Mercury 4hp 2 stroke 100:1 running problem

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andrewdingram

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I have a new Mercury 2 stroke saildrive, runs at 100:1 mix.
Using on integral tank.
I have done about 20 hrs in 2yrs.
Always clean fresh fuel, freshly mixed.

When initially using it on 50:1 mix first 4-5 hrs , no problem, but now running 100:1 have a problem.

When running medium - 3/4 throttle after 20-30 min the engine revs drop and go up/down, like it's about to stall.
I tried to increase or rev the engine but this makes it sound worse.
Decreasing the revs actually makes the engine run smoothly again.

Someone suggested that the 100:1 mix may be too lean and the engine may be seizing up.

As I bought the engine new because the old one died with my family in a tricky spot, and as a relatively new sailor I am quite concerned.

I haven't tried it since on a 50:1 mix because I am nervous about it, especially going out in Portsmouth. It actually played up at the entrance so am not sure of cross-currents or extra load were contributing, or whether it was just sods law.

Any thought please would be very welcome.
 
Is water telltale showing a good flow of water? (To discount overheating issues)

After that, check / clean plug.

After that i'd drain/clean integral tank. May be a bit oily in the bottom.
Strip and clean carb.

That would be the basics.
 
Run it on 50:1.........

is what I was told when I bought a new Mariner. note that this was by an engineer and not the marina where I bought the engine. He said that they gave problems on 100 : 1 but were fine on 50:1. I followed what he said and now some 6 or 7 years later it is still running perfectly. This seems to apply to the Mercury/Mariner range of engines only for some reason, I have a Yamaha that is supposed to run at 100:1 and that is what I use and never have a fuel problem with that.
 
Cliveshep said:
is what I was told when I bought a new Mariner. note that this was by an engineer and not the marina where I bought the engine. He said that they gave problems on 100 : 1 but were fine on 50:1. I followed what he said and now some 6 or 7 years later it is still running perfectly. This seems to apply to the Mercury/Mariner range of engines only for some reason, I have a Yamaha that is supposed to run at 100:1 and that is what I use and never have a fuel problem with that.
Click to expand...

On the other side of the fence my Mariner has been running 100:1 since new (9 years) and never any issues.

Saying that if you're fairly confident that is causing a problem, it's easy to add a little 2-stroke to the mix to lower the ratio.
 
Thanks,
I haven't checked the plug but after so few hours I din't think that should be a problem.
I always clean out all fuel every time and run it dry, so shouldn't be that.
 
Cliveshep said:
is what I was told when I bought a new Mariner. note that this was by an engineer and not the marina where I bought the engine. He said that they gave problems on 100 : 1 but were fine on 50:1. I followed what he said and now some 6 or 7 years later it is still running perfectly. This seems to apply to the Mercury/Mariner range of engines only for some reason, I have a Yamaha that is supposed to run at 100:1 and that is what I use and never have a fuel problem with that.
Click to expand...

Mariner 100 > 1
Tohatsu 50 > 1

same engines both made by Tohatsu
i have a Tohatsu 3.3 & they recommend 50>1 for a reason & not a marketing ploy
 
Thanks,
the mix was a guess by my dad, but I guess I need to experiment.
I thought running it at 50:1 if it was designed for 100:1 may cause the plug to black up or the ports to block prematurely.
If as you say the engines are both identical, I guess it isn't going to be an issue.
 
the mix was a guess by my dad
Click to expand...
Do not guess. Do not risk running with less oil than 100:1. Measure it!**

If it were mine I would run on 50:1 unless plug fouling occurred (very unlikely since you use 50:1 during the break in period) then I would opt for something in between.

in the mid 1980s, about the time I bought my Evinrude, OMC switched to 100:1. After a couple of years they switched them all back to 50:1.

50:1 is often recommended for commercial use when 100:1 is used for leisure use. A lesson there somewhere! (and I think that applies to Yammies, Clive)

**I use a couple of conical laboratory measures. One lives in the garden shed for use at home and another on the boat.

 
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sailorman said:
@ 50>1 it wont wear out
Click to expand...

Amen. Two-strokes like it wet 'n sloppy (reminds me of some former girlfriends). Plenty of fuel, plenty of oil. Starve them of either fuel or oil, even a little bit, and you've got melted metal bits instead of an engine.

Why would you tempt fate? All of my two-strokes -- and I love two-stroke engines, whether in snowmobiles, in hedge trimmers, or whatever -- get 50:1.
 
