Requesting some advice on a 1999 Mercury four cycle, four cylinder, carbureted motor

[Likesspace]

Hi guys,

As the title suggests I have a 1999 Mercury carbureted, four stroke, four cylinder outboard motor.
When I first got the motor it ran like a top.
I could go from idle to WOT with very little hesitation and I had full confidence in the motor getting me back to the marina.
Well starting a couple of days ago my confidence has started to fade. This is about a month after buying a 20' Suntracker pontoon with this motor as part of the package.

A few days ago the motor started to have a bit of a rough idle. Nothing severe, but the gates on my pontoon were starting to rattle while travelling in the no wake zone of our lake. Regardless of that fact, once I would get out of the no wake zone the motor would open up and smooth out.

Then I started to notice that the motor was running rough, even once I exited the no wake zone. Within a couple of days, the motor would run really rough until it warmed up well and then it would smooth out. This happened for a day or two and then things changed again......

For the last two days the motor is running great at an idle and at low rpm but once I try to go to WOT the motor starts to miss and hesitate. Where as before I could back off to about 3/4 throttle, now anything over about half throttle causes the motor to run REALLY rough.

The motor starts immediately whether warm or cold. Tonight I went from WOT to an idle and the motor stalled. Regardless it started right back up and ran great at idle to half throttle while going back to the marina. Anything over half throttle would induce the stuttering and missing that I've already described.

A few days ago I cleaned the ground point of the motor and also sprayed the plug wires with Gibbs brand penetrant. My reason for doing so is because the foam rubber at the back of my cowling is wet which made me think that there might be moisture entering the cowling, causing problems. Gibbs is great at displacing moisture so I figured it was worth a try.

When I first did this the motor ran great again and I thought I had solved the problem. I wasn't sure that the problem was a bad ground or that the Gibbs had done it's job. Well now, two days later the same symptoms have appeared again.

Tomorrow I'm going to try spraying the plug wires with the Gibbs spray, again. If this is a moisture/plug wire problem then it should show itself rather quickly.
If this is not the problem, I'm at a loss.
Does anyone have any ideas why this motor would go from running perfectly to running like crap, pretty much over night?

I'm hoping for something easy and inexpensive but at this point I'm ready to pull the boat from the lake and take it to our local marine repairman and getting a complete tune up.

If any of you can offer some advice it would be greatly appreciated.

Dave

 

[flyingscott]

Need a model# and what Hp. I am not sure mercury made a 4 cylinder 4 strk in 99

 

[Likesspace]

Sorry about that. The motor is a 50 HP bigfoot and the model number is: YM9XM9352G0.
Thanks for the response.

 

[flyingscott]

Need the serial# sorry about that. Check the fuel for water. Does it have enough oil check that and make sure it's not milky.

 

[Likesspace]

Okay Scott here's what I could find....
There is quite a bit if information on my motor tag that is legible but of course the serial number part was scuffed and faded, badly. Here's my best guess from what I could make out: OG95422. There was also another number on a different tag which appeared to be LB255. The model of the motor is: 50ELPT4S and it says it was made in April of 1999. The YM9XM9352G0 number I gave you earlier is the engine family.
I checked the oil and it was well within the full range. It is also still nice and clear, (changed it early last month), and did not have any off smell, like it had been fouled.
I wasn't sure how to tell if there is water in the fuel but I did do the following that appears to have been successful.

I sprayed some Gibbs penetrating oil on the plug wires to act as a moisture dryer. My reason for doing so is because I noticed my cowling seal had slid up into the cowling instead of sealing on the back side. It really appeared to only have soaked the foam rubber at the back of the cowling but I thought that maybe that added moisture was affecting the plug wires.

After letting the Gibbs sit for a while I then wiped off the excess and we took the boat out for a test run. Other than running just a little rough right at first I was able to go from idle to full throttle without any stutters or hiccups. We ran it for probably 10 - 15 minutes without a problem and then put it back in the slip and will try it again later to see if it's still running well.
As I said in my original post, the other time that I sprayed the Gibbs the motor also ran great so at least I'm hopeful that this is my only problem.
I also brought the top cowling home to repair the seal so that this doesn't happen again.

The only other thing that might have been causing a problem is that the spark plug boot from the #2 cylinder was possibly touching the boot on the number 3 cylinder. I don't know that for sure but I did make sure all were separated before taking it out for a test run. I do know that the boots were very close to one another.

I'll give you an update after we check again tonight or tomorrow.

Thanks again for trying to help me.

Dave

 

[flyingscott]

Is the cowling catching on the spark plug wires when you put it on? Make sure they are routed correctly.

 

[Likesspace]

No, I'm pretty sure the cowling is clearing the plug wires. I'm pretty sure that the wires are routed correctly and I did make certain that none are now touching any of the others. We'll be back at the lake either tonight or one day over the weekend and I'll give you an update. Thanks again for your help. It is very much appreciated.