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Old 10 August 2008, 21:09   #1
Locozodiac's Avatar
Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider Boats
Length: 4m +
Engine: Tohatsu 18 /30 HP
Join Date: Sep 2007
Posts: 1,706
Uprating Mariner 25 to 30 HP - Part 2

Thanks Chatts for providing the missing pic, the engine was identified as a disguised Tohatsu brand, was in doubt if it was a rebadged Yamaha or Tohatsu 25, 2 strokes engine. Although on the previous thread 50% of the answer was given as correct, the other 50% vital tech info was missing : This uprate applies only to latest current production made 25/30 models. pic 1, older models don’t apply. Will have to check older parts & service manuals. Here is the complete procedure.

1-Remove 25 Carb from engine, remove flat air restrictor plate from front intake manifold change for a new Tohatsu 30 HP carb gasket part N 346-02011-0. Put carb back, adjust nuts. As a reference, see plate/gasket differences pic 2

2-Go to the left side of the engine, where the throttle linkage is located, underneath the dented flywheel will see spaced vertical marks called “Timming Marks” Usually the Mariner 25 version comes factory adjusted to 20,to check this out turn completely the tiller throttle to full wot and check for: The position at which the mark has stopped in front of the Crank Case Mating Surfaces (a thick metal line, where both cases meets together) pic 3

3-The 30 HP Tohatsu comes factory adjusted to 25, that is 5 more, if not in this final position loose nut CCW that holds stopper A some 3/4 mm CCW away, full throttle again, see if the timing mark has moved to the right to the next 25 position, if not play with stopper A ccw until the 25 has come to stop at the 25 case mating position. Pic 4

4-Take a look at the carb arm & butterfly plate, with 20 will be not at full wot nor the butterfly at horizontal position, with 25 will be. Loose the stopper A CCW sufficient gap until you see the throttle lever pushes all the way the plastic white carb collar/roller for the butterfly to reach 180 position. Once reached this final position, secure the stopper A bolt by placing a + screwdriver on the screw head, retain not to move to any direction to change final adjustment, proceed to adjust/lock the nut properly. Put a drop of hand nail paint between nut & screw to secure/tight well. If throttle grip too lose, adjust by means of throttle grip resistance nut CW Pic 5

5-Will need to fine tune the air/gasoline mixture screw between lean and rich, if in doublt, adjust sligtly rich, with digital tacho adjust the ralenti/idle screw if needed to obtain 1,050 rpm at neutral and 900 rpm at gear. Don’t modify timming nor play around with rods 3 & 4 pic 4. You'll screw it.

Both engines 25 & 30 comes factory equipped with same size/pitched prop, no need to change, only if looking for max top end speed for cruising, racing, etc. Will feel a much smooth engine in the low/medium range throttle and the nice 5 HP difference at full throttle. On the other hand will have better/more end speed, better hole shot and carry more weight, gear, mates than before.

If needing a 30 HP engine buy this Mariner/Tohatsu 25 version and uprate it to 30 just investing peanuts. It’s a well manufactured engineered engine that will give you lot’s of boating satisfaction. You could reverse the same procedure to degrade a 30 to 25 HP, just buy the air restrictor plate Toh N 3P1-03536-0 and de grade timming marks as before uprate. Don’t know about 4 strokes same models procedures, as we don’t import these, to my luck, but not good to the environment, no pollution restrictions are applied here. Buy, change HP rear cowl decals if required by law to do so.

Although the factory conversion is very simple & cheap, both brands are not willing to offer this beneficial 25-30 uprate for free, this benefit will cost the buyer 10/15 % more price as they are getting more power, and that costs extra.

Happy Boating
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Locozodiac is offline   Reply With Quote
Old 11 February 2009, 14:24   #2
Country: UK - England
Town: Torquay
Make: Avon SR4,AX500
Length: 4m +
Engine: Yam 50 Toh 9.8
Join Date: Jan 2009
Posts: 409
Just trying to change the timing to 25 degrees but cant seem to get more than 20 by moving screw. Spoke to a technician at a local dealer who said that to change you must adjust rod 4 in picture 4. Is this correct?

Many thanks

gibbo500 is offline   Reply With Quote
Old 19 June 2011, 09:36   #3
Country: UK - England
Town: Littlehampton
Boat name: Rib
Make: Avon
Length: 3m +
Engine: Jet
Join Date: Oct 2002
Posts: 88
Anyone else done this?

I get stuck at the part adjusting the throttle linkage. Which nut is it that is mean't to be adjusted to open up the throttle a little to the 25 mark?

Many thanks
rarms is offline   Reply With Quote

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