Root cause of Mercury 2-strokes' rough idle

[dandreye]

Hello All,

My 90 ELPTO is quite rough at idle, even stalling occasionally. I've been told rough idle is the nature of the beast and is common for all Mercury 2-strokes as they are optimized for riding. Fair enough but what's the root cause of rough idle? (assuming all components work as designed)

Many thanks in advance,

Dmitriy

 

[schematic]

Re: Root cause of Mercury 2-strokes' rough idle

Rough idle is a subjective statement. If all is normal, 2-stroke idle is acceptable. Sometimes near as good as a 4-stroke. Many things can affect idle quality. Probably too many to mention. The one that we fear the most is uneven compression.

 

[CharlieB]

Re: Root cause of Mercury 2-strokes' rough idle

Compression

Adequate HOT spark and proper timing

Correct idle mixture, clean carbs, idle mix adjusted properly

Once all three are verified the motor should purr.

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

schematic:
CharlieB:

Thanks for your replies.

Re uneven compression: do you mean this results in one or more cylinders not firing occasionally or even if they always fire uneven compression itself disrupts smooth rotation of the crank thereby making idle rougher?

Btw my spark plugs are fairly new BUZHW-2.

 

[schematic]

Re: Root cause of Mercury 2-strokes' rough idle

what's the root cause of rough idle? (assuming all components work as designed)

Dmitriy

Now we're getting into a different train of thought. We will no longer "assume all components work as designed"

Yes, uneven compression will cause rough idle as well as MANY OTHER THINGS.
Check fuel quality, compression and plug appearance first. Then move on to spark quality, idle timing and carb adjustments.

You have to start somewhere....

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

) true.

The compression readings aren't perfect: the lowest reading was 103psi in one of the cylinders when measured about 2 years ago (the engine hasn't done more than 25 hours since then) but I don't longer have the readings in the other 2, I think they were around 110psi, so will have to re-measure them all now. I asked the mechanics who were overhauling the OB whether it was time to put in +0.015 oversize pistons and the answer was "Not yet" (perhaps because the readings were less than 10-15% away from one another), so I agreed with installing brand-new standard size ones. Now thinking if I should have insisted on the oversize ones installed back then.

 

[redjmp]

Re: Root cause of Mercury 2-strokes' rough idle

I vote for intermittent spark!
Old mercs have wiring that rots away...

 

[1nebel0]

Re: Root cause of Mercury 2-strokes' rough idle

You don't say what year but if its a 3 cylinder like my motor it will never idle good...I have spent countless time in the water up to my waist adjusting each carb for best idle and it still doesn't idle smooth.. I am of the opinion and I own 10 boats and as many outboards and none will ever idle like my 115 four stroke on my Pontoon boat...I have fiber reeds in all my motors, and spend more time than anybody I know rebuilding carbs and going over fuel system to insure as much perfection as possible and all of the larger motors have rough idle.. I will say that the large Johnson's seen to run smoother than Mercury as all my motors are Mercury's. I am no mechanic but other than an Optimax your basic two stroke Merc is going to bounce around...just my 02.

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

1nebel0: yep my 90 ELPTO is also a 3-cylinder one made in 2000.

redjmp: during the overhaul 2 years ago I had flywheel replaced along with stator and trigger, CDMs and spark plugs. Later I tried another set of spark plugs (also BUZHW-2). The carbs were cleaned, reed valves looked fine. Are you saying that wire harnesses need checking? I didn't realise anything can happen with them as a lot older Mercury engines than mine are still in use, which sounds like they never die without "help"...


It'd be great to see a video with as perfect idle as can possibly be achieved with these engines (ideally showing the engine at a close distance so that its vibrations can be clearly seen).

 

[ENSIGN]

Re: Root cause of Mercury 2-strokes' rough idle

You were told right,these motors do not have a smooth idle.There's even a warning in the mercury service manuel which states(DO NOT ADJUST CARBORATORS TO ACHIVE A SMOOTH IDLE)This is because the engine will be running to lean at this point.

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

Just found this:
http://forums.iboats.com/showthread.php?t=457228

ENSIGN: in that case maybe it's better to forget about the proper idle mode completely and simply use low rpm mode instead? I guess this could be done by fine tuning the accelerator lever so that it never reaches the position at which carb throttles fully close.

 

[redjmp]

Re: Root cause of Mercury 2-strokes' rough idle

Sorry... I was just replying to the thread title and speaking in general terms of all old mercs not specifically about your model.
I have seen many corroded connections and wires...

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

redjmp: btw if it turns out to be something causing intermittent spark I wonder if a clamp-on tachometer I recently purchased for general purpose car/OB diagnostics will tell me anything or only Mercury tools can reveal it.

 

[redjmp]

Re: Root cause of Mercury 2-strokes' rough idle

You need to build or buy a DVA tester. http://www.tooldiscounter.com/ItemDisplay.cfm?lookup=ESP640&source=froogle&kw=ESP640
While you are at it, get a spark gap tester as well.
Then you can properly troubleshoot your electrical system.
The tach won't really be too much help, you need to verify the correct voltages and check for good spark under load.

The CDI website has all the info on how to test.

 

[merc850]

Re: Root cause of Mercury 2-strokes' rough idle

I hope that you are adjusting the idle while the motor is running in F at about 1100 rpm.

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

redjmp: If I can use this ESP640 tool with a digital multimeter then it'll save me the cost of Mercury #99750 DVA/Multimeter - thank you. Btw is there maybe a similar cheaper alternative to Mercury #825207A2? Don't fancy paying over $70 for what looks like a bunch of wires.

merc850: I was absent while it was being fine tuned for me and so have no technical details...

So to summarize what we have by now: (1) achieve more even compression across all 3 cylinders, (2) verify the presence/quality of spark, (3) if still no help then just increase rpm @ idle and relax )

 

[aussieflash]

Re: Root cause of Mercury 2-strokes' rough idle

You got any footage of it idling?Upload so we can hear it.If its coughing and spattering at idle id have a closer look at reeds.

 

[dandreye]

Re: Root cause of Mercury 2-strokes' rough idle

aussieflash: good idea: I'll see the boat again in a month and take a video of it. In the meantime it'd be great to see other similar engines idling properly and would let me say whether mine is rougher or smoother.