97 4 stroke 9.9 to 15 conversion....????

[jcshep7896]

This is my first Yamaha i have ever owned and its a 97 4 stroke 9.9 and i want to know what it takes to convert into a 15 hp. Im sorry if this has already been answered!! The motor had sat a while from previous owner and seems like carb may need adjusted because it dont seeem to hit top end and when backing out of throttle quickly it will sometimes die. any suggestions??? I thank you for any help!!

[cleddau1]

Welcome to forum!
Suggest a search on the F9.9
On a recent post of mine there is a link kindly provided my Panasonic to download the official workshop manual.
First thing would be a full service and get the motor running sweetly before considering modifications.

[jcshep7896]

Thank ya very much i will try to find it

[cleddau1]

It always helps to state full engine details when posting. There are some very knowledgeable (not me) and helpful folk on this forum

Here is the link:
https://www.vansoutboardparts.com/se...a-shop-manuals

[99yam40]

the 4 stroke is not like the 2 stroke that derated a 15 to make a 9.9.
look at the parts of the 9.9 and the 15 4 strokes to see if you can find any parts the same.

maybe you could put a nitrous kit on it like they do hot rods

[boscoe99]

I am still looking for the fountain of youth while some seek free horse power.

[jcshep7896]

I was just going by what i had heard i didn't mean to ruffle anyones feathers I just knew it was possible on some.

[panasonic]

I was just going by what i had heard i didn't mean to ruffle anyones feathers I just knew it was possible on some.

No worries about ruffling feathers on here...it is usually short lived...

Now how about your engine running badly...

You have fresh gas, clean tank and fuel filter? If it sat around for awhile the carb probably needs cleaning to start.

How do you know its not making full RPM? Do you have a tach?

The wrong prop can also make it not reach full RPM..

I would start with getting the carb clean and go from there.

[99yam40]

so have you seen info somewhere to change a F9.9 to a F15 before?
maybe you should enlighten us on how that would be done

[jcshep7896]

Just seen a guy on youtube who had 2005 4 stroke that supposidly upgraded his 9.9 to a 15. Never said i had the answers, Ive never been well off enough to have a lot of what i want. I always have to make do with what I have and try to make the most of it. I often say if it wasnt for others junk they didnt wanna fix or work on I wouldnt have much. I will say this though if you dont feel like helping then just pass it on by, or are you so ate up with your ego you feel the need to run someone asking a simple question in the ground? If so im sorry sir there is much more to life than belittling ppl, I found this out after becoming disabled at the age of 28 and like you i found joy in very little to nothing and only seen the bad. Thankfully I overcame that anger and realized that it only robbed me of any good time i could of had, so in saying that i hope you are able to see you are much more of a man either by not saying anything or simply helping someone who asked for help out. Im sorry but if that offends you please feel free to pm me your reply, i will gladly discuss it with you there.

[jcshep7896]

Thank you for your input! I was merely going by the sound of the engine. But im also new the 4 strokes. i contacted my dealer and he said run a strong mix of seafoam through it and see if it clears up some. Yes i have fresh gas, oil, plugs and filters i need to check... Im pretty certain once i get my transom raised up that it will help with the top end issues........Thank you very much for your help and input

[99yam40]

the 4 stroke is not like the 2 stroke that derated a 15 to make a 9.9.
look at the parts of the 9.9 and the 15 4 strokes to see if you can find any parts the same.

as I said look at the parts breakdowns of the 2 F motors and see what all parts are different.
not many if any are the same.

on the 2 strokes there are very little different and are easy to convert

I do not see any way to change the F motor

too bad you take comments so badly

[cleddau1]

If we were all rich enough to take our motors to the dealers for service and repair there would be nobody on this forum.
Most of us have old or secondhand engines that we try and repair ourselves with the help of others.
I investigated upgrading a 4 stroke 9.9 to 15 some years ago, bewitched by them have the same cubic capacity.
The easiest and most economic way to do it is to sell the 9.9 and buy a 15.
The F9.9 is an excellent engine with plenty of torque, in some ways better than the later, lighter, smaller capacity 9.9 and depending on your application may be well suited to your needs as it is.
Anyway good luck and enjoy your boating......

[panasonic]

Thank you for your input! I was merely going by the sound of the engine. But im also new the 4 strokes. i contacted my dealer and he said run a strong mix of seafoam through it and see if it clears up some. Yes i have fresh gas, oil, plugs and filters i need to check... Im pretty certain once i get my transom raised up that it will help with the top end issues........Thank you very much for your help and input

How much is the engine to low?

[jcshep7896]

i would say its a good 6" too low.