Converting an OMC 9.9 to 15 HP :  This question seems to pop up quite frequently and can get controversial from those who have never been associated with the conversion and are only going from what they have heard.   As you can see from this information, it is possible.  The main ingredient is a bigger throat carburetor, which are not currently factory available, and used ones are expensive, if even available, (often over $200), then you can probably expect to rebuild it. 

   Some will tell you that you will also need different reed valves and reed valve stops.  NOT SO.   I bought a 9.9 new in 1978 and still used it until I sold it in 2008.  It is the basis initially for writing this article, as I was trying to convert it to 15 hp, and in those days (45 years ago), there was no, or RELIABLE information available.  I had also bought the repair and parts manuals at the same time I purchased this motor.  From this parts manual, I can assure you that the reed valves for either engine are the same for either motor of that year.   However in 1979 a .0125" shim was introduced that was placed between the leaf valve stop plate and the leaf valve itself for the 15hp motors only, apparently to allow them to breathe better at the higher RPMs it achieves.

    The other main ingredient to convert is a round (tuned exhaust) shown below.

    The Real Difference Between the 9.9 & 15hp :  When they designed this series of motors, they designed it as a 15 hp and then detuned it with a different carburetor for the 9.9.   When the 9.9hp and 15hp came into being in 1974 and up thru 1978, the only real difference between the two motors was the carburetor.  In 1979 they added a shim, part #325038, (one for each cylinder), behind the leaf valves and under the stop plate for the 15 hp, apparently to allow leaf valves to open up more at higher RPM, giving the motor more fuel in the air mixture.  

   The owners manual indicates that the operating range  RPM for the 9.9 to be 4500 - 5500 where for the 15hp it is 5500 - 6500 RPM.   So where the 15hp gets it's power over the 9.9 is that the carburetor allows more fuel/air to increase the RPM by 1,000 which equal the extra power.

   Carburetors :   In the LH bottom photo, you will note that the cam roller pivot arm is white plastic and with an adjustment screw.  This would have been a later unit in that series because the early arms were made of steel with no adjustment.  Some of these carburetors are missing the throttle cam roller and arm unit.

    You will note the far right carb is does not have a choke arm.   These carbs are different than the pre 93s and will not interchange.

	Carburetor, 1974 to 1979 metal fuel bowl	Carburetor, 1980 to early 1987 plastic fuel bowl	Late 1987 to 1992 with plastic top & fuel bowl	1993 to 2004
RH VIEW
LH VIEW

 
   Carburetor :  Do not think that you can up the horsepower by simply modifying a existing 9.9hp carburetor by replacing the main jet with a 15hp main jet.   All you accomplish is that the engine is being
fed too much fuel and not enough air to properly mix with it, the engine will then blubber (floods out) at the top end.   It can run OK on the lower end or idle OK because it is using the idle jet then, but it will have less top end power than it had with the original 9.9hp main-jet.   Believe me, as I have been there/done that.  Matter of fact that is what years ago got me into writing the original of this article so I could document and provide something for me to remember (I get forgetful), as I could not readily find any factual information on the subject, only speculation.  To reiterate the above, a 15 hp main jet can be installed in a standard 9.9hp carburetor BUT THE REASULTS WILL BE MISERABLE.

     Carburetor, Early Metal :   Early production motors had all metal carburetors including the fuel float bowl. Then from about 1980 to late 1987 the same basic carburetor as previously was used, but had a black plastic bottom fuel bowl.  This plastic seemed to be an improvement in that if water had entered in the system and got trapped in the bowel, then sat for a while creating RUST inside the carburetor, even enough rust on the outside if used near saltwater to corrode holes in it.  If you have to replace the metal bowel the cost was $24.00 as of 2005.   I have however seen metal fuel bowls on 1982 to 1987 carburetors, however I am not sure if the carburetor was ever changed or just the bowl.  Or there may have been a supply of metal bowls on hand on the dealer's shelves if the plastics had gotten cracked?

   The older floats are made of varnished cork which can get deteriorated over time, especially now with the Ethanol fuel, and may not function properly, if so replace it with the newer black plastic type.

