1999 25hp 3 cylinder mods

[breeves05]

Hello All,
I am new to the site and would like to ask for some help / advice on making some modifications to my newly acquired 1999 25MSHX3 outboard. I have heard these little 25's are sleeping giants and I am trying to unleash the beast within mine!

I have purchased both the performance head kit and the exhaust tuner kit, plus I have downloaded the service manual. The head replacement seems pretty straight forward but the exhaust tuner installation is stumping me.

Is it possible to install the exhaust by removing the lower unit and then dropping the upper case? I tried but could not get the upper case to break loose. I did not get too aggressive with this for fear of damaging something. I am not the most knowledgeable on outboards but it looked like this was an option looking at the exploded diagrams.

If this method is not an option I am going to have to bring it to the shop because I would not be willing to go in from the top side, way more than I am willing to tackle.

Thanks for any insight

[99yam40]

power head will need to come off for exhaust tuner replacement

More HP = more air and fuel in correct ratio and then you have to let it back out

If more HP was wanted why not just buy a larger HP motor?

[breeves05]

Well that's what I was afraid of, thank you for the info.

And to answer your question about upsizing the hp, I fish in a refuge and they cap the motor size at 25hp.

[almetelo]

Thats a expensive mod for sure... That kits are expensive (at least for my pocket) but I understand you may not have other option...

Get the 30HP jets and you'll have 30hp with 25 sticker ;-) or get the Tohatsu M40D carbs and you'll have even more HP but cant say that is for 40 HP

Do one mod at a time if possible and check increase/decrease of power (my opinion)

You'll need a tach and a GPS

Good luck with your mods...

[breeves05]

You're right it has gotten a little pricey (just ask my wife, lol) but I have been saving for some time now. I do intend on increasing the jets because I also have Yamaha 40 carbs to install on it too. The manufacturer of the performance heads informed me that if I install the heads that I would have to at least bore out my existing jets.

Hopefully I will not be the last boat to the fishing hole anymore!!

[almetelo]

Thats the type of things that my wife doesn't need to know the price lol

25 main jets are #96 and 30 are #102
25 slow jets are #54 and 30 are #50
The only diference between these 2 motors are these 6 jets
30 main jet part# 6J8-1492E-51
30 slow jet part# 6J8-1492F-25

The yamaha 40 carbs will do but you'll need to make even more mods...
The yamaha carbs will not fit right to the cages/mainfold like tohatsu carbs that will do...
With Yamaha 40 carbs you'll need a fuel pump while Tohatsu carbs have fuel pump attached like you carbs... The only diference is that on Tohatsu carbs the fuel pump is on bottom carb and on Yamaha 25/30 x3 is on mid carb but you just have to switch the float chamber and the choke and will be plug and play.
On Yamaha 40 the mid carb have diferent main jet size from top and bottom carb while tohatsu have all 3 main jets the same size like you motor (Tohatsu main jets #132)

But YES, Yamaha carbs will do... and if you already have them thats the way to go...

Boyesen reeds B332 may be something to consider if you have money to spend on this project (I never use them but I'm sure will not give you more power like a few claim, just a better response while accelerating)

If you'll do all this mods be prepared to spend more money on a big pitch prop...

After all with money of the sale of your motor plus money spent on mods you could get a good 2nd hand 40 or 50 HP but if you have that restriction and like to have a project on hands go ahead...

I have that model but with 30HP and its a great machine!

Good luck.

[breeves05]

*** Almetelo you are very knowledgeable on the subject and thank you for the priceless info you have so kindly given me.

So let me make sure I follow what you are saying...

I tried the 40 carbs and you were right, they are not plug and play...wish I would have know that before hand! The bottom carb will not fit because the float chamber hits the bottom cawling. You said change the float chamber, are you saying the 25 carb float chamber will fit onto the 40 carb, then it should fit? Will I need to do this for all 3 carbs? And the jets are different sizes, would it be better to make them all the same size as the middle carb?

And for the choke, it looks like the 40 carbs are quite a bit bigger so the knob and linkage will probably be to big/long. Do you have any idea what I will need to correct this problem? Is it possible to modify the existing choke knob and linkage or is it better to get the choke knob and linkage off a 40 hp motor?

