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Old 23 April 2015, 04:37   #1
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Mariner 2st mixture tune

Hi to all,

I just recently bought my 1st boat, equipped with a 1999 3 cylinder Mariner 2st 40HP. The motor runs nice & smooth, impeller is ok with nice water flow, the electric start also works fine. Autolub unit also working ok and already tuned up according to the service manual.

Just one problem, the consumption is way exaggerated! the motor is an alcoholic! something like 17L hour not at full throttle. I don't have a RPM gauge so I don't really know the rotations. The boat glides good and fast, 2 person crew, no extra loads... there is nothing that justifies such a consumption!

Another observation is that the spark plugs get all black really easy! the mixture is extremely rich! I guess?

In the manual I already found how where to tune the idle point (idle screw)... easy, but how do I tune the fuel intake when throttling up? and get a less rich mixture.

Regards to all.
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Old 23 April 2015, 04:53   #2
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A 40hp at full throttle as a rule of thumb will burn 4 Gallons or 18 Litres per hour, so your almost there. Fiddling with the air mixture screws won't affect fuel consumption much, so I would be looking at resetting the carb floats and for any potential fuel pump problems. Also get ignition timing checked. It could be over oiled if premix or problems with autolube.

Begin with the easy things.
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Old 23 April 2015, 05:02   #3
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I got to get myself a gas station!

I'll check the carbs thou... floats etc..
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Old 23 April 2015, 05:04   #4
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Quote:
Originally Posted by RageGlow View Post
Something like 17L hour not at full throttle.
Welcome to boating!
I've heard it likened to showering in bank notes quite a few times...

Even a new 4 stroke 40HP you're looking at 14 LPH WOT.
As a rough guide I use www.boat-fuel-economy.com and then take with a handful of salt. I then feel thankful that I've only got a 9.9 which burns < 4 LPH WOT

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Old 23 April 2015, 05:20   #5
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Old 23 April 2015, 05:28   #6
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While I'd love the comfort of a RIB I just can't afford the fuel! so I enjoy SIBing around instead.

Not quite the same style of boating but still fun and doesn't break the bank either!
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Old 23 April 2015, 05:48   #7
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Get a tinytach type rpm gauge - only £10 on eBay to check how the engine is doing but regardless if you've just bought first thing to do is get it serviced or DIY it.
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Old 23 April 2015, 06:12   #8
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Dammit, I've just fitted an engine hour timer that doesn't have the tach Might have to get one of those too!
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Old 23 April 2015, 10:34   #9
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Do all motor have connections for hour/rpm?
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Old 23 April 2015, 11:13   #10
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the tiny tach, and my cheaper engine timer that doesn't show rpm, both just have a fly lead that wrap around a HT lead. The coil of wire is energised when the spark plug is fired, and as you can't spark the same plug more than once per rpm, if you count the sparks, you can work out the current RPM

Quick and very very easy mod that you can do in less than 3 minutes with 3 cable ties.

So yes, all motors with spark plugs have a connection for the tiny-tach.
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