All of my two-strokes ..........get 50:1.
Click to expand...
50:1 not suitable for all of mine.
Evinrude gets 50:1
Seagulls modified to take 25:1, previously 10:1
Villiers engined lawnmower takes 16:1
Flymo seems to prefer 16:1

Generally outboards pre mid 1960s ran on oiler mixes than 50:1.
 
andrewdingram said:
Thanks,
the mix was a guess by my dad, but I guess I need to experiment.
I thought running it at 50:1 if it was designed for 100:1 may cause the plug to black up or the ports to block prematurely.
If as you say the engines are both identical, I guess it isn't going to be an issue.
Click to expand...

Andrew
The ports wont block up
Stick to 100 to 1
That aint yer prob.
Strip and clean the carb I reckon.
She runs ok for abit.
Then summat in the float bowl is blocking the main jet
Bit of water or cr%p maybe.
As Vic says yer have to measure the oil properly.
Although by hours the eng has not done much
Get a new plug as well.
If She runs ok at low throttle settings that means the pilot jet and orofices are clear and OK
 
If it runs better with the heavier oil mix then it is possible that the top and bottom crankcase oil seals need replacing. The heavier oil mix will be creating a better seal on the worn seals but the downside is that your engine may be smokier and have a risk of the plug fouling. On a 2-stroke engine, stripping down and replacing the seals is not rocket science.
 
Could be gus
I have noticed when crank seals are dodgy that idling and low speed performance suffers.
Andrew informs us that the eng is fairly new.
Seals should be ok
But yer never know
Andy
Does She idle ok?
 
Cliveshep said:
is what I was told when I bought a new Mariner. note that this was by an engineer and not the marina where I bought the engine. He said that they gave problems on 100 : 1 but were fine on 50:1. I followed what he said and now some 6 or 7 years later it is still running perfectly. This seems to apply to the Mercury/Mariner range of engines only for some reason, I have a Yamaha that is supposed to run at 100:1 and that is what I use and never have a fuel problem with that.
Click to expand...

Because they're Tohatsus re-badged.

The 100:1 mix was only brought in to extend the statutory sales-life of 2-T outboards - all run much better on 50:1 than on 100:1.
 
Thanks for your replies

To answer a few queries:-
I always mix the fuel freshly and to exact ratio with decent marine oil.
I had put any engine defects like seals etc out of my head because I have had this problem practically since day 1.
She idles beautifully, problem only appears above 1/2 revs, changing the revs up or down a little doesn't help, going back to idle makes her run perfectly again for a bit.

I put carb or fuel problems out of my head because I have always used fresh fuel from a new plastic container (washed out with petrol first to remove any plastic shavings) and use the internal fuel tank only.

I run strimmers frequently without any problems, even abuse them a bit, but have really stuck to the rules on this engine.

Thanks
 
OK I though you were saying that your Dad was guessing the amount of oil added. Not what you meant!

You are using a 2 stroke outboard engine oil to TCW3 spec ? (Not just a "decent marine oil" )

I think you start from scratch.

New plug. (you carry a spare anyway dont you?) Check spark, which should be a good bright blue and jump 3/8 - 7/16".

I reckon though it's a fuel or carb problem. It runs for some while before the problem occurs so check the tank venting and the fuel system for a good uninterrupted flow to the carb.
It sounds as though it is being starved of fuel.
Does the engine have provision to connect a remote tank ... if so look at the fuel system critically to see if there could be a problem there. I forget the exact layout but could the pump which lifts the fuel from a remote tank be sucking in air and pushing it into the system thereby starving the engine of fuel. Try a remote tank if you have one.

If fuel system is OK then I think I would turn my attention to the carb. Does it have a high speed mixture adjustment? Might the high speed jet be partially blocked? Float level or needle valve?

Something is not right and you cannot dismiss every thing on the basis that it is all new ... after all its been like this from day one .

Can but plough through the possibilities. Easy things first.

Try posting the question on the Mercury board at iBoats
 
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Dockhead said:
Amen. Two-strokes like it wet 'n sloppy (reminds me of some former girlfriends). Plenty of fuel, plenty of oil. Starve them of either fuel or oil, even a little bit, and you've got melted metal bits instead of an engine.

Why would you tempt fate? All of my two-strokes -- and I love two-stroke engines, whether in snowmobiles, in hedge trimmers, or whatever -- get 50:1.
Click to expand...

I bet her ports were never blocked up.
 
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