# Mercury 25hp 2-Stroke Acceleration Problem…



## HaMm3r

Ok guys, riddle me this. The last couple of trips my 1999 Merc 25 two stroke has been acting a little strange. I have a theory as to what’s going on with it, but want some other opinions before moving forward. So here’s the deal…

Two trips ago I noticed that the motor would bog down a bit while accelerating. Basically it would idle great and run perfect at low RPM, but when I’d get to about half throttle it would hesitate, then struggle for a few seconds before suddenly getting a surge of power and continuing to accelerate smoothly up to the max RPM. Top end was great, with plenty of power and speed.

The next trip, I added seafoam to the tank but the problem persisted, although it wasn’t quite as bad as the prior outing.

This last trip the problem was worse than ever before. In fact, the mid-throttle bogging down actually stalled the motor a couple of times.  I was able to work through it and salvage the trip by adjusting the fuel mixture. I tried various settings, but ultimately running the motor a half turn richer seemed to help more than anything. Again, it started, idled and ran at WOT perfectly, but half speed…forget about it!

Now, I should add that before this last trip, I tried doing a deep seafoam treatment to the engine using the following procedure.
_“I also tried your suggestion on seafoaming my motor today, except I took it one step further. In addition to what you did, I pulled the fuel line off the filter on the carb side, ran the motor until it was dry, choked it and ran it a few seconds more til it was totally out of fuel in the carb. Then I stuck a funnel in the fuel line and poured the seafoam in. With plugs disconnected I then cranked the motor until all the seafoam was sucked in. Repeated that about 9 or 10 times, then used your straw method to pour seafoam into the air intake. Let that sit for about 3 hours, then started it up and burned all the seafoam out.”_

So what do you guys think?


----------



## Creek Runner

> Ok guys, riddle me this. The last couple of trips my 1999 Merc 25 two stroke has been acting a little strange. I have a theory as to what’s going on with it, but want some other opinions before moving forward. So here’s the deal…
> 
> Two trips ago I noticed that the motor would bog down a bit while accelerating. Basically it would idle great and run perfect at low RPM, but when I’d get to about half throttle it would hesitate, then struggle for a few seconds before suddenly getting a surge of power and continuing to accelerate smoothly up to the max RPM. Top end was great, with plenty of power and speed.
> 
> The next trip, I added seafoam to the tank but the problem persisted, although it wasn’t quite as bad as the prior outing.
> 
> This last trip the problem was worse than ever before. In fact, the mid-throttle bogging down actually stalled the motor a couple of times.  I was able to work through it and salvage the trip by adjusting the fuel mixture. I tried various settings, but ultimately running the motor a half turn richer seemed to help more than anything. Again, it started, idled and ran at WOT perfectly, but half speed…forget about it!
> 
> Now, I should add that before this last trip, I tried doing a deep seafoam treatment to the engine using the following procedure.
> _“I also tried your suggestion on seafoaming my motor today, except I took it one step further. In addition to what you did, I pulled the fuel line off the filter on the carb side, ran the motor until it was dry, choked it and ran it a few seconds more til it was totally out of fuel in the carb. Then I stuck a funnel in the fuel line and poured the seafoam in. With plugs disconnected I then cranked the motor until all the seafoam was sucked in. Repeated that about 9 or 10 times, then used your straw method to pour seafoam into the air intake. Let that sit for about 3 hours, then started it up and burned all the seafoam out.”_
> 
> So what do you guys think?


Who is Your?

Never do that agan unless you want to buy a new engine![smiley=badidea.gif] Your engine is pre-mix oil/gas, in order for your engine to recieve the proper lubercation it requires fuel because that's where the oil is, the last 1-2 min the engine was running you created a very lean condition!!!  It only takes a sec, to score a piston!!!

Pull the carb, see if there is any trash sounds like a float sticking a little? 

