2003 Merc 9.9 4-stroke rough idle when warm and more... help!

[whaleboat]

I have a 2003 Mercury 9.9 4-stroke having some persistent issues. I have removed the carb several times over the past couple of weeks to clean it out, as I have done before with good results. Not this time, and no visible defects or problems that I can see in the carb. Here is what is happening:

The engine starts with a couple of pulls with the choke on, and keeps idling very well after the choke is off after several minutes. It will idle fine until I take it up to a moderate speed, maybe 1/2 throttle. It will run at this speed for 10-20 seconds, then sound like it's missing sporadically but not too rapidly, then the rpm will start falling a little bit as the engine looses power. When I throttle down back to idle, the engine idles very rough and sometimes dies... unless I turn the choke on. With the choke on, it will continue to idle pretty well and operate at low speeds. After use, and the engine cools, it will start up again and idle fine with or without choke, so the issue seems to be when the engine is hot, usually spurred by a moderate-speed run.

I'm thinking this is a carb issue, although I am almost sure it is cleaned out completely. Seems like it is running too lean when it's warmed-up. Is this as simple as a carb clean, or could it be something else? Please let me know what you think, the help is much appreciated. Thanks.

 

[jerryjerry05]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Fill out the profile.You could be close to someone who can help.Could be water in the gas.Or the fuel pump needs rebuilding.Check for old/soft hoses.
Do a compression/spark test.Get the manual.For spec's to set the carb.J

 

[whaleboat]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Thanks for the info. I will add some info: the gas is pretty new, and while this motor has been in the shop I have been using the same fuel tank with a newer 8hp merc 4-stroke, with no problems. I also rebuilt the fuel pump and replaced the fuel filter and several hoses in this last course of maintenance, with no change in the issue. I will dig up the manual and make sure the carb is set properly, and also check the spark and compression. Thanks.

 

[olddaddy]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

I have a 1999 9.9 merc that is driving me crazy. It revs up and drops off (pulses up and down) at low idle. This makes it near impossible to troll with.

I have a manual and checked it...have clean plugs, checked the oil level (this is the top of the list when trouble shooting this problem, odd) clean fuel filter, drained carb, sprayed carb cleaner through carb, made sure ALL hoses were clamped tight...

I am using absolutely new gas, yesterday's purchase, and have a full water separator to boot.

Does anybody have an ideas. This is my first 4 stroke. I am so far not a fan.

Help please

 

[whaleboat]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Well I have a bit of an update... I ran the motor for an extended period of about 10-15 mins, running the motor at different RPMS between 1/4 trottle and maybe 1/2 throttle or so, for several minutes on end. For whatever reason it seems the motor is running smoother now without bogging down and idling bad, but I did discover a new issue.

I have just finished rebuilding the waterpump after the telltale started to sputter right before I pulled the motor from the boat. After running the motor with the new pump I got much better pressure, back to where it should be. Once I ran the motor for about 5 mins though, up to a decent cruising speed, the telltale died back to a sputter. Seems the water pressure dies when the motor gets up to temp. I let it cool and ran it again, and sure enough, good water pressure until everything warmed up.

I figured maybe a problem with the thermostat, so I pulled it, but found no obvious problem with it (not stuck open or anything). I did however find a lot of deposits in the visible passages, including one passage out of the head that was totally clogged. Maybe this cooling issue had been contributing to the poor performance? In any event, this gives me another problem to address, I will update when I put in the new thermostat and clean out the passages as best I can.

If anyone has another idea of why I could get good water pressure when cold but poor when hot, please let me know. Thanks!

 

[Jack49]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

I purchased a mercury 9.9 4 stroke for a pontoon boat 4 years ago. I am not a mechanic which will become very obvious as I ramble on. From day one the engine had a bit of a ringing sound in idle - no big deal. It ran smooth and was relatively easy to start. Engine was used very lightly - 3/4 throttle at max. Stalling issues developed and the local dealer replaced the carb. and blamed ethanol fuel for the problem. Immediately a secondary issue arose regarding water and oil mixture burning out the ball bearings in the lower casing.

I have had nothing but merc's (50hp, 65hp, 70,hp and a 2 stroke 4hp) I have used and abused all of them and had nothing done except for very occassional tune ups. I contributed to the problem by not changing the engine oil - I did change the gear oil each year.

My choices are to rebuild the lower portion of the engine - or get what I can for the old engine and buy a new non-merc 4 stroke or a used 2 stroke.

The boat is used in a small fresh water lake and the engine got about 10 hours per year - maybe less.

any advice would be appreciated

Also my mechanic said there was an engine temperature sensor that would slow the motor down when the lower part would get too hot and eventually shut it down completely. Not sure if he is blowing smoke or not.

 

[Star]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Sure sounds like a fuel pump problem, I know that you manualy pumped the priming bulb. A full carb bowl should run for one to two minutes at 1500rpm even if the pump wasn't working. I have seen the float needle stick if you leave the tank hooked up on a hot day with the vent closed. That carb isn't that fancy and I belive the tech that told the guy that ehtonal ruined the carb was not telling the truth!!! Good Luck

 

[scuba04070]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

i have a 99 9.9 four stroke as well and im having kinda the same problem, when i finally get it going it runs for a couple of seconds then the carb fills up then dies i then need to drain the carb and restart it then when i give it gas it finally went then the problem went away until i got it out in the lake it ran for an hour then just ran like crap then quit. i then pulled till my arm pulled off and it just wouldnt start, until the end of the day thats whenn it finally started!!!! man im getting tired of it, i bought the four stroke for a kicker do to the rise in gas. any help would be great.

 

[Maxz695]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Maintanance beats repair and diagnosis do maintanance EVERY YEAR Dont wait til something breaks thats repair not maintanance. Do your water pumps Do your fuel pumps Change the oil in the lower unit Change your spark plugs check all wiring replace bad wiring. Rebuild carbs check coil wires make sure the tight and not cracked. check your compression check you timing check your carb adjustments THIS IS Maintanace to avoid REPAIR!!!!

 

[Silvertip]

Re: 2003 Merc 9.9 4-stroke rough idle when warm and more... help!

Carb rebuilding should not have to be a "routine" maintenance item PROVIDED the fuel system gets the attention it deserves. Carburetor issues begin when the fuel system is ignored. Understanding the difference between fuel treatment and fuel system treatment is essential. Whether you burn E-10 or non-ethanol blended fuel, you still need to at least periodically run a fuel SYSTEM treatment like SeaFoam in a tank of fuel. Use it in your car, truck, bikes, and lawn equipment. It keeps the system clean. For long term storage or if you don't use a lot of fuel, you need to add a fuel stabilizer like Stabil (and SeaFoam does that too). It prevents the fuel from going bad. Ruining a 4-stroke because of failure to change the engine oil is worth 40 lashes with a wet noodle. For Shame!