   If the float bowl gasket on the carburetor is black, it is the older composition cork material used for bowl gaskets, the float will usually be cork also, then you are probably way overdue for a overhaul.  There could be pieces of this cork gasket floating around inside the bowl, occasionally plugging the main jet.  The float should be OK unless the varnish has deteriorated.

  On the early carburetors which used all steel float needles, the little float needle valve clips did not exist then.  They have become necessary because the modern soft tip needles can stick shut.

   You may see float needles used a nylon tapered tip for the actual shut off seal even on the early production 1974 but these may be replaced parts over the lifetime of the motor.

   When ordering carburetor repair kits for these older carburetors, you will find a small white Nylon funnel that is actually a needle bearing.  Somewhere along the line early on, the carburetor was apparently redesigned internally to better support the needle.   So your carburetor may or may not use it, in any case you can not see inside well enough to tell if one is there or not, and if one is, how do you easily remove it without damaging the metal carburetor, so I simply do not use it.  And it is not shown in the illustration below as #21 is the outer rubber needle thread seal. 

   These older metal topped pre 87 carburetors idle jet needle is 2.500" Over All Length, as compared to the later plastic topped needles as shown below in that section. The body of these needles were made of brass while the tapered needle itself is made of Stainless Steel. The straight (front) steel section is .092" diameter and is .300" long. The tapered needle part begins at the rear edge of the straight part but is .063" diameter, tapering a distance of .240" farther back to it's terminal end of .028" with the end being flat. Overall length of this steel section is .530". The overall diameter of the brass threads are .250" with a thread pitch of 28 TPI, with the smaller brass diameter of .156" to where the stainless steel is permanently inserted.

Carburetor, Late :  Late 1987 and newer production carburetors up thru 1992 were completely different (but compatible) having the bottom fuel bowl and top made of heavy black plastic type material.  The choke lever is the same in these newer carburetors even though the motors used the cable twist grip which utilized a totally different throttle system.

   They were apparently designed for easier manufacture as the plastic top has the idle jet in it along with the juncture between this top and the main body serves as a passage (depending on the the gasket) for the fuel from one chamber to the other.   These carburetors were a great improvement over the previous metal topped carbs, being a lot easier to work on.

   The choke butterfly is the same as the previous models, but the throttle plate is smaller.  The internal throat of the 9.9hp is about .500 dia. while the outlet hole into the manifold is .750 dia. (don’t confuse these).  

    If you see a carburetor advertised on e-Bay that does not have the above RH side choke lever, but only a shaft protruding with a roll pin installed, it will be for a 1993 or later motor, as the choke lever is totally different for these later series motors.

   There then was another twist that you may encounter, that is a remote controlled motor that has a electric choke.  On these there is a small solenoid with a wire running up to the choke lever on the carburetor for activation when you push the remote choke button.

    It is my experience however that the 15 hp does not idle down quite as slow or as smooth as the 9.9 hp does.  It should not really make that much difference as at an idle they use a different idle jet than the main jet, except that the 15 has a larger throat size and apparently needs slightly more fuel even at an idle (trolling speed).  Maybe this is just me, and need to do more research.   So if you intend to troll for trout at a SLOW speed, you may want to stay with the 9.9 carburetor, however if you want it for a backup motor and salmon troller (which usually requires a slightly higher troll speed) then consider the 15 hp.  The change to aftermarket Boyeson reed valves may help in this situation however.

   The outside visual dimensions of both carburetors appear the same, unless you look INSIDE the throat from the front.  If you look in from the rear, you will have to position the throttle plate to a horizontal position to be able to see the throat very well.   The 9.9 hp carburetor’s INTERNAL THROAT is .625 dia., 15 hp internal throat is .875 dia. up until late 1987 when they went to a plastic topped carburetor.    NOTE this is not measured from either the from or back but the internal throat that you can see on the LH photo below.   The outlet or rear throat dia. is the same on both (.994") to match the manifold port dia.   The idle jet venturi system is also slightly different between the two.  The part number for the the pre 1987  9.9 hp Carburetor Assembly  is #388936, while the 15 hp Carburetor Assembly is #338273.   This increased throat dia. and larger main jet allowed it to breathe better and to pull more fuel in, increasing the RPM about 1000 RPM, hence more HP. 