And while I am at it, I have tried to remove the powerhead to install my exhaust tuner but I am having trouble getting it to come off. I have removed the 6 bolts (the ones hidden by the plastic skirt) and I still had no luck. Are there any other bolts I could be missing?

Thanks again for any advice you can give.

[almetelo]

Hi!

When I mean interchange float chamber is on Tohatsu carbs. The bottom carb #3 float chamber that have the fuel pump with mid carb #2 float chamber so that fuel pump be on mid carb like is on your carbs (maybe this will make no diference but I prefer leave all things closest as possible from what it was). Your carbs parts are no interchange with Tohatsu or Yamaha 40 carbs.
About Yamaha 40 carbs I can't help you... I never used them and you have to do your home work by reading what you can find on the net googling "yamaha 25 or 30 3 cylinder with yamaha 40 carburetors" or something else then try to find the best solution... will be a harder work (or not) because you need to adapte the carbs to the cages/mainfold while with Tohatsu carbs this is not necessary.

About the choke knob will be better leave it like he is... (my 2 cent) if necessary just try to adapt the linkage.

To remove the block is just necessary remove that 6 bolts and unplug some linkage as shift, I used a chisel to lift the block.

Yamaha have at least 2 sets of carburetors of 3 cylinder 2 stroke carbs and that will depend of model that your carbs came from. On latest model carb #1 and #3 have #115 jets while mid carb #2 have jet #118. If you should use all #115, #118 or leave it that way I donīt know... (Youīll need to do your home work by testing that)

Lets try to get some help from the experts:

Guys why the mid carb on some models have diferent jet size? The one where fuel pump is connected) Thanks!

[99yam40]

I am not sure why Yamaha jets different cylinders on the same motor with different sized jets, but I am sure they needed to do it for proper burn.

Maybe it has to do with the different temperature the cylinders run or the way they breath.
Yamaha even designed some motors with different compression (CC of the head space) on the different cylinders on same motor. I would think that would call for different jets, but am not sure

[breeves05]

Thank you guys for all the advice.

I was able to install the exhaust tuner yesterday, man what a chore. I also installed the performance head. The internals I was able to see look great to the uneducated eye. I didn't get a chance to crank it yesterday to ensure all has been put back together properly. I didn't have any "spare parts" so I am glad about that. I have some work to do on the carbs, first got to research like you mentioned above. If nothing else I will order the 30 hp jets to install.

Again, thank you gentlemen for all of the help in figuring out this outboard. I need all the help I can get, lol.

[almetelo]

Nice you could do it...
Hope you handle the 40 yammy carbs... if not, sell that carbs and get the tohatsu but are much harder to find (at least on ebay US) alredy found them at a resonable price there but at the time didnīt get them because costums would that my fun away. lol

Let us know the results of your mods. (would be nice so we could learn a bit more with your experience)

Do you have the Service Manual for your motor?

Good luck with your mods.

[breeves05]

Well I have not had any luck with the 40 carbs. The bodies of the 40 carbs are to big to fit all three on the intake manifold. And the main jets have different thread patterns so I cannot change those out either. I will have to take a different approach, so for the short term I have ordered the 30 hp main jets to install in the 25 carbs.

I will put the 40 carbs up for sale (freshly rebuilt might I add ) and then try to find the Tohatsu carbs. Is there a specific year / model Tohatsu 40 that I need to be looking for?

Oh, and yes I have the Service Manual for the motor plus I joined the Selco site. They offer some great photos and additional information.

For now it's just waiting for the new jets to come in....

[almetelo]

I think you also should order the slow/pilot jets but you can always probe only with main jets and see what will happen...

The Tohatsu model is M40D and guess M40D2 also will do but not TLDI
Maybe the Tohatsu 50HP (M50D) will do but I'm not sure if the bores (not jets) are the size of 40HP model. Even if the bores are the same size you have always to go for the 40 jets because 50HP jets will not work.

Good luck

[almetelo]

Hope you have this in mind...

If mods will no work or not work as you wish I'M NOT RESPONSIBLE for that...
I'm just giving you some tips trying to help for what your are looking for...

Good luck with your mods...

[breeves05]

I definitely understand that and I do appreciate all the advice. At the end of the day it is my decision to make the modifications, no one else's.

Thanks again.