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Old 11 June 2018, 12:05   #1
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Tohatsu 25hk

I habe a Tohatsu 25hk two stroke (i belive it is a 2003 mod) and i would like to get it op to 30hp. I have heard that you only need to change the gasket between the carbuerator and the cylinder. Is this true or do i also need to change the main jet?
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Old 11 June 2018, 16:54   #2
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Welcome to the forum.

You need to do a little detective work to answer your question. Tohastu outboards, especially 2-stroke can be tweaked. My M60C uses the same block of the M70C and the only difference is the main advance arm needs adjustment to allow full throttle opening.

On the 25hp, 2-cylinder block you'll need to cross check against the 30hp to ensure it doesnt have an increased exhaust manifold, restricted carbs, or indeed a different inlet reed valve.

2003 25hp
https://www.boats.net/catalog/nissan...-stroke-nissan

2003 30hp
https://www.boats.net/catalog/nissan...-stroke-nissan
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Old 11 June 2018, 17:17   #3
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Thank you on a norwegian site they explained that it is a gasket restricktor (didn't manage to upload it here), if there is a gasket type restrictor on my engine will i need to upjet?
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Old 11 June 2018, 20:03   #4
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Quote:
Originally Posted by jonaseriksen01 View Post
Thank you on a norwegian site they explained that it is a gasket restricktor (didn't manage to upload it here), if there is a gasket type restrictor on my engine will i need to upjet?


The only way to be sure is to get the parts diagrams out and compare side by side.
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Old 13 June 2018, 02:14   #5
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I think you are correct in saying it's just a gasket behind the carb, you can either order a 30hp or just open up the 25hp I believe. I've never done but remember reading on another forum, maybe worth checking the parts book for both engines, just quickly check exhaust part numbers and maybe jets too
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Old 01 June 2019, 07:40   #6
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Hi guys.

Did we ever get to the bottom of how to do this?

I've just acquired a YL380Dl with an old Tohatsu 25Hp 2 stroke and have been through all the carb parts list and there doesn't seem to be any difference at all in the 25-30hp parts.

Could it just be a throttle restrictor to stop full RPM?
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Old 02 June 2019, 14:19   #7
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25HP to 30HP Factory Conversion.-

There's 2 issues associated with upgrading a Nissan/Tohatsu 25 to 30 HP :
-It's a carb gasket swap, buy the 30 HP model, has wider air intake.
-Time advance on both are different, the 25 model works with 20, the 30 model works with 25 timing.
-Both models shares same exact parts except the carb gasket.

Adjustment Procedure :
-Remove cowl.
-With left arm finger press butterfly's pawl fully down to open the butterfly fully.
-Hold it there, with right hand throttle grip back and forth from 3/4 to full grip.
-Check if hearing a slight banging going on between cam actuator and white roller.
-If the slight banging is not heard means the carb remains adjusted to the 25 model.
-Without releasing pawl nor throttle grip.
-Stand at middle right side of motor and visually check at which timing mark is timing plate stopping at ?
-If stopped at the 20 timing, will need to adjust carb and timing to 30 HP specs.
-If stopped at the 25 mark leave it there, make sure has stopped at middle of both mating surfaces as in pic, if not will need a slight adjustment.

Note: could have a case in which carb doesn't open fully but motor was delivered with 25 setting, as have found out checking may 25 HP motors.

What's the prop number stamped on current prop ? Report your findings to go from there step by step.

Happy Boating
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Old 03 June 2019, 12:17   #8
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Update, forgot to mentioned that after the motor cowl has ben removed, with throttle grip fully turned to min throttle position, must move gear lever to forward position, from there can continue with the process...

Happy Boating
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Old 15 July 2019, 12:15   #9
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Hi guys

Quick update.

As per Locozodiac has explained.

Very simple operation to change the carb gasket and its exactly as Loco's pictures.
New Wider open gasket fitted.

I test fired the engine on muffs and all started fine.
I've adjusted the timing one notch as instructed.

Not sure if its important but after adjusting one notch forward I realised I still have another notch marked on the timing marks to advance more.

This would either mean my engine was set at 15 degrees by the previous owner and I've adjusted to 20 or there is another setting marked on my engine to maybe go to 30 degrees?

Haven't had a chance to get it in the water yet to open it up (first week of August I'll have it in the sea in Cornwall) but it fires up fine and ticks over on the muffs perfect.
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Old 15 July 2019, 17:16   #10
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Counting from 0, you should advance the timing 6 full (l) marks till reaches 25 timing mark as previously described. Don't recall if there's a seventh timing mark pass the 25 one. If so don't use it , motor will have excessive time advance for the max fuel intake achieved at 25 deg for the 30 HP motor.

If in doubt, post a clear picture shot in front of both mating surfaces. Which is the current pitch number stamped on prop body ? Is it a 13 ?

