Merc 90 FourStroke carburetor rebuild

[pcrussell50]

solved: turned out the rings were all carboned up. It's been great now ever since



My fishboat has a 2000, carbureted Merc 90hp FourStroke.

6 months ago, it started: -hanging at mid-power, not wanting to transition to the WOT fuel circuit without coaxing and easing it up there.


This was no biggie to me because it ran great in every other phase, and the pacific ocean is usually too rough to go full speed in the little, flat dead rise 17' Whaler.


But now:
-it would barely idle anymore because of a bad misfire


Sooo--yesterday I yanked all four of the carbs, one for each cylinder, and disassembled them. They looked spotlessly clean except for one thing--carb#3--from the top. The little rubber-tipped needle connected to the float, was stuck in it's bore, with hard crud all over it. I forget, but I think it was jammed shut.


the carb bodies with all the little bits are in the sonic cleaner soaking and vibrating in kerosene.


observations:
I guess the #3 cylinder was misfiring because the float bowl couldn't fill properly.


seems weird to me that every other part of the carbs looked sooo clean except that one float needle.


every tank had sta-bil and quickkleen in it.


the gunk on the needle was hard and mostly white--not the typical color of varnish fuel or chunks of sta-bil.


man these EPA carbs have some tiiiny passages in them.


I'm diggin' the o-ring-style assembly over the old skool paper gaskets on my carb'ed crossflows .


I have been religious about starting it on the muffs every week, but it sat for nearly a year by the PO without running, (but with sta-bil and quickleen).

[Capt.Insane-o]

BE SURE you remove the pilot jets and clean them DO NOT try to poke anything through them though.

[racer]

The carbs do plug easily once rebuilt you will need a vaccum mate to sync them.

[pcrussell50]

got one, al--the old skool kind though... looks like four old thermometers in one housing.
pilot jets are out--cooking and vibrating. did NOT run anything through any orifices.

but what about nylon guitar string? (not steel)

-peter

[Capt.Insane-o]

no....

[pcrussell50]

Going through old threads cleaning them up with resolutions to problems.

This one was [solved] a month or so later in 2012 (yes about ten years ago )

Since this was a Mercaha and there is a Merc dealer nearby, I took it to them. The head mechanic is/was a young guy named Tony. (He has a tattoo of a little wrench on his hand). Even though he is a Yamaha factory mechanic he recognizes this power head as the Yamaha F100 that it is. Plus he’s very wise and grass rootsy and old school like an old man who spent his life fixing carb motors.

He starts any poor running complaints with a leak down test and found a couple of cylinders super fast leakers. 80-90% IIRC from ten years ago. Rings not valves. So either an overhaul or stuck rings. So he advised a super duper de carbon shock treatment just to make sure. And sure enough that is what did it. Whew! I really didn’t want to spring for an overhaul or a new engine.

I think I carboned it up by idling it on the muffs weekly thinking I was doing it good. I did that too much without getting it out in the water with some speed and load.

Anyway, since then, the motor has been running great. I have a motorcycle carb balancer and do that every three years or so. And drain the carbs for the off season (Sept-May for us). With that it’s been flawless ever since.

Spoke to a boat mechanic in fishing village San Carlos, Mexico who said (in broken English) these motors are desirable there because they are good with fuel but don’t require high tech maintenance.

-Peter

[25two.stroke]

They are damn good motors if you keep the gas clean. Simple to fix. Carb problems are really all they have aside from the occasional ignition issue. I don't know what all the individuals above were saying about not putting anything through the main and idle jets to clear them...there is nothing wrong with doing that. WE do it all the time and does not damage the jets. good way to clear all the junk not just 90% of it like carb cleaner might do.
The engines do have corrosion issues (like other 2000s yamaha blocks...) mostly in the adapter plates, which can cause exhaust leaks or water in the oil.
They also DO NOT like running cold, ever. It will get water in the oil and carbon up the rings. If the thermostat sticks open or if you idle in the driveway but don't let it run long enough to take it up to operating temp then it will damage the engine. Running it in the driveway regularly is good like you were doing, but you would probably need to run it longer and above idle a little.
Like the engines. Wish they had smartcraft.

