue. But If its not the coils........then what? I know the powerpack or switchboard gives power to the coils, but it seems that if the powerpack went out, you wouldn't have any fire at all. But, thats just an assumption.

Am I on the right track? Any guidance would be appreciated. I have great respect for all of you who take the time out of your day to answer and share your knowledge for the benefit of the community.

When nothing is going right, GO FISHING

Thanks Again
Dennis Bonvillian

 

[Texasmark]

I have had temperature problems with coil packs like your engine has on a 2002 90 triple which is basically your engine. I've had them not working cold and then working when they warm up and vice versa. Good luck on that. You mentioned that you were filled with glee with your 4700 rpms when you got it. The 75 is rated 4750 to 5250 for WOT operation. You are below that range. My experience with Merc 2 strokers is that they love to run. My 90 just got a little more gas and air to push it on up to 5500 WOT max rated. I ran above that most of the time in my little light weight boat. It loved it.

Assuming you have been running 4700 for a long time, might consider a new prop, same make and model, just 2 inches less pitch. Your engine will love you for it.

Also adding Sea Foam to the gas/gas mix if you premix, has proven itself to me and lots of members of this forum (it's where I learned about it) for keeping carbon under control. Your engine is loop charged and the design uses only 2 piston rings per cylinder and the 2 rings are wedge shaped, pressure expanding to force better sealing. Running lower than recommended rpms at WOT (lugging if you will) and especially for an older engine, translates to carbon buildup in the combustion chamber...piston and rings included, and that can prevent the rings from expanding as they were designed. Directions on the can. Might run some SF for awhile before you decide on another prop and see if it cleans things up.

 

[Bonvillian]

Thanks for the Reply Texasmark. Before I began having the problem, I was turning right at 5000 rpms. Since then I could never get over 4000 exept for the first time I cleaned the carbs. It ran great at 5000 and after about 15 minutes, it dropped to 3500 missing and sputtering all over again. I'm thinking/hoping that this all has to do with a bad coil or coils and I'll get my WOT range back after changing them. I should know more on whether the coils fix the problem or not within a couple days. I'll be sure to update.

Since I cleaned my carbs the first time, I've been running strictly non-ethanol gas. Should I still be using an additive like seafoam? If it does help out on the engine side of things I'll will definitely start using it.

Thanks Again.

Dennis

 

[jimmbo]

Have you checked all ignition wiring for good connections? Heat/corrosion can wreck havoc with the lower voltage wiring too. Is your fuel tank venting correctly? Have you ruled out a fuel tank pickup pipe problem, by running on different fuel tank?( seems to run fine for about 15 - 20 minutes, then develops problems.) I am assuming you head out for your tests with a freshly topped up tank?