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Old 28 August 2013, 13:36   #1
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Country: UK - England
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Engine problems

Very confused and hoping symptoms are familiar to someone!

Have an old 1994 60hp Merc which is generally well behaved but developed fault on Monday will run on idle and WOT fine but anything in between just dies!? Have tried it on muffs again ok on idle but not fast idle also tried opening choke while running but no joy.

Any thoughts welcome

Cheers ........
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Old 29 August 2013, 09:17   #2
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lean idle mix screw.

Quick test - if you blip the choke as you open the throttle does it go OK?

If so, & If it suddenly started doing this I would guess you have some cr@p in the passageway / jet. That'll be a carbs off & clean job.

Alternatively you might get away with richening up the idle a bit. Give the idle mix screw a 1/8 turn towards rich running

(if none of what I just said makes sense, post back with a serial number & I'll find an exploded view of your carbs to point you at the right adjuster)
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Old 29 August 2013, 10:06   #3
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Hi .... Thanks for your post, you sound quite assured it is carbs, I was scratching my head because it is apparently fine at WOT. We did blip the choke last night while running but very difficult to tell the difference with muffs and not helped by the fact I am working abroad so watching my son do the test on Skype!

I assume you suggest making same idle mixture adjustment to all three carbs regardless that it is likely only one has crap in it?

Thanks again for the advice
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Old 29 August 2013, 10:27   #4
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check the colour of the spark plug electrodes, are they the same colour? if one if differant colour you could try it in a differant cylinder but I suspect dirt (we called it shit) in the carburetor, if it is dirt clean all the carbs and replace any fuel filters as they are obviously not workin properly.I also always set the idle mixture screw at one and a half turns out from lightly seated it is now running on the rich side which is better for it. Better richer than to lean (weak)
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Old 29 August 2013, 11:14   #5
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Running on muffs does not give the exhaust back pressure it sees when the engine is in the water, so what seems like the perfect set- up on muffs might not even start when in the water.

The fact it idles means there is enough fuel getitng in to keep it going under next to no load. Somewhere between 1200 & 2000 rpm (depends on the engine) the fuelling moves form the idle jets to the main jet, so once you get it going it will (should!) happily run. This is what you are experiencing.

If the idle is is set a touch on the lean side, when you open the throttle the lean mix isn't quite enough to keep it going, as the acceleration puts a fair load on the engine.

If you imagine the fuel going in releases a certain amount of energy, which at idle has to overcome three main things: 1) the exhaust back pressure 2) bearing & piston ring friction & 3) compressing at least one other cyl in time for the next "bang".

So, as you accelerate, these three don't go away, but added to the pile of energy consumption is the power needed to spin the prop & accelerate the boat. If you are running slightly lean, the carb can't predict what you are about to do (i.e. open the throttle) & react fast enough, and so the "spare" energy to push the boat along from the lean mix is not enough to overcome the extra power demands, so it stalls.

Ironically the bit where the idle jet can't let enough through is a tiny proportion of the operating regime, hence by richening the mix and yes, your idle becomes a bit less eficient, but at least when you open the throttle, enough petrol can be dragged through the crab to allow the engine to spin faster & let the main jet take over.


The reason I am pretty certain?
I've had it happen to me on three engines, most recent time when I upped the pitch of my prop & the "ideal" idle mix settig for the old prop wasn't enough to get the bigger pitch prop (& therefore load) prop past that crucial point.


+1 for Uncle Al's comments.
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