1997 Mercury 2 stroke 200 gets up to WOT then loses RPM's

[Blake_F]

Hey all! I Have a 1997 Mercury 200 Carburated 2 stroke. ill try to be as detailed as possible and any help would be much appreciated

ISSUE: so the issue that I am having is when I'm on plane and get up to WOT. it will start right up and idle fine in gear and in neutral. When I push the throttle all the way down it gets on plane no problem and when I trim it up, it gets up to WOT. After running for about 10-15 seconds or so at WOT, rpms start to slowly drop and it loses speed. ill come off pad then punch it again and it will do the same thing with no issues of getting on plane or getting up to WOT, then it will drop back down.

I had my co-angler squeeze the primer bulb when it would start to drop and the rpms came back a little but nothing that opened my eyes real big so see a forsure solution.

DIAGNOSTICS: that weekend I went into the rabit hole on checking everything I could check. thinking it was a fuel issue I put clear fuel lines from my tank all the way to my low pressure fuel pump to check for any air bubbles. Fuel flow looked good and it it fresh fuel with seafoam. however I did not check my fuel pump because I rebuilt it last year but I will check it next just cause its easy to do. I checked the carbs and they looked extremely clean with no clogged jets. also at idle held my hand over the intake of the carbs and all of them were extremely responsive which was good.

Thought of the possibility of overheating (no alarms were going off). but my thermostats seem to be opening and closing just like they should and im getting correct water pressure at idle and WOT.

So then I proceeded to check spark. did a drop test on all 6 cylinders. when I pulled the boots off the top 2 cylinders getting fed from the top carburetor, there was no idle change but it had obvious spark. which had me scratching my head because the carb is having no issue getting fuel or sending it. I've read about some motors running on 4 cylinders at idle then 6 when in gear. but there is no way that's the case with a Carbureted outboard (right)?

after looking at everything I can think of, im looking into the stator, but im not the best reading OHMs or Voltage and knowing what's good or bad.

Anybody had something like this before? any tips on trouble shooting would be awesome,

trying to get back out on the water asap, bass are spawning!!!!!

 

[Chris1956]

Actually, Mercury makes a 4 cylinder that runs on top two carbs at idle. I do not think Mercury ever made a V6 that did that though.

You might examine your spark plugs. If some are real clean, perhaps water is getting into the cylinders.

The primer bulb doesn't collapse, right? You could switch the top coil with the third coil and see if the non-firing follows. If it does, perhaps the coil is bad or the switchbox is bad.

 

[Faztbullet]

After running for about 10-15 seconds or so at WOT, rpms start to slowly drop and it loses speed.

This is a fuel issue not ignition. Check your fuel lines and filters and pump

 

[jimmbo]

DIAGNOSTICS: that weekend I went into the rabit hole on checking everything I could check. thinking it was a fuel issue I put clear fuel lines from my tank all the way to my low pressure fuel pump to check for any air bubbles. Fuel flow looked good and it it fresh fuel with seafoam. however I did not check my fuel pump because I rebuilt it last year but I will check it next just cause its easy to do. I checked the carbs and they looked extremely clean with no clogged jets. also at idle held my hand over the intake of the carbs and all of them were extremely responsive which was good.

So then I proceeded to check spark. did a drop test on all 6 cylinders. when I pulled the boots off the top 2 cylinders getting fed from the top carburetor, there was no idle change but it had obvious spark. which had me scratching my head because the carb is having no issue getting fuel or sending it.

In the Paragraphs above, are some clues as to where the Problem might be

You are assuming the Pump is fine, cause you rebuilt it.
There was no change in how the engine ran when 2 of the Plugs were disabled.
How did you determine the Carbs were clean Internally?
Where did you verify the Fuel Flow as being Adequate?

