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Old 23 September 2020, 01:00   #1
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Mercury 15 hp 2 Stroke Back fireing

We had a lovely day out on the River Hamble on Monday 22nd but had some issues with the outboard which is a Mercury 15 hp 2 Stroke. Although this issue is disconcerting it never let us down the whole day.

The issue is that at low revs it backfires every now and then but when running fast and on the plane it is never an issue. If you watch the little video below, Turn your sound up you can clearly hear it back fireing

https://youtu.be/AzZ-BrOX4Fs

I was wondering if anyone has ideas of what it could be
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Old 23 September 2020, 01:07   #2
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Having read quite a bit online I have found others who had this problem with the same engine and the issues seemed to range from bad plugs to the most interesting one which was frayed wires to the Kill switch that touched together as the engine vibrated and in effect operated the Kill. I have checked mine and they seem ok.

When I got home I noticed that the idle was very high on the engine so I adjusted this down to a better level and also as suggested on one thread I found gave the mixture screw on the carb a quarter turn to the left which should make it run more rich.

I have now ran it up in a tub and it seems to not now backfire as it used to.

I may have found a very simple fix but I am not sure. Have others had this issue and do you think I may have stumbled upon the fix. If I have an explanation would be interesting

Video below of it running now in a tub

https://youtu.be/osXAbi7lH34
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Old 23 September 2020, 01:08   #3
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Here are the 2 videos embedded to make it easier to view



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Old 23 September 2020, 02:41   #4
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Quote:
Originally Posted by smallribber View Post
...I noticed that the idle was very high on the engine so I adjusted this down to a better level and also as suggested on one thread I found gave the mixture screw on the carb a quarter turn to the left which should make it run more rich....

I have now ran it up in a tub and it seems to not now backfire as it used to.
You cannot just turn the screws a bit this way or that without knowing the base level. You need to set the idle and mixture up correctly from the start as per workshop level.
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Old 23 September 2020, 03:56   #5
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You cannot just turn the screws a bit this way or that without knowing the base level. You need to set the idle and mixture up correctly from the start as per workshop level.
Well it seems to be a lot better. I will wait till I test it for real before I change it further.

This is a second hand engine so have no booklet on it I am afraid and have no idea what the chap before me did to it.

Overall apart from this it is a very good engine in my opinion
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Old 23 September 2020, 05:15   #6
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Perhaps the lemonade bottles have put some fizz into the petrol & it's burping......
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Old 23 September 2020, 05:25   #7
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Well it seems to be a lot better. I will wait till I test it for real before I change it further.

This is a second hand engine so have no booklet on it I am afraid and have no idea what the chap before me did to it.

Overall apart from this it is a very good engine in my opinion
Basic owners manual:

http://download.brunswick-marine.com/download/dosearch/@isnew/desc?mod=4&lang=EN&categories=OUTBOARD$2006$2-Stroke$

Workshop manual:

Boatinfo - Mercury Service Manual for 6-8-9,9-10-15 hp
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Old 23 September 2020, 05:43   #8
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Originally Posted by Max... View Post
Basic owners manual:

http://download.brunswick-marine.com/download/dosearch/@isnew/desc?mod=4&lang=EN&categories=OUTBOARD$2006$2-Stroke$

Workshop manual:

Boatinfo - Mercury Service Manual for 6-8-9,9-10-15 hp
Many thanks
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Old 23 September 2020, 05:44   #9
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Perhaps the lemonade bottles have put some fizz into the petrol & it's burping......

I wondered the same but I managed to get all the way up the Hamble and back and still had about 8 ltrs left in the main tank so the Bottle was never needed........ Nice to have though
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Old 23 September 2020, 09:02   #10
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I've had three versions of that engine and never suffered that particular regular "cough"... the most common one is an initial cough as you open the throttle from idle.

You may have hit upon the answer by chance but...

Important to know spark plugs are good and correct grade... also that fuel is weeks/months and not years old... and the correct mixture of proper 2-stroke oil... not something drained out of the lawn mower (I know you like to experiment).

Ideally timing settings should be checked as per the manual Max linked but they are rarely wrong so you could skip that.

Setting the idle and mixture speed should be done in a tub or on the water not on muffs.

