crazy upgrade idea

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czuniga

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Hello.

Crazy idea so I'm trying to upgrade my 1980 9.9 Johnson to a 15 HP. These is an idea until a bigger hand tiller comes along.. so I called my local marine place to ask for parts and a quote on labor.. guys tells me wecan get a ccarb that shouldn't be hard to find but the exhaust or megaphone might be tough to find..so he suggested we can bore a couple of holes on the existing exhaut as long its above the water line. Has anyone ever heard this before
 
I don't think you need the exhaust horn on a 1980 to make it a 15hp. All you need is the carburetor and the shim that was added in 1979 to the 15's. It goes behind the leaf valves & under the stop plate. It is part number #325038 and it is .0125" thick.

I did this mod on my 87' and actually bought (and still have) the shims not knowing if I needed them but they were already in my 87'. If I were you I would do this mod myself unless you don't mind paying the extra money for a marina to do the work. To get to the exhaust you will have to remove the powerhead. And to be honest when doing mine I didn't see enough difference in the exhausts to actually make a difference. But I put the round 15hp one on anyway since I was already in there and already had bought it.

Oh an this is not a crazy idea by the way. I did it and had a very noticeable improvement. I went from not being able to get my boat on plane to planing at a decent clip. You will notice it to if done right : )
 
Look on ebay for the exhaust horn, I paid $40 bucks for mine off there. And I can tell you if is the correct one if you give me the number off ebay if you aren't sure if it is the correct one. Because in 85-86 they went to prop rated HP so maybe if you really want 15hp (at the prop) you will need this exhaust probably. But I believe in 1980 the 9.9 and 15hp used the same exhaust horn. But I doubt they were really making 10-15 hp back then, hence the changes to the same motor over the years to get a few more ponies : )
 
Where does this shim goes? I will look into it. The crazy ideaq was to drill a hole on the existing ehaust. Without replacing it with a new one. He suggested lets try the carb and if needed lets drill the existing exhaust
 
Ok well if you remove the power head (which is required to get to the exhaust horn) then you may as well put the correct one on instead of drilling it out. The shim's go between the leaf stops and the leaf valves. What it does is allows the leaf valves to open a little further for better air flow.

Here is a link that will help you (about 1/4 of the way down the page)
https://www.sschapterpsa.com/ramblings/Johnson%209.9_15.htm
 
I have been researching and the exhaust is the same for the year of the outboard I have. I just need to change the carb to a 15 HP. And the shims if I wnna go the extra mile. Do I need to remove the powehead to get to these shims. I think I could do it myself. Does anyone have a pic of how to install them. Or a 15 HP carb up for sale. I've been on eBay for a while and they are. Not easy to find
 
Do not drill out the exhaust...will just make the engine louder and drive people crazy.....does nothing for horsepower.
Yes, find the megaphone and you will notice a difference. The change came about around 1981 or 1982 (look it up if you care to) and along with the carb it makes a stout 15hp engine. While there change out the water tube grommets at the top of the water tube as well. These have a nasty habit of collapsing around the water tube and restricting cooling water to the powerhead. They collapse due to overheats and, if around salt water, corrosion.
 
As per my research the megaphone is the same. It did does not change for my engine since its a 1980 so ill be Hunting for a 15 HP carb I thnk its the same carb from 1980 TO 1986 I Believe. Also its recommended toinstall shims. Just have no idea where these are suppose to go. Do I need yo remove the pwerhead for this.
 
Yes, you are correct in your research, however if you want more horsepower than the shims and carb will give you then add the megaphone. It is a direct replacement for the inner exhaust pipe on your older version engine.
The shims are spacers that are placed between the reeds and the reed stops to allow the reeds to open a little farther. You will have to remove the intake and reed boxes and disassemble the reeds from the boxes to accomplish this. Much more seat of the pants horsepower will be gained by going with the megaphone. Make sure you utilize loctite on re-assembly if you do install the shims. If you don't necessarily trust the information coming your way it may do you well to look at the profiles of those who offer information. Some of us have "been there ,done that".
 
FYI your motor is going to become a lot louder when you put the 15hp exhaust horn on. Not to mention your engine RPM will increase by 500-750 rpm.
 
would i need to exchange the horn. I ive read a couple of threads it only give you about .5 of hp increase. Im wondering if for my 1980 the carb is the only difference. ive been looking for some online. i guess ill start with that and give it a try.
 
Okay....lets go over this one more time. Prior to around 1982 there was no exhaust megaphone, only the lowly exhaust pipe. The changes were in the carb and reeds. OMC Engineering took advantage of the tuning in that engine and developed a megaphone or (tuner) to enhance the midrange and top end horsepower of the engine platform. That particular engine was an early one to make it to propshaft horsepower with these new modifications and it was on the plus side of allowable. To make your engine into a later model 15hp you will need to add the tuner or megaphone. The horsepower difference is seat of the pants in some boats in acceleration in particular. Much greater than the .5 horsepower you may have read (was that person in OMC Engineering and have access to the dyno sheets? I doubt it ). To date I have updated at least 6 engines that I can think of with the carb and tuner and all customers have had a nice grin factor. There is no audible difference either with or without the tuner that I have heard and have run tons of these engines over the past 35 plus years. There will be no difference in idle quality with the addition of the tuner and carb. The job is an easy one to do. From here on it is up to you.....
 
Found the round exhaust on a salvage place online. Couldn't find anything on eBay. Guy is suppose to try to find me a carb to go with it. Ill make sureto compare the carbs before I install them. I'm also checking out the shims online. I think ill be aable to tackle it once I get the parts.
 
One more question is there a gasket or something I need to buy to install the exhaust
 
I will humbly disagree with you pappy about there being no audible difference between the 9.9 and 15hp due to the exhaust horn. The 15hp round horn is a few inches shorter and does not seal to the lower unit like the 9.9 does. This causes less of the exhaust to be pushed through the prop which increases the exhaust sound that comes out of the port in the back of the midsection. The 15hp exhaust creates less back pressure this way also, giving more HP. But the noise increased noticeably on my 87' when I did this mod, and I expected it to anyway given the different exhaust. Also your engine is going to run 500-750 RPM higher which also increases noise.

Well worth it though for an extra 5 ponies : )
 
The only increase in noise level I can think of would be coming as a result of the increase in RPM and therefore a pitch change and possible increase in noise level. At higher RPM almost all noise heard is coming from the intake and reed noise so that should increase with additioonal RPM. The idle reliefs in the exhaust housing passed the same Db level no matter the inner exhaust exhaust due to the fact that the exhaust gas has to pass through a wall of water. What can make them louder is to drop the static water level thereby dropping the wall of water. If you started yours on a hose for the first time after modifications or in a shallower tank you may have heard a difference (and that stuck into your memory) but the wall of water designed into it did not care which inner exhaust was in it. Next time you have one completely apart look at the way the water and exhaust flows and you will see what I am talking about. That tuner cannot have an effect on it unless you change the static water level at the same time. I have one apart at the shop and will see if I can get a photo of it for you if you care to take this any farther. Don't know if the photo will show well enough but I will look tomorrow for you.
 
I would like to continue this but I do not want to gum up this gentlemens thread with our debate. I will PM you : )
 

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