1985 Mercury 75 - Fine on land, dying at idle in water

[EchoNovember]

Not trying to turn this into an oil thread :facepalm: but Walmart doesn't make oil and all oil will have an API certification label. The label means it meets the minimum requirements for the given oil type. No APT label, don't buy. with label, it should be fine

Why not? We could always use another oil thread. Just like motorcycle forums need more tire threads. What is the boating equivalent for that, anyway? Lol.

I use Wal-Mart as a synonym for crappy, low quality items. It may meet minimum standards, but I don't like minimum standards, never have. I find it causes problems and usually leaves me feeling the minimum level is far too level. If I knew more about chemistry and oil engineering, I might feel differently.

But, as I've already said, I'm satisfied that it wasn't the oil. I'll be switching oil to reduce smoke output, though. I'll be towing skiers and tubers, so I don't want to smoke them out.

 

[EchoNovember]

I didn't get the compression tester tonight, picked up epoxy for the transom project, though. It's on another thread. I'm going to try to get that done soon, though.

 

[DavidMoore]

As others have said you have to get the compression tested, should really have been the first thing to be checked even if just to rule it out of the equation. With a cheap tester don't worry about the indicated values because they can be wildly inaccurate, you just want them all to be about the same.

That said, whilst it could be many things, my gut feeling is you should look at what Roscoe said in post 8. Either a lean condition or your idle speed is set too low, or both

In which case I would advise to get a manual and synch the carbs with each other and then with the ignition timing, the procedures are in the book. You might find the manual for your engine for free online at boatinfo.no.
But you'll need a tach, no ideas on how to set the idle speed without one unless you just have a feel for it.

Or try a quick fix:
If you didn't make adjustments when cleaning the carbs you I don't see how you cleaned the low speed jets. Counting the number of turns, turn each one in until lightly seated then back them off the same number of turns, it's an easy way to clear a blockage, no help then try opening them up an extra 1/8 to 1/4 of a turn and see if it makes a difference, it won't hurt anything and you can always put them back how they were. This could treat a lean condition but may lower the idle speed slightly.

If it's idle speed problem bump it up a bit, just remember this is done with ignition timing so use the low speed timing screw to do the adjustments, it might be all it needs. It's no problem to error on the high side so long as you can shift smoothly.

If a quick fix works then great, but consider that a proper link and sync will result in a better running longer lasting engine, so I would do it sooner rather than later.

 

[EchoNovember]

I have a Clymer service manual for the engine. I'll look into how to get that done. I should have a $12 tester arriving tomorrow, and I'm still going to try to borrow one from an auto parts store tonight to get the test done sooner.

 

[EchoNovember]

Okay, so the tester is arriving today. Once it gets here I'll get compression numbers.

Been researching this like crazy. Please correct me if I am wrong, but it looks like if the compression is fine than it's either my idle is too low, or it's just running too lean. Or, third option, it's an old two-stroke and I just need to let it warm up longer with the throttle turned up.

Again please correct me. I bought this boat a month ago and at the time I only knew a couple nautical terms and how to drive a boat from my dad renting a pontoon when I was 12 and letting me drive it. I've been crash course studying ever since. I know a little bit about engines as I do all my own work on my car and motorcycles, but my only two-stroke knowledge comes from a 50cc scooter I had. My knowledge of choking an engine comes from my first motorcycle. That's it. I'm learning more and more every single day, and although I am a fast study I always miss things because I don't always know what to research. They say 90% of being smart is knowing what you're dumb at. I'm dumb with marine engines. ^_^

Oh, I did find out the "push to choke" doesn't really choke it. It mainly squirts a little fuel into the throats to help with starting. That's why it's common to have people recommend pushing it in several times before starting, it puts more fuel into the throats to help it start easier. I was doing this during testing with Seafoam spray. A little Seafoam sprayed in the throats worked like starter fluid, but I don't want to do that every time I start it. I think it burns hotter than gas, and I don't want to burn a hole in a piston or cylinder.

 

[EchoNovember]

Good news! Compression is 151-150-150-149. Inline spark tester shows a spark on every wire.

Now I look at adjusting the carbs, right?

 

[MH Hawker]

yep

 

[EchoNovember]

Okay, I set up a super scientific back pressure simulator. It worked well enough to kill the engine the first couple times it started. Then it ran fine. The engine was already warmed up from running on just muffs. Since the bucket wasn't tall enough for the water intake to sit submerged, I still had to use the muffs.



Is water supposed to spray out of here? It was water along with higher speed exhaust.



When I revved up the engine, the amount of water and force it came out with increased.



According to non-touch tachometer, my RPM dry and in the bucket is about 1,200 RPM at idle.

We need to finish rebuilding the transom before we can splash it and truly test it in the water.

 

[alldodge]

Yes, that's calld the pee hole and tells you your pumping water thru the motor. If you ever run the motor and don't see water coming out the hole, shut it down and find out why

 

[EchoNovember]

I didn't mean the tell tale/pisser valve/pee hole. I meant the exhaust vents, where the exhaust comes out when the pressure is high enough.

 

[EchoNovember]

Random thought... Is there a sticker of that kid who pisses on Ford, Chevy, etc., I can get to center over that tell tale? Or something a little classier like those pissing fountains?

 

[MH Hawker]

i use a plastic trash can, muffs wont work on my motor and try that it will get you a lot closer and you may still have to tweak it on the lake

 

[EchoNovember]

That bin? That was the best I could do. I bought the largest can Home Depot had, and it didn't fit. Our largest garbage service wasn't big enough, either.

 

[MH Hawker]

one of the blue barrels cut down may work

 

[racerone]

Very little if any backpressure with that set-up.

 

[EchoNovember]

Very little if any backpressure with that set-up.

Yeah, I need something bigger. At the very least, it acted like a muffler, keeping the neighbors happier.

 

[Mercurylips]

Take the time to remove the propeller and the thrust washer. This will keep more water in a regular inexpensive plastic garbage can. I don't have a clue as to what may be going on with your motor. I have been thru the wars with mine. Expect the unexpected .

 

[EchoNovember]

I found one off the wall solution where the symptoms match mine: heavy carbon deposits in the exhaust. I'm hoping to be able to take off the lower unit tomorrow to check, but it's likely going to have to wait until next week. We're finishing up the transom replacement so that takes priority. We leave Thursday for our trip, so time is of the essence. I'll be tinkering with the carb adjustments while on vacation with the boat in the water, so we'll see what happens.

 

[EchoNovember]

Transom is back in, trim piece on, just finishing up bolting the important bits together and will be putting the motor back on and resuming troubleshooting. Ordered a fuel pump kit. Going to replace the diaphragm and check valves, filter screen, and o-rings. Also going to replace the engine fuel lines preemptively. I have no idea when they were last replaced, if ever, so I figured it would be a good idea to.

Separately, I will be taking off the lower unit, changing the gearcase oil, impeller, and propeller, and checking the exhaust tube for carbon build up. I have a snake camera I can send up the exhaust to get a look at the top near the engine to see if I will need to pull the powerhead to clean out lots of buildup.

Is there any truth to cheap/low quality oil causes excess carbon buildup? When looking into what causes carbon buildup I found posts on iBoats, thehulltruth, marineengine, and other sites that had people claiming cheap/low quality oil can cause these issues.

 

[EchoNovember]

Oh yeah, and I never took the boat with. Bad weather and it wasn't ready. So, no tinkering was done yet. Just planning on preventive maintenance and some rebuilds. I may never know exactly what the issue is as I am likely going to get everything done in one go, though I will try to test it after each "fix" to see if that solves the issue.