    For those of you who think you are a good machinist and can bore out the throat of a 9.9 to make it a 15hp, well NOT REALLY.  The throats are made different, you may be able to take MAYBE ¼ of the needed diameter, but would you get any noticeable gain?   Then the mainjet tube from the float chamber up thru the center of the throat and into the upper section is different on the 15.  And I have NEVER been able to remove these tubes without ruining them (or getting so close to it that I gave up), so without being able to do that, you could not bore it out.   Try it if you want and let me know how it comes out, but be prepared to buy another carburetor as your backup plan.

 
     The photos below show the differences in the carb throats.  In the RH photo, looking in from the front behind the choke, the light blue is the throat size for the 9.9 hp, while the dark blue represents the 15 hp carburetor, represented by this CAD drawing of pre 87 carbs.  The one on the right was snatched off the OMC 9.9/15hp group board on the internet.   Considerably more air flow.   In redneck terms, the 9.9hp will be about the size of a nickle, while the 15hp will be closer to quarter.

   A problem in buying a used carburetor is that the part number may be hard to read as it is only stenciled on.  
And many people, even marine mechanics, do not see enough of these to be really sure which is which unless they see them side by side.   The reason for this is that when these motors were sold new, say in 1978, the 9.9 went for $647.75, while the 15hp was about another $150.   Needless to say way more 9.9's were sold than the 15's by possibly 80%, therefore considerably less used 15's parts are available.

   FYI, here are the specs on the carburetor mainjet/orifice = 9.9hp Part Number 321742 - ID Number 40 - Orifice Size 0.040 inches - 5/16-24 thread size, 15hp Part Number 322293 - ID Number 54 - Orifice Size 0.054 inches - 5/16-24 thread size.  Either one will screw into the other body, but will not function unless the throat size is correct for the HP.  AGAIN BEEN THERE - DONE THAT early on in my learning experience.

   If you do decide to up you HP by changing to a 15hp carburetor, it might be best to try to stay in as close a year range as possible because there were changes in the carb arm cam roller diameter.  As the replacement carburetor you get may have a different size roller, you will have to "link and sync" the carb roller to the timing plate, otherwise you may not achieve the high end performance that you anticipated.  Or swap the carburetor roller arm assembly off your 9.9 to the 15hp carburetor.

   I have not truly tested a before /after on the same boat, same water conditions.  But from information of those that have, the consensus is that it may not be worth the price or effort to change the exhaust tube when upgrading on the older motors, if trying to up the hp to the full blown 15hp, UNLESS you can find a used 15 hp tuned exhaust tube.  Remember that the older (pre 1981) motors still used the same 9.9hp square exhaust housing on both motors.    I have compared the early 9.9hp exhaust tube with the later and 15hp tube.  The mounting flange where it bolts to the powerhead is slightly different, but it appears that the tuned (round) 15hp can be installed and would fit.   But just by changing the housing itself, you will not show many benefits.  Tuned exhausts are well known in racing two stroke motorcycles and are very effective at specific RPMs but they need to be matched to the carburetor also.  

    Also if your boat is too large, heavy, or you and your fishing buddy plus gear are overloading the boat, your conversion benefits will most likely not show.  However if you have a lighter boat and it just needs a nudge to get it onto a plane, then possibly going to the 15hp would help.  HOWEVER loading the boat (repositioning your fuel tank and battery farther forward) or re-proping, may be just as important, along with possibly adding trim fins on the motor’s anti-cavitation plate to gain you that extra bit of jumping it up onto a plane.  Once it is on a plane, any extra speed will be minimal and only proportional to the RPM increase that they say the 15hp has.

    I will not go into exact part numbers for this conversion, as depending on your year of motor, a simple conversion (to factory 15hp AT THAT TIME) could mean just a carb on say a 1977.  But if you wanted to wring every last bit out of it, you could also change the exhaust tube (3 types), reed valve stop shims and possibly the prop. Then if you had a 1990  9.9hp which is basically a detuned 15hp, then just a carb may do it for you.

   In changing carbs, the older metal topped carb was used from 1974 until early 1987 when the plastic topped carbs came into being.   If I was going to attempt this conversion, I would try to stay within the same range of carbs.  HOWEVER it would be entirely possible to use an older metal topped carb (more availability, and more trouble free) on the later motors (87-92).

