88 Mercury Bogging down at full throttle

It starts and idles fine, sometimes it'll run without an issue topping out around 40 miles an hour on my boat others it bogs down and won't go over 20. Even when it bogs down it runs smooth, it just doesn't have the power to push it any further. I don't have a tach on board to say the absolute RPM drop, but suffice to say it's running at 50% power. I can throttle it down to an idle without issue, it doesn't stall or run rough. Once it starts bogging down it doesn't seem to recover unless it's shut down for an extended period of time.

The plugs are new and clean and I have strong spark to each cylinder and strong compression (120psi +/- per cylinder)

It's fed from a new fuel tank and I rebuilt the fuel pump after bringing it back to life from an extended period of dormancy. Thinking it felt more like a fuel delivery issue I've replaced the fuel line, fuel filter, and most recently cleaned the carbs top to bottom to no avail.

Has anyone had any experience with this problem with this motor? I have the full factory service manual on it, but I have not found this fact pattern in any of the troubleshooting sections.

..yer loosing spark when hot and by the time ya check for spark there is spark.

Check for spark when it bogs.. .like as quick as ya can...if ya haven't done it that way yet...


...or shoot the head with a temp gun to see which jug is out...then check for spark...

Found a bad coil with the multi-meter last night. Tested the stator and the trigger, as well to cover my bases knowing I can't test the switch box and found both were within spec. One of the 3 coils appears to not be original, now a 2nd has died, and the motor doesn't have too many hours on it, so I'm suspicious there's an underlying issue that's killing the coils.

Replacing that and the plug wires to start. Switchbox is sort of a diagnosis by default by my understanding...so if the problem persists/resurfaces I'm going to replace that. At least I'm only gambling a $25 coil at this point.

Did you ever get this figured out?
I had same issues on my 88 35hp and it turned out to be an issue with the fuel pump. The expanding pins that hold the diaphrams in are a little larger in the current rebuild kits thus not allowing the push pin to expand the fingers and allow the pin to come out and and sucked into the inlet of the fuel filter causing the motor to bog at 3/4 to full throttle. I drilled the two holes in the fuel pump block one size larger and never had another issue.

Yeah, it was an electrical issue, though after three rounds of failing to ID the underlying culprit part I'd had enough of 1.5hr back & forth trips to the lake just to test the boat killing what little free time I have so I replaced everything (stator, trigger, switch box, coils, rectifier) in order to be certain I had a reliable system going forward. I was able to get CDI parts (OEM) for the rectifier, stator and trigger on Amazon, and I used well reviewed Kemimoto aftermarket components for the switchbox and coils and the total price worked out to $460 before tax.

In the process I upgraded the rectifier to one with voltage regulation to protect the digital electronics in the boat per CDI's recommendation, which wasn't even something I'd been aware of being an issue before starting this project.






So the gremlin lies somewhere in this collection of components. I've held onto them in the hope that I can eventually ID the problem, but no luck so far.

Glad it’s fixed!

very common

i did a lot of them when i worked at mercury/mariner dealer

their electrical componentry is subpar

i even had a 90hp 1987 i think, same issues, same symptoms, same "fix".....replace the switchbox, stator, all 3 coils, and eventually the wiring harness and it ran fine until the fuel pump started to die then it leaked fuel into the crankcase while engine was off, made it nearly impossible to start it after running it. of course usually i was a few miles from civilization by then. got rid of that whole boat with the motor after fixing the fuel pump then the 'new' owner said the pt&t died. glad it became someone else's money pit

never known a mercury that didn't need something

Yup. The rest of the story goes right along those lines...I wasn't going to gripe about it, but so much for that!

I only bothered in the first place because prior to all this it idled nice and smooth, has 125psi on all 3 cylinders (+/-) and the lower unit shifts nice and smooth. There is no PT/T on this motor, one less thing to go wrong at least. I went down the electrical road after rebuilding the carbs, fuel pump and replacing the fuel lines thinking it was too smooth to be an electrical issue.



Well, I'm tearing all the ignition parts off and sending them back to Amazon if they'll take them. It started up with the Mercury sneeze, I confirmed with the guy at the local repair shop the "smoking gun" just about confirms a reed valve is the culprit, as it was with my 25hp. Only on my 25hp the reed block was just behind the carbs, easy access, easy fix. On this motor they're on the crank bearing carriers. Dumbest design conceivable and way beyond my comfort zone for tearing into the internals.

Maybe someday I'll try it, if I'm smart I'll just part it out and be done with it and buy a new one.

onthewater102 said:

Yeah, it was an electrical issue, though after three rounds of failing to ID the underlying culprit part I'd had enough of 1.5hr back & forth trips to the lake just to test the boat killing what little free time I have so I replaced everything (stator, trigger, switch box, coils, rectifier) in order to be certain I had a reliable system going forward. I was able to get CDI parts (OEM) for the rectifier, stator and trigger on Amazon, and I used well reviewed Kemimoto aftermarket components for the switchbox and coils and the total price worked out to $460 before tax.

In the process I upgraded the rectifier to one with voltage regulation to protect the digital electronics in the boat per CDI's recommendation, which wasn't even something I'd been aware of being an issue before starting this project.

So the gremlin lies somewhere in this collection of components. I've held onto them in the hope that I can eventually ID the problem, but no luck so far.

Click to expand...

Which rectifier did you get?

I got the one that was recommended for my 84 60 Merc. It didn't really fit in a good spot, and would get hot and start overcharging just like the OEM one once the battery was topped up.

I ended up having to move it off of the outboard and on it's own heat sink to work properly.

https://www.amazon.com/gp/product/B008F8WZWA/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1