25hp Tohatsu to 30hp

henryfreston · 22 September 2012, 16:47

Tomorrow I am going to pick up a 4m honwave SIB with a 2006 25hp 2 stroke tohatsu on the back. Can these engines be modified to a 30hp on a reasonable budget? Sorry to be fussy but I would need specifics e.g. if there was a restrictor plate then where is it/what does it look like/maybe a picture

sorry I am new to engines so if some just said a random part I would not know where it was.

Another thing was, what do you think of these engines? Are they reliable and would it push the SIB well? Would you be able to pull ringos and kneeboarders?

thanks, Henry. P.S. I am new

maxitope · 22 September 2012, 17:58

i have a honwave t40 with tohatsu 20 4stroke goes well 5up .proper goes 2up would pull ringo no problem

henryfreston · 22 September 2012, 21:28

thanks for the reply buddy

Locozodiac · 23 September 2012, 00:04

Henry, you must change carb gasket, buy the 30 HP model and depending on the model advance +5º timing mark. A 30 HP will fly a 4 mt sib, just inflate well to factory recommended working pressure to perform as expected. There is plenty info on this section about modifying Tohatsu 25 to 30 HP , including pics of the procedure. A matter to search.

Happy Boating

lightning · 23 September 2012, 19:17

If I were you I'd try the boat out with the engine on it first, you are worrying about the performance and have not even tried it out on the water yet!
Get the boat out on the water and see how it performs, you may find it goes like a rocket and you don't need that extra 5bhp.

Locozodiac · 24 September 2012, 17:01

The extra 5HP is at the end of the trottle, you can throttle less and behave like a 25 HP. If carrying more weight or total max allowed passengers, a must have. Both 25 and 30 models weights same, just 25 with restriction. For $ 7.50 go for it....

Happy Sibbing

J_HOU · 20 November 2012, 12:30

Hello.
I am returning to boating after a long period away from the water. I am also new to forums.

Now that I have got that out of the way, I would like to know if it is possible to cost effectively modify a Mercury / Tohatsu 25hp 4 stroke motor to 30hp. In particular the EFI models.

I hope that someone can help.

Best regards.

clydeoutboards · 20 November 2012, 13:10

Hi

Not worth it and you haev a great package so enjoy.

Dave

J_HOU · 20 November 2012, 18:16

Hello Dave.
I was just having a wonder. Thank you for your extremely quick response.

On motor vehicles with EFI a small increase in bhp can be obtained, 10 to 15%, by de-restricting the air inlet tract because the Mass Air Flow Meter and Oxygen Sensor compensate for the increase in volumetric air flow whilst still staying within the original Engine Fuel Map. So, if the Inlet, of for that matter Exhaust, Tract had been restricted by using a reduced area gasket or by some other easily defeated means an easy 5hp could be gained.

You can even purchase a re-mapped chip for the Engine Management Module on motor vehicles. It looks like outboards have a little catching up to do.

9D280 · 20 November 2012, 21:27

Welcome to ribnet!

Thing with outboards is the market (compared to cars) is miniscule, hence not designing everything "bespoke". There are plenty ways to "de tune", as you say, a restrictor gasket, hard stops on the throttle mechanism, even different cams on the spark advance follower (Older machines the throttle advances the spark - the throttle is generally driven off that)

Newer machines, when you look at the relatively small market & cost differences, you can kind of see why they might want you to buy the bigger engine to get your extra gee gees! Have a trawl through the forums here - you will quickly find who resprayed what into their own livery, and what might be easily re-powered. Some it;s a siomple djustement, others you might as well buy the bigger engine by the time you have replaced the parts!

Thing with EFI is there is also a compooter to re-tune too.... What's yours hanging on? You may find 25 is plenty!

Pop back with Qs once you've had a search- plenty of opinion on here, average it out & you'll get a reasonable answer!

(use "advanced" to keep it to the engine forums - Loco knows a fair bit about the Tohatsus

)

J_HOU · 21 November 2012, 12:56

Hello 9D280.
Thank you for the welcome.

The outboard is on a 5.35 metre SIB. I will have a trawl through the site to see what information I can glean.

The boat is mainly used for fishing with 4 persons on board plus fishing tackle and all the usual boat equipment the approximate weight of "everything" including the boat is 750kg.

The outboard seems to be propped correctly and goes ok through all the rev range. I just thought that if there was an easy way to get hold of an extra 5hp then I might just do it this winter.

Best regards.

Locozodiac · 21 November 2012, 15:12

The conversion is a cheap piece of cake with wipped cream, extra 5 HP which is at the end of throttle grip will give better hole shot when more loaded and slight final top end speed. Depending on the Letter/Number next to 25, could find the following :

25 HP with restricted carb gasket and restricted time advance to 20º
25 HP with restricted carb gasket and no restricted TA to 25º

Second is faster to convert as is already timed to 25º, just swap carb gaskets for the 30 model.

Happy Boating

J_HOU · 21 November 2012, 20:11

Hello Locozodiac.
My Tohatsu is a MFS 25B EFI Four Stroke. I thought that the modification that you have posted was for the 2 Stroke models. Unless both Four Stroke and 2 Stroke models have the same modification to increase the BHP to 30HP.

I hope that they do.

Locozodiac · 22 November 2012, 14:01

Quote:

Originally Posted by J_HOU

Hope they do.

Sorry, NOPE, it's only for 2 strokes. 4 strokes 25 & 30 models functions with different CPU parameters. If being same CPU will need factory tech data to reprogram, if not, a complete CPU swap from 25 HP 30 model is mandatory, and it's expensive. 4 strokes engines have an electronic advance system, not mechanical as found in 2 strokes.

Happy Boating

J_HOU · 22 November 2012, 14:17

Hello Locozodiac.
Thank you for the fast reply. Yep, it looks like I will not be doing the 30hp modification this winter.

I might just fit some fins to to the outboard, or, speak to some people I have worked with in the past who reprogram EMU's for 4 Stroke engines. The problem with re-mapping the EMU is that I cannot use a rolling road.

I think that it is too much work for an extra 5hp though.

Best regards.

Best regards.