1986 Mercury 4hp 2 stroke running rough after half throttle {link for video}

So to start off here, I picked up this little engine a couple weeks ago knowing that it bogged under load. Seller said that all he did was an impeller change so I was hoping it was a carb issue.

When I took it out on the water the first time I noticed the amount of smoke coming from the engine and the smell of a rich fuel mix and that at WOT it would take a bit to get up to the rpm's, bogging a bit, then would sporadically sound like it was misfiring.

First thing I did was check spark. I noticed that it indeed was getting a consistent spark but the plug was a bit fouled. I changed the plug took it out again for a little ride and checked plug again, again it looked a little oily. I should mention I'm running 50:1

I should say that it's only a single cylinder engine. I decided before I did anything else I'd check compression and was getting 100psi

So then I decided I'd disassemble the carb and give it a good clean. I soaked it pretty good, making sure to clean out all the Jets and passageways. All of the gaskets seemed in good shape and the fuel pump also seemed like it was in good shape aswell.

I tried to find a fuel mix screw but it doesn't seem to have one, only a idle mixture screw.

Fuel lines in the engine look new and also the one coming from the tank, and all of the wires are in good shape. I'll also add on here that I do have some seafoam in the tank, so that might be why it's smoking so much.

My guess now is that it may have something to do with timing or that maybe the spark isn't getting enough voltage under load. But to be honest I have no idea how to fix that, but would learn if I knew that was the problem.. I don't wanna tinker with it if I dont have too. All in all the engines running too rich in my opinion and misfiring.

If someone with a little experience can watch the video I'll upload and give me their opinion it would be much appreciated!
If it's not a good enough video just let me know and I re-upload one thats better.

Thanks in advance!

Do you have a test prop on that motor? If not then you should not attempt to run it at that high RPM without a load. Also running in that tank is contributing to the over rich mixture.

Use a spark tester to see if the gap is at least 3/16" they are cheap. And yes, the Sea foam will create smoke !! Go ahead and flood the carb by spraying sea foam in the carb while running then shut it down. Let it sit for 30 minutes then fire it back up. After a short run, switch to your regular 50-1 mix with fresh fuel and see if it still smokes a lot. At idle, you will get smoke, but not like it is on fire ! Compression seems good, check spark, it could be weak. Get a factory rebuild kit for the carb, it will give you the proper settings for the carb to get it running plus the fresh gaskets will seal everything up. How good is the fuel line from the tank ? Any cracks or bad connectors will suck air and create fuel mix issues and look inside the tank for crud. Good luck.....

MDCrappie said:

Do you have a test prop on that motor? If not then you should not attempt to run it at that high RPM without a load. Also running in that tank is contributing to the over rich mixture.

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I appreciate all the replys. Yes it was fitted with a prop but I can't see how that would harm the drive considering it was not in gear and the engine should be relatively cool considering the impeller was sucking up fresh cold water.

I hate when threads go cold so I thought I'd come on here and tell everyone what resolved my issue.

After cleaning the carb a thousand times, soaking the Jets in carb clean for a couple hours it resulted in the same issue.

I gave up initially and found that my neighbor had a nice 9.9 Chrysler that started first pull with two other identical engines for parts all for $150 CAD!

After I had a bit less of a reliance and careless for the Merc I decided to rip apart the fly wheel and see if my coils had gone bad.. nope..

Before I gave up and stuck it on marketplace for parts I thought I'd give gapping the single point to spec of .020...

Anddd voila the thing runs great! The engine idles perfect, no hesitation when accelerating, at any throttle it's not misfiring anymore, and it doesn't smoke like it's about to catch fire haha.

I had a huge feeling off the bat it had something to do with the ignition system but just wasn't confident that I wouldn't mess the entire engine up..

Anyway thanks again, I really appreciate the time y'all took to respond!

merc4hp1986 said:

I appreciate all the replys. Yes it was fitted with a prop but I can't see how that would harm the drive considering it was not in gear and the engine should be relatively cool considering the impeller was sucking up fresh cold water.

I hate when threads go cold so I thought I'd come on here and tell everyone what resolved my issue.

After cleaning the carb a thousand times, soaking the Jets in carb clean for a couple hours it resulted in the same issue.

I gave up initially and found that my neighbor had a nice 9.9 Chrysler that started first pull with two other identical engines for parts all for $150 CAD!

After I had a bit less of a reliance and careless for the Merc I decided to rip apart the fly wheel and see if my coils had gone bad.. nope..

Before I gave up and stuck it on marketplace for parts I thought I'd give gapping the single point to spec of .020...

Anddd voila the thing runs great! The engine idles perfect, no hesitation when accelerating, at any throttle it's not misfiring anymore, and it doesn't smoke like it's about to catch fire haha.

I had a huge feeling off the bat it had something to do with the ignition system but just wasn't confident that I wouldn't mess the entire engine up..

Anyway thanks again, I really appreciate the time y'all took to respond!

Click to expand...

Wasn't asking if you had a prop, but if you had a test prop. A test prop puts a "load" on the motor same as if you were running it on a boat and in gear. With a test prop the motor needs to be put into gear. You should NEVER throttle up an outboard like that in neutral........if you want it to last.
It not whether or not the motor is getting hot, but the unrestrained RPM's. I would equate it to you running down a 45 degree hill as fast as you can and then trying to stop on command.