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Old 31 August 2008, 11:37   #1
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Mariner 25hp upgrade?

I have just bought a 2006 Mariner 25hp 2 stroke and have read a few quotes here that it can be upgraded to 30hp. Is that correct and if so is it worth the extra costs please?
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Old 31 August 2008, 14:24   #2
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Hello, Yes you can, its very simple. Just remove the restrictor from behind the carb, and fit a 30hp carb gasket. But if your engine still has any warranty left you could void whats left of it if anything goes wrong.
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Old 31 August 2008, 15:05   #3
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Thanks turbodeisel, I will check if any warranty, but then is it worth doing, i.e performance and fuel use etc. I intend to put a prop guard on
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Old 01 September 2008, 11:31   #4
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Thanks turbodeisel, I will check if any warranty, but then is it worth doing, i.e performance and fuel use etc. I intend to put a prop guard on
Have no experince with prop guards, been reading that slows you down a bit, even have to change prop pitch to compensate, in that case update to a 30 HP for that big size Quicksilver 430 if planning to use prop guards.

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Old 01 September 2008, 15:01   #5
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Thanks turbodeisel, I will check if any warranty, but then is it worth doing, i.e performance and fuel use etc. I intend to put a prop guard on
Yes itw worth it!. Prop guards can take anything upto a third of your hp depending on which type you go for!!!
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Old 02 September 2008, 00:53   #6
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Hello, Yes you can, its very simple. Just remove the restrictor from behind the carb, and fit a 30hp carb gasket. But if your engine still has any warranty left you could void whats left of it if anything goes wrong.
I notice you work for Barrus- is it correct that you have to advance the ignition by 5 deg too, to get the extra power.
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Old 02 September 2008, 01:39   #7
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so which prop guard should i buy to prevent this power loss you mention?
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Old 02 September 2008, 11:22   #8
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I notice you work for Barrus- is it correct that you have to advance the ignition by 5 deg too, to get the extra power.
If you engine is currently adjusted to 20º, you won't have a full open throttle.

If you engine is already delivered to 25º just change the carb gasket by a 30 model (it's much air wider than restricted 25 gasket)

If your engine is adjusted to 20º, advance to 25º to full throttle, and see if the butterfly is at 180º, adjust by means of stopper. Will have the extra 5 HP at the end for better hole shot, carrying more weight, use on larger boat, etc.

So not to get mixt-up, don't know if all Mariners/Mercury/Tohatsu's 25's are being delivered worldwide to 20º/25º, a matter to check out on each one's individual engine. All cowl 30's are delivered to 25º.
Note: this info is on the 25/30, 2 strokes tech service manual.

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Old 02 September 2008, 11:28   #9
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so which prop guard should i buy to prevent this power loss you mention?
Very thricky question: a matter of experiment under trial/error with different pitch propellers untill full wot max rpm is achieved for that engine model.
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Old 02 September 2008, 11:48   #10
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Cheers for everyones knowledge. Engine good after service and just out of warranty, it is fixed to 30hp and have fitted a guard called propguard, an 11inch model for this engine, so will have to see how it performs when next out and then tinker with your ideas. Cheers Ian
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Old 02 September 2008, 15:03   #11
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Yes itw worth it!. Prop guards can take anything upto a third of your hp depending on which type you go for!!!
They are all horrible!!. The stainless steel ones are pretty good, but not cheap. And Plastic ones are ok but dont last very long. There is no easy answer!!
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Old 02 September 2008, 16:31   #12
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Re propguards
The setup the beach lifeguards on their 30 mariners on the Arancias looks pretty good along with a stainless prop . They certainly take a hammering and still seem to kick out the power with 4 people aboard , I could take a look for you re pitch and prop type for you on Sunday if you like . If I can get a look between training sessions
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Old 02 September 2008, 19:12   #13
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Cheers for everyones knowledge. Engine good after service and just out of warranty, it is fixed to 30hp and have fitted a guard called propguard, an 11inch model for this engine, so will have to see how it performs when next out and then tinker with your ideas. Cheers Ian
Hola Ian

Just to get the nail out, your 25 Mariner was factory delivered adjusted to: 20º or 25º timming ? Which one ?

Have you changed the 25 air restrictor gasket for the open air 30 gasket ?

To have a well converted 30 HP engine both issues works together.

Remove the prop guard, with same prop, why don't you water test the 30 converted engine and let us know if it was better than when at 25 at full throttle. Once adjusted to 30 HP must fine tune engine lean/rich, gas/air mixture needle, slightly towards rich.

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Old 12 September 2008, 12:11   #14
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Check my post on here about durajets - I think that they may soon be replacing prop guards!

The prop guards they use on the beach lifeguard boats are stainless steel racing guards which are matched to the prop and the engine - normally a tohatsu.

Having used them you get very little power lost as the gap between the edge of the prop and the guard is minimal. Also the metal around the prop is very wide and creates a kind of duct.

There are definitely a big difference between the performance of different guards out there.


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Old 12 September 2008, 12:50   #15
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But if your engine still has any warranty left you could void whats left of it if anything goes wrong.
And who is there at Barrus with the intelligence to spot that it had been modded?
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