'96 125 hp... fuel pump?

[Kludge59]

I've got a '96 125 hp Mercury OB. Here are the symptoms:<br />* Hard to get started at first<br />* Once started, it runs ok, with good hp, but noticebly rough <br />* It doesn't seem to missfire, just runs rough<br />* No problem revving to WOT<br />* When at idle, about every 30 sec it coughs, spits smoke, and then returns to normal idle<br />* When I turn it off and turn the key within 30 sec or so, it starts immediately<br />* If I wait a few minutes and restart, it takes a while to crank.<br /> <br />So far, I have replaced the sparks (with proper gap), and replaced the fuel, and after running it for 30 min, the problem is still there.<br /><br />I think it's the fuel system... but beyond that, I need help...

 

[roscoe]

Re: '96 125 hp... fuel pump?

Check compression and check for spark to each cylinder, especially when it "coughs".<br /><br />Check sparkplugs for appearance, (fuel, gas, clean condition?).

 

[Kludge59]

Re: '96 125 hp... fuel pump?

Thanks for the response...! Good suggestions.<br /><br />So you're saying that this looks more electrical than fuel related? If the compression is low, does that point to a cracked cylinder or leaky valve? Again, the sparks are new... so you're saying pull them again and check them for signs of non-spark?

 

[outboard tech]

Re: '96 125 hp... fuel pump?

Are you new to this motor, if so, do you understand it only idles on the top 2 cyls. The bottom 2 engage either with the accelerator pump, or when rpms reach approx. 1800 rpm. Since you have trouble cold starting; I would wonder about your cold start procedure. If these symptoms are on the hose, they aren't telling you much. But the sneezing may be an idle setting too high, causing the bottom 2 clys to intermittantly get fuel and try to fire.

 

[Clams Canino]

Re: '96 125 hp... fuel pump?

Originally posted by outboard tech:<br /> causing the bottom 2 clys to intermittantly get fuel and try to fire.

Out of curiosity Ben, what happens when one of these motors is trolled for say 8 hours at 1000 RPM? Do those poor bottom 2 cylinders just run dry with no fuel or oil?<br /><br />-W

 

[Kludge59]

Re: '96 125 hp... fuel pump?

Ben...<br />I'm using the cold-start procedure right out of the owner's manual. It idles around 1000. I'm primarily worried about the rough performance... and just mentioning the tough start after it's off for a few minutes as a sympton that could hopefully point to the real problem.<br /><br />Doesn't it appear that something in the fuel system is causing the cylinders to "de-prime"? Could the vacuum pulse to the carb cause this?

 

[outboard tech]

Re: '96 125 hp... fuel pump?

Clams, it has a recirc/bleed system;plus residual oil clings to bearings and journals. You can troll it for 10,000 hours on the top 2 cyls.The same way you can idle an outboard for hours with no oil in it as long as the motor is never loaded.----Kludge, do you mean 1000rpm in gear, and on the lake---that's about 250 too high and will cause enough venturi vaccuum to pull some fuel through 3 and 4. If you mean 1000 on the hose, that's nothing to go by, but the same rusult can occur because there's no backpressure on the exhaust.I'm not sure what deprime means, but rough running and tough warm re- starting might mean the thing is rich--if indeed it's even a fuel related problem. You could have one of your lowspeed charge coils down on the stator, and once you get above 3000 rpm the high speed takes over and fires that missing cyl, hence the good top end performance. You could compression down on a cly. also.

 

[Kludge59]

Re: '96 125 hp... fuel pump?

Ben... you're really teaching me a lot here. The silver lining with this problem is that I'm becoming much smarter about this engine!<br /><br />Now I know what you mean by "on the hose". Yes, sitting in the water at idle it's turning 1000rpm. But I can see where a little gas might get pulled into one of the two other cyls and cause a cough. It turns about 800 when in gear and idling.<br /><br />By de-prime I mean that the gas is, for some reason, not available at the cylinders after the engine is off for a few minutes. That's why I thought this was a fuel pump or carb related problem. But it sounds like you think it may be related to something else?<br /><br />Thanks...