OB should be warmed up and make sure choke/fast idle is backed right off. Best if you have a small tach to know the speed you are setting.

OB needs to be in forward gear and start with the idle mixture screw wound in clockwise to gently seat and then out 1.5 turns for a basic position. Set idle speed to 750-800 rpm.

Turn idle mixture screw in gradually until the revs drop and it hesitate and note this position of the screw. Then turn out screw until it hunts and revs drop noting position. Set mixture screw to a midpoint of these positions. Stop engine and screw mixture screw in until it gently seats and note how many turns in this is then turn back out to the midpoint.

If this midpoint is about 1.5 turns out then often any coughs or sneezes can be eliminated by turning out another quarter of a turn (anticlockwise)... if the midpoint is around 1.75 turns out that may be fine but you can always try another quarter turn out if not.

After this recheck the idle speed (in forward gear remember) as it may have risen.

If the coughs/sneezes continued I'd be having the carb apart for cleaning and new gaskets.

However this is what happened to my Mercury 15hp and I've not seen it mentioned on the net elsewhere... post #8 here...

https://www.rib.net/forum/f36/marine...arb-67447.html

If that pipe was just starting to split and opening/closing with engine vibration it could cause an intermittent weak mixture.
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Old 24 September 2020, 00:55   #11
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Originally Posted by Fenlander View Post
I've had three versions of that engine and never suffered that particular regular "cough"... the most common one is an initial cough as you open the throttle from idle.

You may have hit upon the answer by chance but...

Important to know spark plugs are good and correct grade... also that fuel is weeks/months and not years old... and the correct mixture of proper 2-stroke oil... not something drained out of the lawn mower (I know you like to experiment).

Ideally timing settings should be checked as per the manual Max linked but they are rarely wrong so you could skip that.

Setting the idle and mixture speed should be done in a tub or on the water not on muffs.

OB should be warmed up and make sure choke/fast idle is backed right off. Best if you have a small tach to know the speed you are setting.

OB needs to be in forward gear and start with the idle mixture screw wound in clockwise to gently seat and then out 1.5 turns for a basic position. Set idle speed to 750-800 rpm.

Turn idle mixture screw in gradually until the revs drop and it hesitate and note this position of the screw. Then turn out screw until it hunts and revs drop noting position. Set mixture screw to a midpoint of these positions. Stop engine and screw mixture screw in until it gently seats and note how many turns in this is then turn back out to the midpoint.

If this midpoint is about 1.5 turns out then often any coughs or sneezes can be eliminated by turning out another quarter of a turn (anticlockwise)... if the midpoint is around 1.75 turns out that may be fine but you can always try another quarter turn out if not.

After this recheck the idle speed (in forward gear remember) as it may have risen.

If the coughs/sneezes continued I'd be having the carb apart for cleaning and new gaskets.

However this is what happened to my Mercury 15hp and I've not seen it mentioned on the net elsewhere... post #8 here...

https://www.rib.net/forum/f36/marine...arb-67447.html

If that pipe was just starting to split and opening/closing with engine vibration it could cause an intermittent weak mixture.
Thanks Fenlander. I am sure that information will prove very useful if I need to adjust further. I am going to give it a proper test on the water as it is first to see if I have sorted the issue.

Only thing I would say is I dont buy into the whole new fuel thing and thisnk this is more applicable to newer engines.

When you have the number of engines I have it simply is not feasible to remove and get rid of what some might call old fuel. My experience over many years also is this is not a real issue
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Old 24 September 2020, 03:43   #12
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>>> what some might call old fuel. My experience over many years also is this is not a real issue...

Yep I too in land based devices have used some quite old fuel OK but the reason for my comment above is if you are setting out to problem solve a running fault it's important to start with known base conditions and good fuel is one.
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Old 24 September 2020, 06:55   #13
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>>> what some might call old fuel. My experience over many years also is this is not a real issue...

Yep I too in land based devices have used some quite old fuel OK but the reason for my comment above is if you are setting out to problem solve a running fault it's important to start with known base conditions and good fuel is one.
I would not deny that. It was the thought of what I would do with over 300 ltrs of fuel that is a rough estimate of what is in my various boats at any one time.

Plus of course the several thousands of ltrs in lemonade bottles
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