Surging Mercury (with audio)

[l008com]

In a previous post, I figured out that my oil leak was coming from my carbs, not a leaky oil line. The consensus was that fuel was leaking in to the carb, evaporating, and slowly filling the bowls with oil.

Since figuring this out, I've been disconnecting the fuel line when I'm done boating this year and letting the engine run until it's out of gas. It has run better since I started doing this, and a lot less smokey.

But I want to be sure - or at least sure-ish - that my problem is all carb related. Over the past few years, the engine has idled very poorly. Sometimes it won't even idle without dying. Last night it could idle but its very rough, and it surges a lot. In years past, it would run ok but when I was idling in some chop, the engine would start to get smokey and sometimes even stall out. Then it would restart and be ok. And through all of this, if I can get it running and in to get, and GUN it, after a few seconds, it runs LIKE A CHAMP at speed. It doesn't "fix" the problems, like it still idles like poop once i slow down. But it runs so good at like half throttle or more.

So I already plan on trying to rebuild the three carbs this winter.
The point of THIS post is first, to make sure I don't have two problems at once, and something else that might be going on. And second, I would like to better understand exactly what is causing the problems I'm having.
I don't necessarily need to understand why cleaning/rebuilding the carbs will fix the problems I'm having, but it would be nice to Like the low RPM surging, I know a bit about engines, but I have no idea what is causing that!

The engine is a 1989 Mercury 90 HP 3-cylinder 2-cycle. It has the autoblend built in, so its straight gas in it's gas tank.
Here is audio of it idling in neutral last night:

This was after driving it around for 3 to 4 hours.

So what do you think? If you want more of the boat's history, let me know. It's had an interesting and long life and hopefully it keeps going for a while longer.

 

[racerone]

It sounds awful------Step #1 is a compression test.----Post the actual values here.----And does spark jump a gap of at least 3/8" on each lead ?

 

[Faztbullet]

Thats sounds more like a ignition problem. Take a timing light and hook to each cylinder to see which is missing

 

[l008com]

Would any of those problems - poor compression, poor spark, poor ignition, go away at high rpms? I'm going to be packing it up later today, I'll at least take a look at the plugs and see if they're in bad shape. I think they were good last time I checked. For the rest, it might have to wait until spring.

 

[racerone]

Your motor is a simple " 2 stage , air processing machine"----Air is pulled into the crankcase.---Compressed and then transferred to the cylinder.-----It is then compressed again.----A spark fires off the compressed fuel air mixture.------So if there are internal leaks there is more time for leaks to cause idling issues.---Spark gets stronger as flywheel spins faster.------I suggest you do the trouble shooting now.-----In spring shops will be busy.----New and used motors will be more expensive.-----Help yourself, see post #2 on this.

 

[jimmbo]

Have you had the Carbs off? If you did, after putting them back on, did you do what is called a Link and Sync? That is making sure all the Carbs are fully closed at Idle, and all are properly synced in regards to the Spark Advance.

 

[l008com]

Nope I have never taken the carbs off, though I'm going to try to soon. I think my carbs need a rebuild even if I ALSO have some other issues going on.

So here's a picture of my spark plugs. They don't look too bad, but they do look surprisingly dirty. And wet, which is weird since I ran the motor out of gas two days before I took this picture.

Also I guess it's probably also relevatn to note the overall situation this boat is in. It's a 90 HP screamer on a 15' boat. It grew up on a lake in NH taking us water skiing all day long. BUT now it's on it's second life where it boats around Boston Harbor and Boston's rivers. EVERYTHING is a no-wake zone. I try to give it a little when I can, but its very rare. This engine always hates idling and idling is pretty much all I ever do in it.
So with that in mind, it doesn't surprise me that the plugs are dirty. Back when I used to use the boat a lot more than I do now, I also used to put a small amount of seafoam in almost every tank of gas to help keep it clean inside.

Anyway, this is what the plugs look like:

 

[racerone]

Try a different style of plug.

 

[Black58]

Do you have a repair manual? If not get one. Do each step suggested above, plus drain your tank and in the early spring fill with fresh fuel. You might smile after doing the suggestions above. I read and followed them and this year my 115 is running great.