   The larger throated carburetor will gain the most in increased RPM, (about 1000 RPM more, according to OMC) which in this case also relates to more horsepower.  If you insist on squeezing that last ounce of energy, then maybe the aftermarket reed valves, polishing and deburring the intake manifold, replacing the exhaust housing will be of some benefit.  But, if you are trying to push it this far, at wide open throttle for an extended period of time, my thoughts are that then the rest of the engine may not stand up for long, so you might just consider going to a larger motor, say a 20hp.

    Props are listed the same for both the 9.9hp and 15hp motors, however depending on the weight of your boat, it may be advantageous to try a lesser pitch version.    The 1991 OMC parts manual lists the following aluminum 3 blade props  --   9" X 10",  9"X 11",   9 1/4" X 8",  9 1/2" X 10",  10" X 5",  10" X 7", & a stainless in 9 3/8" x 9".   The most common prop would be the 9" X 10", which would probably be fine for say a lightweight aluminum14’ or 16’ boat, with it be minimally loaded.   For a heavier boat a larger diameter, and lessor pitched prop could help.

   Also, as mentioned, the inner round exhaust tube was added after about 1981 for the 15hp motors.  This exhaust tube on the 15hp has a circular cross-section and gets wider at the bottom, (like a long tapered funnel) called a TUNED EXHAUST as opposed to the narrow, rectangular shape on the 9.9hp.  This was probably copied somewhat off the tuned 2 stroke racing motorcycle exhausts of that era.   Part of the principle was that the burned gasses also carried noise waves from the motor, if these waves created a backpressure where the actual exhaust gasses put noise pressure at the intake ports at the programmed RPM to block fuel leakage at the same time having a more efficient exhaust so it could escape more easily.  When a motor breathes better, this equals more horsepower.  This is a rather complicated system using sound waves.

   Then in about 1987 another exhaust tube was brought out, this one is a short rounded unit and is used for both the 9.9hp and the 15hp motors.  My reasoning for the 87 date is that the 86 parts list still shows the old housings, where the 1990 list has the newer one.  However I have a 1989 that has the longer tube.  This motor underwent many changes in 1987 as to thermostat location, fuel pump, twist grip throttle, etc. where they actually produced a full 15hp but they detuned it for the 9.9hp, as compared to the other way around up to then.

   On the later motors from late 1987 on thru the 1992 versions, when they changed carburetors, the inner exhaust tube is quite a bit shorter than the early ones and this same part is identical for both the 9.9hp and the 15hp versions.  Why, I am not sure at this point in time, unless it simply saves money on parts inventory and they found they gained enough by the other minor changes.

   So, In Summary of Converting to 15 HP :  If I have overloaded you with information on the differences between the 9.9 and 15hp, sorry.   I guess it is just how far you want to go in changing parts to do a partial or a total conversion.  Any changes will probably make a difference, (whether measurable to us commoners or not) but to do the full blown conversion, ALL will need to be utilized.  However depending on the year of motor you have and your needs, a full blown total conversion may not be cost effective.  In this day and age, 40 some years later, it may be best to try to find a non-running 15hp made after 1987 and purchase it for parts.   However you need to realize that 15hp motors in this series were not anywhere near as plentiful as the cheaper 9.9hp (at that time).

    I have not listed part nomenclature and number for each change that they did over the 18 years of production of this series because of the various changes and some could have been superseded by other changes.  If you are that interested you will have to go back and re-read what I have posted here, maybe many times.  And then finding the correct part may be another issue.  I personally would have to really think a pretty about doing it to say a 1974 -1976 motor, whereas if it was a 1988 +, that may be a more worthwhile project.

    A reader recently made the conversion on a 1976 9.9hp Johnson using the carburetor off a 1980 15hp Evinrude.  "The carburetor change upgrade results were fantastic.   21 mph GPS top speed with 9.9hp went to 27mph with the 15hp conversion on the same boat". 

The reed valve shim and the 15hp exhaust will add a bit, but are not necessary to make a marked improvement to an old 15.    Do not get confused with people saying the intakes are different, or the head is different or the ignition is different ......... THEY ARE NOT.   90% is all CARB".