But my rule is always, and I mean always do a comp. check 1st


----------



## HaMm3r

> Who is Your?
> 
> Never do that agan unless you want to buy a new engine![smiley=badidea.gif] Your engine is pre-mix oil/gas, in order for your engine to recieve the proper lubercation it requires fuel because that's where the oil is, the last 1-2 min the engine was running you created a very lean condition!!!  It only takes a sec, to score a piston!!!
> 
> Pull the carb, see if there is any trash sounds like a float sticking a little?
> 
> But my rule is always, and I mean always do a comp. check 1st


Hey Creek, thanks for the response. You'll have to explain to me how this is any different than running out of fuel during regular, day to day operation, which happens to people all the time. I've done it, you probably have too. I know you're a marine mechanic, so I'd honestly like to know the answer so I don't make such a mistake again.

I already did a compression check, btw. It was 120 on top and 118 below. Like I said, I already knew what I would do next, but wanted some more opinions. A lot of folks on here, like you, know more than I do about outboards and I like taking advantage of that.


----------



## Swamp

I think he was saying that when you ran the pure seafoam through the engine the motor was now "running" on the cleaner instead of fuel/oil mix.  The seafoam has no oil in it to lubricate the pistons and it _can_ only take few strokes to cause damage.  As the fuel ran out and the pure seafoam got sucked in less and less lubricant was available so you were probably running on a fraction of the required lubricant when combustion finally failed.  It is a very risky thing to do especially when you consider that one cylinder often fires a few more times after the other shuts down.  When you just run out of fuel mix the engine stops when it can no longer fire but it still was receiving the full amount of lubricant at every stroke till the end since the fuel mix was not diluted.  

Next time add the max(no more) recommended amount of seafoam to some fuel/oil mix and run it through normally.  It's going to smoke.  Then pull and clean the carb to get rid of any junk the seafoam may have loosened in the carb or the hoses leading to it.

Granted I'll admit at the end of the day I'm just a hack who has learned by doing and making mistakes.  Now I did work as a tech in a small engine shop many moons ago but it was learn as you go place.  Listen to Creekrunner he has not only had real training, but has a LOT more experience than a few years working on chainsaws 25+ yrs ago and few dozen or so outboards over the years.

Swamp


----------



## HaMm3r

Ah, well that makes sense Swamp. Thankfully, I never ran the motor with just cleaner. I only cranked it with the plugs disconnected to get the fuel pump to draw in the seafoam.


----------



## Swamp

That is better than running it but you may still have less than ideal amounts of lubricant in the cylinders because you replaced the mix with pure seafoam. That metal to metal contact without lubricant is not a good thing. It is probably unnecessary but I'd "fog" the cylinders and hand crank a few times before restarting. Also push any remaining seafoam out of the fuel lines with fuel mix.


----------



## Creek Runner

> Who is Your?
> 
> Never do that agan unless you want to buy a new engine![smiley=badidea.gif] Your engine is pre-mix oil/gas, in order for your engine to recieve the proper lubercation it requires fuel because that's where the oil is, the last 1-2 min the engine was running you created a very lean condition!!!  It only takes a sec, to score a piston!!!
> 
> Pull the carb, see if there is any trash sounds like a float sticking a little?
> 
> But my rule is always, and I mean always do a comp. check 1st
> 
> 
> 
> Hey Creek, thanks for the response. You'll have to explain to me how this is any different than running out of fuel during regular, day to day operation, which happens to people all the time. I've done it, you probably have too. I know you're a marine mechanic, so I'd honestly like to know the answer so I don't make such a mistake again.
> 
> I already did a compression check, btw. It was 120 on top and 118 below. Like I said, I already knew what I would do next, but wanted some more opinions. A lot of folks on here, like you, know more than I do about outboards and I like taking advantage of that.
Click to expand...

It's not really any different other than one hopefully is by mistake and the other is on purpose. Here is an example of the damage that can be caused by running a 2stroke pre-mix engine out of gas.