[Baker343]

They are damn good motors if you keep the gas clean. Simple to fix. Carb problems are really all they have aside from the occasional ignition issue. I don't know what all the fools above were saying about not putting anything through the main and idle jets to clear them...there is nothing wrong with doing that. WE do it all the time and does not damage the jets. good way to clear all the junk not just 90% of it like carb cleaner might do.
The engines do have corrosion issues (like other 2000s yamaha blocks...) mostly in the adapter plates, which can cause exhaust leaks or water in the oil.
They also DO NOT like running cold, ever. It will get water in the oil and carbon up the rings. If the thermostat sticks open or if you idle in the driveway but don't let it run long enough to take it up to operating temp then it will damage the engine. Running it in the driveway regularly is good like you were doing, but you would probably need to run it longer and above idle a little.
Like the engines. Wish they had smartcraft.

I appreciate your comments on some of the things that you appear to have some knowledge on. But the members you call fools no longer contribute their knowledge because of comments like this. They help build and make this site relevant for people like me who want to ask and learn.

[Dave S]

Call me a Fool......Big snot........If members are thin skinned......so be it...... it's just a boatin' site........I have seen this place go down in members...... just life..... no ones fault.......

[pcrussell50]

That’s two of you who warned of the adapter plate between the Yammie head and the Merc leg. Never thought of considering that. I guess I’ll start to look for water or exhaust fouling of the oil. Any other telltales of it going bad that I should be looking for?

As to fuel and small passages: The previous owner already had a water separating filter system on it when I bought it in January 2011. It’s probably paid for itself in spades during that time.

Giant overweight FourStroke 90.

Shameless proud daddy plug: My two little monkeys climbing around on it. Shameless proud husband plug: Mom/wife has trailering skills. She has gone to go get the truck and trailer. On top of that, the truck is a manual tranny. :-)



-Peter

[25two.stroke]

I appreciate your comments on some of the things that you appear to have some knowledge on. But the members you call fools no longer contribute their knowledge because of comments like this. They help build and make this site relevant for people like me who want to ask and learn.

Call me a Fool......Big snot........If members are thin skinned......so be it...... it's just a boatin' site........I have seen this place go down in members...... just life..... no ones fault.......

Sorry, I call everyone fools. People I like...people I don't like. Its just a saying. I will try to remember to not say it if its offensive to someone. I do not mean fool as in lack of knowledge, I meant guy or person or anyone. However, I truly do not know why anyone would be concerned about putting a rigid object through a jet. It works great and doesnt damage???

[25two.stroke]

That’s two of you who warned of the adapter plate between the Yammie head and the Merc leg. Never thought of considering that. I guess I’ll start to look for water or exhaust fouling of the oil. Any other telltales of it going bad that I should be looking for?

As to fuel and small passages: The previous owner already had a water separating filter system on it when I bought it in January 2011. It’s probably paid for itself in spades during that time.

Giant overweight FourStroke 90.

Shameless proud daddy plug: My two little monkeys climbing around on it. Shameless proud husband plug: Mom/wife has trailering skills. She has gone to go get the truck and trailer. On top of that, the truck is a manual tranny. :-)



-Peter

Nah, not much you can tell from the outside. They die from the inside out.

[pcrussell50]

Nah, not much you can tell from the outside. They die from the inside out.

Any difference, in fresh water vs. salt? This boat hasn’t seen salt since 2013.

-Peter

[25two.stroke]

Any difference, in fresh water vs. salt? This boat hasn’t seen salt since 2013.

-Peter

Yeah freshwater the adapter plates will not corrode. The thermostats can still stick and you get water in the oil, but thats an easy fix when it happens.

[Baker343]

25two.stroke it's all good. The reason I found this post was my Pops has a 2005 90hp 4-stroke on a pontoon and we have water in the crank case. We bought it used and didn't initially see water in the oil.
We did do a compression test before purchase and all were even, can't exactly remember 110-120? We will be doing a leak down test before we move forward to throw parts at it.
Doesn't look like a salt water motor to me and it was a trade in at a boat dealer. Any thoughts thermostat/housing, base gasket?