   Another testimony taken from the i-Boats forum, "I had a 9.9 Johnson 2 stroke on a 16' Grumman V hull, it would start to get on plane but that's about it, it needed just a bit more power.  This was in a bare 4 bench seat hull with two 250 lb men aboard and a full 6 gallon tank.  I eventually converted it to a 15hp by changing the carburetor and exhaust tube, that did the trick but, it still only did about 18 mph".

   So take your pick.  What they are not being informative about is the boat (the hull design could be critical) and how much gear they have aboard.

Change in HP Ratings :  The Japanese imports were rated at HP at the prop, while the US motors were rated at the crankshaft, which related to about 10-12% loss in the official power.   This created comparison issues for the US manufacturers.  In 1981 the marine industry started to move from powerhead rating, to a prop rating.  OMC did this in 1984 with the industry moving this direction in1985 was the first year all outboards had to meet the new prop-shaft specs.   OMC saw this coming and as explained below, phased theirs in over a few years.   Apparently the 9.9s were close to meeting this requirement, but not the 15s, as the 9.9s were actually a detuned 15.  And as explained below, OMC finally just added all the improvements to both motors apparently as a matter of simplifying production.

   In 1981 there was a change made in the inner exhaust tubes for the 15 hp OMCs only.  It was a sort of tuned round exhaust housing, allowing the engine to again breathe better at high RPM, which  in addition to the larger throated carburetor, and the leaf valve shim gained about another 10%  in overall HP to put these OMC engines on par with the competitive Japanese imports.  The 9.9 hp square inner exhaust housing with no shim for the leaf valve, stayed the same as it was.  A guess here is that they needed to use up the older parts and at the same time have a noticeable difference between the two HP motors.  The big difference in the performance between the tuned exhaust 9.9 and 15 hp will be in the last 1/3 of the throttle range where the increased fuel/air flow of the carburetor will be readily apparent with the increase in RPM. 

   In the photos below, the early 9.9 hp exhaust tube are on the left, with the round tuned 15 hp on the right.  The copper tube on the tuned one is so the engine will disperse the exhaust water more down than as compared to the older 74-76 motors that needed the water exhausting higher to be able to exit the blubber hole before the overboard water indicator (pee hole) was used.

   Later motors from the factory, at about 1990 the 9.9hp uses the 15hp exhaust housing.  The guess was economy in using the same parts, and it was better to over rate than to under rate their motors. 

   For an explanation of possible water tube grommets deteriorating where they attach to these exhaust tubes, possibly causing water blockage CLICK HERE for a link to my water circulation problems article. 

   You will note that there were 3 different ONGOING 15 hp modifications.   (1) The 15 hp from 1974 to early 1987, with just the different metal topped carburetor.  (2) The added leaf valve shim in 1979.  (3) And the added tuned exhaust system in 1981, which also included the 2 previous modifications.  The 1979 parts manual lists a shim, part #325038 to go between the leaf valve and the stop for the 15 hp only.  This shim sells for $2.94 each.  It was carried on up into 1992 for the 15 hp, which apparently allows the 15 hp to breathe a little better.   These leaf valves are the same for all motors they are used on.

   To reiterate, when doing a conversion from 9.9 to 15 hp, yes you could just change the carburetor, and depending on the year of motor you may have, it may equal what the factory did for that year.  But if you want to do the whole thing using information they developed over the life of these motors, you may also want to add the reed valve shims AND the later round tuned exhaust that was developed for the 15hp.  Obviously if you are converting a late motor, say an 1987 to 1992, the reed shims and round exhaust should already be on your motor.

   If you are decided to install the aftermarket Boyeson reeds in these motors, you DO NOT use the 15 hp or later 9.9 spacers because the Boyeson reeds are a dual reed system with a primary and secondary reed that stack on top of each other.  The primary reed has a hole in the center.   And they DO NOT come with new reed stops for these motors, so you need to use the existing factory ones.
  

  Converting the 1993 - 2007 :   Here, these later versions would only need to change the carb, (NOTE – THE MOTOR AND CARB ARE TOTALLY DIFFERENT than the pre-93s) as these are new enough to have exhaust tubes all the same, (they just detuned a full-fledged 15 to a 9.9) at the time of manufacture.

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Originally stated 03-01--2025, Last Updated 03-22-2025
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