Upgrading a 15hp yamaha 4 stroke outboard

[Xtremefreak]

hey guys

i just bought a yamaha 4 stroke 15hp out board last year and now are looking for some upgrades that might give it a bit more power i am not interested in buying a new motor at all because i only use it 6 weeks a year and i have to carry it my self so a 20hp 4 stroke will be to heavy and 2 strokes are out of the question as they will be banned shortly. but any way i have herd that the only difference between a 10-15-20 yamaha motors is that they just have different throttle limits and that they have smaller or larger orifice in the carburetor so i am just after some clarification on these. is this true? if so can you direct me to a website or store that might be able to do this for me or give me instructions so i can do it by myself

thanks Alec

 

[Doggy]

The different size carbies is about right plus or minus a few other things as well. I had this issue with my old 12ft ski boat. The max size of motor was a 115hp and we only had a 140hp off an old boat we had but physically both motors were the same size and weight. Was just some internals changed so it might very well be the same for you

 

[Jon]

What exactly is the problem?
Is it slow to get up on the plane?
Not enough top speed?
Maybe too much weight or is the boat waterlogged?(fibreglass only though)
Possibly re-propping with a more suitable propeller may fix your problem and avoid any service issues further down the track.
Ring Solas props in Queensland for a solution.

 

[Xtremefreak]

the motor is fine i am just after a bit more power we have put a bigger prop on it and one of those hydrofoils to stop it sucking air down (forget the proper word) we have just herd that the only difference is the throttle and the carby and i was just wondering if people can clarify these things for my because i don't want to go out and and buy some thing and a few days later the engine blows up in my face

 

[3viltoast3r]

Does the carby have air/fuel mistuxe settings? i would investigatee there first

Is the thermo stat running it warm? i believe they are a small valve type thing that screws in and as it heats up it expands letting more water into the cooling system, i suggest you take it out and saw off the valve (did this on a 10hp evinrude from 100years ago) just make sure the part which screws in goes back in. (BTW that was on a 2stroke so it might be different)

perhaps you could try and get the carby of the 20hp monster and put it on yours if the mounts are the same. otherwise just all the usual things to improve the enginee IE good oil/premium fuel/new sparks/keep it cleaned/grease it/etc etc

We have a 15hp Yamaha for Surf Life Saving and on the inflatable boat (hard bottom) it absolutely rockets.

 

[sawtell]

What exactly is the problem?
Is it slow to get up on the plane?
Not enough top speed?
Maybe too much weight or is the boat waterlogged?(fibreglass only though)
Possibly re-propping with a more suitable propeller may fix your problem and avoid any service issues further down the track.
Ring Solas props in Queensland for a solution.

the motor is fine i am just after a bit more power we have put a bigger prop on it and one of those hydrofoils to stop it sucking air down (forget the proper word)

yeah didnt answer his question, whats the problem with the power, do you want to be able to hoon quick from stand still, or go faster top speeds?
Id say since you have put new prop and what not on, you want to hoon from nothing to top speed quickly?

we have just herd that the only difference is the throttle and the carby and i was just wondering if people can clarify these things for my because i don't want to go out and and buy some thing and a few days later the engine blows up in my face

how about contacting manufactureererererere

 

[Xtremefreak]

We have a 15hp Yamaha for Surf Life Saving and on the inflatable boat (hard bottom) it absolutely rockets.

yer that is what we got it is a 8 seater fully inflatable quicksilver rubber ducky from mercury it has a hard deck floor that you can take out and put it back in.



id say just we need it to get it up on the plane quicker

 

[Ty]

yer that is what we got it is a 8 seater fully inflatable quicksilver rubber ducky from mercury it has a hard deck floor that you can take out and put it back in.



id say just we need it to get it up on the plane quicker

If just getting it up on the plane is an issue have you looked at moving some of the weight around the boat, a 20L fuel tank placed up in the bow area can really effect getting on the plane and top speed when you get it there.

and to add some actual fact to the carby rumour.

http://www.yamaha-motor.com/outboard/products/subcatspecs/5/specs.aspx

it appears that the 15hp and the 20hp are indeed the same motor with some tuning differences (same displacement, compression and RPM range).

 

[Jon]

the motor is fine i am just after a bit more power we have put a bigger prop on it and one of those hydrofoils to stop it sucking air down

2 seperate issues here
1. If you put a bigger Diameter? Higher pitch? prop on it already you may have over propped it. Basically smaller pitch ie less angle on the blades = quicker take off and heavy load planing but shorter top end speed as the motor runs out of puff. Think of it as doing a dh course in the granny ring.Higher pitch(more angle) will stop the motor from revving out and ultimately damage it.
Bigger diameter will just overload the motor and end up damaging it.
2.If your prop sucks air from the surface then the motor is too high on the transom. The ventilation plate should be inline or no more than 1" above the bottom of the keel in the normal running position. Any higher and the plate wont work and the prop will suck air .
If neither of these are wrong then you really need a bigger motor.
Give us a side on pic of boat and motor for more accurate set up info.
Hope this helps

EDIT: Set up of motor height is different for inflatables,not to sure on that topic but as Ty said it may be the loading of the boat and make sure the chambers are fully inflated as any bending in the boat will cause the motor to tuck under the transom and push the bow down causing extra drag.

