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Old 11 August 2008, 02:09   #1
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Country: Other
Town: Lima-Peru
Boat name: Nautile
Make: Sea Rider 450 Rib
Length: 4m +
Engine: Tohatsu 5/18/30 HP
Join Date: Sep 2007
Posts: 1,998
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 Nº 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 20º,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 25º, that is 5º 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 25º position, if not play with stopper A ccw until the 25º has come to stop at the 25º case mating position. Pic 4

4-Take a look at the carb arm & butterfly plate, with 20º will be not at full wot nor the butterfly at horizontal position, with 25º 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 180º 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 Nº 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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Old 11 February 2009, 19:24   #2
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Country: UK - England
Town: Plymouth
Make: Avon SR4,AX500
Length: 4m +
Engine: Yam 50 2 Stroke
Join Date: Jan 2009
Posts: 418
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
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Old 19 June 2011, 14:36   #3
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Country: UK - England
Town: Chichester
Boat name: Rib
Make: Avon 450
Length: 3m +
Engine: Yammy 50hp
Join Date: Oct 2002
Posts: 96
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
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Old 30 May 2022, 00:06   #4
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Country: Canada
Town: Westerose
Boat name: Knot Stolen
Make: Lund
Length: 4m +
Engine: Sea Pro
Join Date: May 2022
Posts: 4
Reading this post as I am interested in upgrading my engine but am hesitant to start since the part about adjusting the timing is concerning. Has anyone completed this upgrade with success that can clarify timing adjustment step?
I have sent a message to Locozodiac also ... hoping he replies.
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Old 05 June 2022, 16:33   #5
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Country: Canada
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Removed factory gasket
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Made gasket
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Installed gasket
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Throttle adjustment. After installing new gasket idle speed increased by approximately 500 to 700 RPM so I turned screw counter clockwise with engine running and in forward gear (tied to dock) until it nearly stalled then turned screw clockwise to increase RPM slightly.
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Air/Fuel mixture adjustment turned 1/2 turn out to richen mixture
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After installing gasket and adjusting air/fuel mixture and throttle I took the boat out on the water and it ran perfectly. Achieved a very noticeable power increase for hole shot and mid throttle acceleration.

Top speed may have gained 1 KPH but did not expect any change in top speed anyway.

I did not adjust timing nor feel I need to. Process of adjusting timing is unclear and inaccurate according to previous posts in this thread.

Tools required: 10mm wrench and flat screwdriver.
Time required: 1 hour including 10 minutes to cut new gasket.
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