2002 Mercury 90 four stroke low idle after startup

[Basshouse]

My 2002 carburetted 90 hp FS (owned from 2010) always drops idle to stalling at startup. First enrichment circuit kicks in and engine starts normal but when enrichment automatically stops the engine has tendency to stall. Normal idle is 850 but when started it drops to abt 500/600 and most times I need manipulating the throttle lever to prevent it from stalling. After a few minutes at higher rpm (1200-1500) I can, with fingers crossed, cast away with just enough rpm without stalling.

Once underway, after a few minutes, at 2000 rpm I can put engine in neutral/shift fwd/reverse and all is fine. Rpm then around 850 and rest of the day no problem.

- New fuel lines/fuel & water filter/ fresh fuel/ oil looks ok/ impellor ok/gear oil ok.
- Allways use dryfuel/quick clean.
- Last year carbs cleaned

It's just everytime at startup (warm or cold outside). What could it be?

 

[Basshouse]

Re: 2002 Mercury 90 four stroke low idle after startup

No replies yet but my thoughts right now:

Change spark plugs? (can improper plugs react differently when cold and normal operating temperature?)
Change engine oil? (looks ok but could it be that viscose when cold is too thick that engine has trouble to make proper rpm when idling cold?)

Any tips much appreciated!

 

[Basshouse]

Re: 2002 Mercury 90 four stroke low idle after startup

No replies yet but my thoughts right now:

Change spark plugs? (can improper plugs react differently when cold and normal operating temperature?)
Change engine oil? (looks ok but could it be that viscose when cold is too thick that engine has trouble to make proper rpm when idling cold?)

Any tips much appreciated!

Another thought: Could it be that for some reason only 3 of the 4 spark plugs are properly working and when warmer all 4 are working?

Again, any tips most welcome!

 

[1nebel0]

Re: 2002 Mercury 90 four stroke low idle after startup

Your idle mixture sku's are off.....turn in each screw to lightly seat...then turn out slowly for best idle..You will need to do this for each carb and then go back and tweek them all...you may have to turn the idle timing down if the idle is too fast one you get it right... Also my 115h.p four stroke 2004 will die a couple of times when cold and it's fuel injected..Just got the injectors back from being cleaned and flowed and the motor runs much better...With carbs same problem with ethanol eating floats and needles...My buddy's 15hp. four stroke had a plastic float that was bent due to ethanol melting the float..The dealer told be that is why it is flooding and he was right..They see it all the time and I would not have imagine that a plastic float would warp from the fuel but it did and wouldn't start due to flooding..there is no adjustment on these floats but there is a spec and it was way off spec so it got tossed...Good Luck.....

 

[Basshouse]

Re: 2002 Mercury 90 four stroke low idle after startup

Your idle mixture sku's are off.....turn in each screw to lightly seat...then turn out slowly for best idle..You will need to do this for each carb and then go back and tweek them all...you may have to turn the idle timing down if the idle is too fast one you get it right... Also my 115h.p four stroke 2004 will die a couple of times when cold and it's fuel injected..Just got the injectors back from being cleaned and flowed and the motor runs much better...With carbs same problem with ethanol eating floats and needles...My buddy's 15hp. four stroke had a plastic float that was bent due to ethanol melting the float..The dealer told be that is why it is flooding and he was right..They see it all the time and I would not have imagine that a plastic float would warp from the fuel but it did and wouldn't start due to flooding..there is no adjustment on these floats but there is a spec and it was way off spec so it got tossed...Good Luck.....

Thanks 1nebel0!
Re-tweaked all 4 pilotscrews as best as I could. One thing I forgot before I put boat in the water was synchronising after carb cleaning with mixture of carb cleaner and fresh fuel. Synchronised engine whilst in the water ( good to do) and carburetor nr4 had more vacuum than other 3. Adjusted carburetor synch.screws untill equal vacuum on all 4.
Now wait untill cooled down and see if startup idling is smoother (should do...)

 

[jdlloyd_iboat]

Re: 2002 Mercury 90 four stroke low idle after startup

Basshouse, I have a similar problem. My fuel lines had all disintegrated this past year and were clogging an in line filter and the engine fuel filter (I have a 2003 75 HP 4 stroke). Fuel in tank was old but was working fairly well except for idle. Cleaning filters and replacing lines improved operation somewhat. Worked fairly well at top speeds. I added new fuel with ethanol treatment and idle worsened to a point where engine will not continue to idle once started. How did your last tried solution work? I suspect my carburetors are needing rebuild but am hesitant as I have never worked on this engine since new. Engine has 130 hours since new so it is very low use on this engine and I left fuel over winter without winterization. I think I will be paying for this neglect. I am studying the manual to see how entailed it is to remove and rebuild the carbs.

 

[Basshouse]

Re: 2002 Mercury 90 four stroke low idle after startup

Basshouse, I have a similar problem. My fuel lines had all disintegrated this past year and were clogging an in line filter and the engine fuel filter (I have a 2003 75 HP 4 stroke). Fuel in tank was old but was working fairly well except for idle. Cleaning filters and replacing lines improved operation somewhat. Worked fairly well at top speeds. I added new fuel with ethanol treatment and idle worsened to a point where engine will not continue to idle once started. How did your last tried solution work? I suspect my carburetors are needing rebuild but am hesitant as I have never worked on this engine since new. Engine has 130 hours since new so it is very low use on this engine and I left fuel over winter without winterization. I think I will be paying for this neglect. I am studying the manual to see how entailed it is to remove and rebuild the carbs.

Hi jdlloyd_iboat, Have only ran the engine 3 times after I adjusted pilot screws and synchronised carburetors and up to now look good. On startup when old still need to wait a couple of minutes for the engine to idle normally but after that it's fine!