Happy Boating
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Old 16 July 2019, 05:07   #11
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Hi Loco

After replacing the gasket I moved the timing ONE mark forward of where it was already set.
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Old 16 July 2019, 05:10   #12
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Old 16 July 2019, 11:14   #13
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Forgot to talk about the prop.

I've checked over the prop and I can't find ANY markings on there to tell me the pitch, model, size.

It's not in the best condition, one blade has a very slight mishap to it.

Might be an idea to just get a new one.

Any suggestions as to what to get.

I'm not interested in any thing special. Just a good balance for a family fun boat.
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Old 16 July 2019, 13:30   #14
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Hi DM, something doesn't add up comparing my pic to yours. On yours, are there 8 timing marks in total ? On mine being a 2018 model see only 7 marks.

Which is your boat type, is it a Rib or Sib one, its lenght, number of passengers you usually go out with ? If the diam and pitch are not stamped on prop body (where all blades meet) must be stamped on any of the blades.

It's important to know, have found out with a tach that factory delivered prop if being 13 pitched is way overpropped and motor runs towards to the lugging side of its wot rpm range powering light applications such as a 430 Sib with 2 adults up.

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Old 16 July 2019, 14:32   #15
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Hi Loco

My picture and yours match. My zero point is covered in grease.

My engine is much older than yours. 1999 Model M25C3

I have moved one mark forward with timing, but I still have one more mark to go.
Maybe the timing was set low before I got the engine? It ran ok when I tested it.


The engine is on my Zodiac 380 DL yatchline.

Its mostly for 2 adults and 2 big kids to find beaches and have picnics.
But it wouldn't be a rib if it wasn't fast, that's the reason I wanted the full 30hp :-)
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Old 19 July 2019, 13:33   #16
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Hi DM,

Ok, the grease was 0, right now the timing is adjusted somewhere 22-23 needs to be fully set at 25 (last timing mark)

The Tohatsu 25-30 throttle system is somewhat imprecise compared to the 18 HP and their younger HP brothers, so needs to be spot on adjusted to work fine through the whole throttle range. It's important to follow these points to the letter.

-Remove cowl, turn throttle grip to min idle (turtle side)
-Move gear lever to forward.
-With left arm finger press butterfly's pawl fully down to open the butterfly fully.
-Hold it there, with right hand throttle grip to full grip (rabbit side)
Ask a mate "to hold" both position for you...

Turn bolt (1) CCW to loosen it say 2 mm back.
Press down and remove black plastic (*) from timing plate.
Turn (*) 1/2 turn CCW as to shorten the distance of the arm as in pic.
Insert back (*) and check if timing plate has advanced to the 25 timing.
If not turn (*) one more 1/2 turn, tight nut against (*) when finished.
Report if all went OK...

Did you find the pitch number stamped on any of both mentioned prop places ?

Happy Boating
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Old 19 July 2019, 14:03   #17
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Hey Loco

Thanks for following up on this. I appreciate the effort!

Just to follow up a bit. My engine is on remotes.

I do understand exactly what you mean and will get it all set on my next play time with the engine.

On the point of the prop, it would seem after scraping off some paint the only mark reads
11 1/4

According to Tohatsu themselves the prop is 11 1/4 pitch and that is standard for both the
M25C4 and the M30A4

The two engines have identical parts according to the spares sheet and the only difference is the carb gasket and timing adjustment.

I've just ordered a standard 11 1/4 inch pitch prop to see how we will get on with trials.

Fenlander on this site has pointed me to a prop refurbisher that when I have a chance will send the old one to and have them refurb and adjust to 12" pitch. I think with the extra 5hp of the engine it will be a good test to compare with the standard prop.

Thanks
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Old 20 July 2019, 09:22   #18
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DM,

A pleasure to contribute to the cause of good boating. Being a remote motor, disconnect the throttle cable on the motor side and move the advancer arm where the cable connects to with hand till carb is fully open, hold it there, move timing advance to 25.

Regarding props, Tohatsu usually delivers new motors with a 13 pitch prop which is over propped. Have changed all my Boating Club's Tohatsu 30 HP motors for OEM 11 pitch props, a day/night faster hole shot/top speed difference.

To achieve a well converted 25 to a 30 HP it's a combination of carb gasket swap, correct time advance and proper prop pitch. Tohatsu motors achieves their 30 HP range at full wot of 5800 rpm as loaded.

Don't send the old prop yet to a prop shop for a repitch, a 12 pitch is better than a 13, a 11 is better than a 12 and a 11 is much better than a 13 LOL!!

With timing advance well set and a new OEM 11 prop will see some more improvement. Trim motor to 90, distribute deck load evenly as in pic to get the max out of your Rib combo.

Happy Boating
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Old 23 July 2019, 03:17   #19
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Thanks Loco

All done.

Link arm adjusted to exactly 25 degrees. I had to adjust the stop screw on the arm as well because when throttle was return to idle timing was then lower then 0. It went down to -4 degrees.

New 11 1/4 pitch prop fitted.

Now just need to get the boat in the water and start testing!
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