 

[Xtremefreak]

thanks for that guys you have been a big help jon i will post up a pick next week of the boat to show you. i don't want to contact yamaha because they wont tell me much they are probably making a few extra thousand from just changing the carby size and not telling people.

 

[gibbo4]

I'd say put a turbo on it... which is my answer to anything.

Or, watch the Top Gear Episode Series 8 Episode 3.. that should give you a few ideas and you may not have to spent much money on it at all...

 

[Ty]

thanks for that guys you have been a big help jon i will post up a pick next week of the boat to show you. i don't want to contact yamaha because they wont tell me much they are probably making a few extra thousand from just changing the carby size and not telling people.

just tell them you have the 20hp model and need a new carby and chip, it'll still cost you an arm and leg though, i needed just the plastic air intake cover on top of the coweling (not the actual coweling)for my 60hp 4 stroke and they wanted $310 for a 150mm by 150mm bit of plastic, i don't reckon the duct tape looks too bad now.

 

[TonyG]

the motor is fine i am just after a bit more power we have put a bigger prop on it and one of those hydrofoils to stop it sucking air down (forget the proper word) we have just herd that the only difference is the throttle and the carby and i was just wondering if people can clarify these things for my because i don't want to go out and and buy some thing and a few days later the engine blows up in my face

Cavitiate (sp) I think is the word you were thinking of. As for the 2 strokes being banned, are you talking about where you are, or in general? If it’s in general I think you are mistaken as an Evinrude E-TEK motors pass the pollution tests better than most 4 strokes. E-teks are lighter, faster and have lower emissions than Honda, Yamaha and even the Suzuki’s. I’d check this before you go making any modifications to your Yamaha, as that sounds to me like it is destined for disaster.
I fish a lot of impoundments and I know for a fact that on lakes such as Lake Lenthal (just out from Hervey Bay) you can only use E-teks and 4 strokes (and I have a feeling the new Mercury Optimax too).
So my advice before you completely rule that option out would be to check that out first.

Edit – expanding on my comments. I’m not knocking the 4 Strokes. I’m actually looking to get one myself, but it will be on a bigger heavier boat that wont be as burdened with a heavier motor. I’ve spoken to a lot of pro’s on their motors and most generally use the Yamaha or the Suzuki 4’s as they are cheaper to service and maintain, and generally have a longer life. So obviously a lot of factors to consider.

 

[Xtremefreak]

i have asked a guy from yamaha and he said the only difference is a bigger carby and a bigger exhaust but the cost of getting an upgrade for that cost i think $1300 and that is all you need to do. so we are just looking for a blown up 4 stroke yamaha and pinching the pipe and carby off it but i think it might be hard trying to find one as they last for ages the 4 stroke's

cheers any way feller's

 

[demo9pro]

Better off just trading it in on a new 20hp or selling the 15 privately.

I was about to get a 40hp honda, then I found out its the same block as the 50hp and when I enquired about upgrading jets/carby/manifolds etc it was going to cost $1500.

And to get the 50hp instead of the 40hp it was only $800, so I went that way.

 

[adragon]

Change

Jetneedle
Yamaha part #:
6AH-14947-00-00 (for F20)
6AH-14947-10-00 (for F15)

Mainjet
63D-14943-81-00 (#118 for F20)
676-14943-19-00 (# 80 for F15)

 

[duett]

yamaha15 4 stroke outboard

I just put our new used boat and motor in the water and I find the 15 is a little slow to get up on the plane and I would like it to go faster alsne of the letters here says the only difference is a larger needle and main jet in the 20hp.That should not should not be very expensive or difficult to do.Is there any reason why one should not do this in order to gain 5 more horsepower?Thanks for your help .

 

[willsy01]

2 strokes are out of the question as they will be banned shortly

Excuse me?

EDIT: Just realised how old this is.....

 

[maxwolfie]

I just put our new used boat and motor in the water and I find the 15 is a little slow to get up on the plane and I would like it to go faster alsne of the letters here says the only difference is a larger needle and main jet in the 20hp.That should not should not be very expensive or difficult to do.Is there any reason why one should not do this in order to gain 5 more horsepower?Thanks for your help .

First it's ambulance applicants and now Yamaha outboard owners.

Funny how a mountain biking forum comes up as "most relevant" in Google

 

[Spanky_Ham]

+

= FASTER!

Supercharger FTW

s