At the dealership we sold a 15 Mercury to an older fellow, (brand new) we installed it on his boat (Gheenoe we were also a dealer for them) along with a tach and an hour meter. Approx 6-7 months later he brought the engine back in  saying it wouldn't idle at the dock and had a loss of power. The engine had >7hrs on it. My junior tech at the time assumed that it was thrash in the carb, and took it off and cleaned it although there wasn't any evidence of thrash or varnish/gumming of stale fuel. In the test tank it still didn't perform correctly so he came and got me. I asked him what was the diagnosis and to show me the work order, I required all compression test to be written on the engine head and work order with date performed. I said what was the compression it's not on the work order he said I didn't do one, I explained to him what is rule number 1.   15min later he came back and said #1 cylinder only has 65lbs I said that explains the loss of power and idling issue and to pull the head. Cylinder wall and piston were badly scorned I called the customer and asked how he mixes his gas and oil ( thought that was going to be the issue) and what his overall procedure was when running and storing the engine. 2-3 min into talking he said and after I'm done flushing the engine I disconnect the fuel hose and speed the engine up to a fast idle 1300rpm or so until it runs out of gas, (and there we have the cause). The engine had been used less than a dozen times, and it now needed to be rebuilt. The last 1-2 minutes the engine is running it's running on fumes  @ a lean condition which again can cause major mechanical failure . I called Mercury and the provided a new power head under good faith, and I covered the labor for the Customer. I felt as this was my fault, and we all learned something from this experience. My junior tech never forgot rule #1 again,  My customer learned the correct way to do things, and I learned that I needed to add this to my pre-delivery check list with clients as If would have gone over this it hopefully would have been avoided. 

Now that being said I know that people do this every day and never have a problem. But it is an incorrect way of doing things. Here is another example, mercury and Yamaha program motors, these are motors sold to fishing camps and then returned to the factory gone through and sold to dealers at discounted price. They almost always have to have the carbs gone through why? Because they only drain the fuel out of the bowls they don't run them out of gas because it can cause major mechanical failure. They would rather pay the dealer warranty reimbursement for R&R carbs, than R&R a power head with in the 1st year. 

Sorry for such a long post.


----------



## Creek Runner

> Who is Your?
> 
> Never do that agan unless you want to buy a new engine![smiley=badidea.gif] Your engine is pre-mix oil/gas, in order for your engine to recieve the proper lubercation it requires fuel because that's where the oil is, the last 1-2 min the engine was running you created a very lean condition!!!  It only takes a sec, to score a piston!!!
> 
> Pull the carb, see if there is any trash sounds like a float sticking a little?
> 
> But my rule is always, and I mean always do a comp. check 1st
> 
> 
> 
> Hey Creek, thanks for the response. You'll have to explain to me how this is any different than running out of fuel during regular, day to day operation, which happens to people all the time. I've done it, you probably have too. I know you're a marine mechanic, so I'd honestly like to know the answer so I don't make such a mistake again.
> 
> I already did a compression check, btw. It was 120 on top and 118 below. Like I said, I already knew what I would do next, but wanted some more opinions. A lot of folks on here, like you, know more than I do about outboards and I like taking advantage of that.
Click to expand...


By the way I am a Marine Technician not a Mechanic,

A Technician  Trouble shoots, diagnosis, and repairs

A Mechanic swaps parts, until it's fixed or takes it to a Technician.

lol, J/k no really I'm serious lol!


----------



## cutrunner

Lifes more comfortable under the shade tree!!!

Lol jk


----------



## HaMm3r

Hey Creek, thanks for the very thorough explanation! What you're describing certainly makes sense, so let me ask you another related question. Would this also apply to hand-cranking a motor, either with the pull cord or with the electric starter?

For example, if the next time I wanted to flush the carb in a similar manner, what if instead of running the motor dry first, I just hook up the funnel, disconnect the plugs and then turn the motor "manually" until I feel that the fuel in the carb has been sufficiently replaced or diluted with the cleaner?


----------



## HaMm3r

Update - So I starting tearing into the carb tonight and almost immediately I found this gunk jammed in the main jet. Very likely it's the cause of all my acceleration woes. The first pic shows it's blocking about half the jet and you can see some of it hanging down in the second photo.

















I've got a complete rebuild kit, so I'll be cleaning and replacing everything before it goes back on the block.


----------