 

[Blake_F]

UPDATE:

thank you for the responses. tested my fuel pump and it was all good. changed all the fuel lines and fuel filter. and primer bulb is not collapsing and holding pressure well while running.

went out today on the water and it didn't slow down but was idling a little rough and made kind of like a gargling noice when running on plane. my ignition system seems to be doing its job, I was idling at 12 volts when when I was on plane it was reading 14 volts.

I took my air intake off on the water to visually look in the carbs. when my buddy turned the engine over my bottom carb was pouring out fuel. looks like the float isn't doing its job. I think I found my issue! ordering a carb kit for it!

also my carbs seem to all be sweating on the outside quite a bit. is this normal for outboards? not sure how long its been doing that but the motor has been running great for the past couple months until this power loss issue.

 

[jimmbo]

Improper Float setting, Heavy Float, leaky Needle and Seat, loose Seat, too much Fuel Pressure, any of which can cause a High Fuel Level.

Incomplete Cleaning of all the Fuel Passages can cause Leanness especially in the Idle and Off-Idle circuits. That is why Carb kits contain those little Discs and Lead Balls.
Cleaning a Carb isn't spraying a bit of Cleaner down the Throat, or taking it off and removing the Bowl, the jets and or Mixture Needles, then soaking or spraying Carb Cleaner in them. Removal of the Discs and Lead Plugs is how you get to where the Gunk hides and blocks Passages

Condensate happens a lot on Carbs, as the Pressure Drop in the Venturi removes Heat from the Carb Body, and the chilled air near or even in the Carb, cold Air can't hold as much Water in suspension as warmer Air. It is what you see when you take anything Cold out of the Fridge on a Humid Day... It is quite Humid on a Lake, River, Ocean

 

[Blake_F]

Improper Float setting, Heavy Float, leaky Needle and Seat, loose Seat, too much Fuel Pressure, any of which can cause a High Fuel Level.

Incomplete Cleaning of all the Fuel Passages can cause Leanness especially in the Idle and Off-Idle circuits. That is why Carb kits contain those little Discs and Lead Balls.
Cleaning a Carb isn't spraying a bit of Cleaner down the Throat, or taking it off and removing the Bowl, the jets and or Mixture Needles, then soaking or spraying Carb Cleaner in them. Removal of the Discs and Lead Plugs is how you get to where the Gunk hides and blocks Passages

Condensate happens a lot on Carbs, as the Pressure Drop in the Venturi removes Heat from the Carb Body, and the chilled air near or even in the Carb, cold Air can't hold as much Water in suspension as warmer Air. It is what you see when you take anything Cold out of the Fridge on a Humid Day... It is quite Humid on a Lake, River, Ocean

Awesome thank you for the info! Once I get the carb rebuild and take it out I’ll let you all know how she runs!

 

[Blake_F]

Hey guys! Little update.

So I rebuilt the carb that was having the issues. Took it out 2 days ago and it was great all day. No issues what so ever. Went out today again abd all was good on the first run. After fishing for a while started it up and when I tried to get on plane it was super slow for a couple seconds then all of a sudden it woild just take off like it should. Did this 3 times today. Once it kicked in full power it was great up and down wide open throttle

Any ideas on why when I give it full throttle to go on plane it’s slow and lags for a couple seconds then all of a sudden gets its surge of power? I’m thinking maybe a plug fowling out? I’m gonna change them anyway just cause it’s cheap and easy

 

[Chris1956]

Check/rebuild your fuel pump. If the diaphragm leaks, raw fuel gets into the crankcase and causes fouling of the cylinders. On a good motor the fouling will clear up and the motor will run well at high speed.

 

[Blake_F]

Check/rebuild your fuel pump. If the diaphragm leaks, raw fuel gets into the crankcase and causes fouling of the cylinders. On a good motor the fouling will clear up and the motor will run well at high speed.

I tested the fuel pump when I rebuilt the carb. There were no leaks whatsoever. So fuel pump is doing its job. I’ve already put new lines and fuel filter on as well