converting a 9.9 2stroke yamaha to a 15hp 2stroke yamaha

[boater1234]

Well i went out and bought a motor i just had to have for a steal i think anyway.I bought a new 9.9hp 2stroke yamaha out of the box for $2000.I can't find them anywere around here in fl but one marina,central marine and they wanted over $2800.

I bought it because i am going to convert it to a 15hp yamaha.

Here is the question?I have talked to a yamaha rep and he told me the only difference between the 9.9/15 is the reed valves and restricter plate behind the carb which is only a $75 part and you gain 5hp.Is this true if anyone knows?We went over all the main parts of the motor on the phone,the carb was the same as well as everything else but that one part.He said change that out and it will get the full flow of air to make it a 15hp.It sounds to good to be true but i hope he knows what he was talking about.

He also said it was a piece of cake to do.Has anyone done this before?

This was like a one of a kind deal i may of never found again so i jumped on it.I have a 20hp 4stroke merc for my bigger boat and now a 9.9 for a smaller 1236 jon boat.I would think it's worth the $75 to make it a 15.I will wait till the warranty is up though as it will void it yamaha said.The guy also said you may not see much difference between a 9.9/15 even if i changed it as i find that hard to belive.But he said it can't hurt for the price it costs.Any help on this would be great,thanks.

 

[boater1234]

Re: converting a 9.9 2stroke yamaha to a 15hp 2stroke yamaha

I also wanted to ask a question on the oil/gas ratio.I know most yamaha's say run on 100:1 but you here alot of people also say run it at 50:1.I will be doing mostly wide open throttle with some idleing and very little cruising.If i do decide to stay with the 100:1 ratio will it be ok for sure?Also i will be using it alot so it will never sit for more then a week to 2 at the very most so storage will never be a problem.Should i just maybe go at 75:1 just to be on the safe side or there should be no issues.I know everyone has their opinion but i just want to know if i will have any engine failure do to a lack of oil at the 100:1 ratio.I'm sure if it was a serious problem yamaha would change it but even the marina i bought it at said to go 50:1 as just his suggestion.I mean at 100:1 that would be great as you will use half the normal oil and very little smoke if any as i use full synthetic oil.Maybe also since i am using a full synthetic oil it may also help lubricate it better also for better peace of mind.

Has anyone had a yamaha engine fail on 100:1 here?My thinking is to break it in on 25:1 for the 10hrs then go to a 75:1 ratio to keep it right in the middle.Please someone correct me if i'm doing wrong as i don't want to damage this motor as i plan to keep it for a long time and give it to my daughter in the future for her own little boat.Thanks for any help.

 

[boater1234]

Re: converting a 9.9 2stroke yamaha to a 15hp 2stroke yamaha

Well i guess no help on this one.It's hard to belive no one has ever done a 9.9 to a 15 conversion here.Anyone on this subject?

 

[99yam40]

Re: converting a 9.9 2stroke yamaha to a 15hp 2stroke yamaha

Sorry no one has the answer, but I have always bought the motor I wanted, instead of buying one and the tried to change it.
But good luck with you prodject and I hope you can accomplish what you want without too much problems.
If you do end up finding out what it takes please post it so other will know for sure what is required, but that will be years down the road if you wait for the warranty to expire before you try

 

[boater1234]

Re: converting a 9.9 2stroke yamaha to a 15hp 2stroke yamaha

Well 99yam40 i would of bought the 15hp yamaha but the difference in price was insane $1000 difference.I got the 9.9 at a yamaha dealer that had it for a couple of yrs on the rack so it was only the $2000 and that was the very last 2stroke they had,the 15 was at another marina here in fl and it was over $3000.The place that has the 15's also has the 9.9 yamaha for a crazy price of $2800,i got mine 30 miles away for $1000 cheaper.They are raping people because they know people will pay for it and there is nothing anyone can do about it.I called yamaha and the only difference was the reed vales and restricter plate behind the carb that seperates the 15 and 9.9.I also went and bought a book at my dealer and confirmed that is 100% accurate,the carb as well as everything else on both motors is exactly the same right down the nitty gritty.The restricter plate is the big thing as it restricts the air flow and knocks the hp down to a 9.9.Also you can't get a new service manual off the shelf you have to order it from yamaha for $75.The cost to make it from a 9.9 to a 15 is $75 in parts and either mine or the marina's labor.I will just wait till my warranty is up and then change the reeds and plate myself to make it a 15 as it will void my warranty.I know i will probally never even have to use the warranty but just in case something does happen i will still have it.If i modify the 9.9 to a 15 it will void it according to yamaha.