Outboard carburetor nozzle/jet consumption?

[ryanonthebeach]

The Yamaha 9.9 4 stroke outboard engine looks identical to the 15 except for a few bits and bobs on the carburetor, Diaphragm, nozzle and jet, same bore stroke etc.. If I upgraded it to a 15 would it use more fuel at the same RPM, all other things being equal?
I’m assuming an outboard’s governor varies fuel flow to keep a constant RPM. Are there any other factors related to the jet that would decrease use more fuel?

Because of the size of the boat and the speed it will operate at, the prop that came with the outboard is perfect for 15 rather than 9.9. So it will hold the RPM’s down a bit too much at 9.9, either I’ve got to buy a new prop with less pitch or upgrade the valve to 15.

By my calculations (with the stock propeller) at 3700 RPM I would be using 5HP with around 7.7 HP available from the engine. With the 15 jet would it use the same gas at this speed/HP as with the 9.9 jet?

At the upper end, the stock prop has a bit too much pitch, so will peak early and will hold the RPM’s down at around 4700 in fair conditions which is below the middle of the operating range of 4500-5500, but with 15HP available it’ll be peak at 5450 RPM’s which is close to max of 5500 and will allow a decent margin for rough conditions.
Here are the details of the engine installation prop etc.
Propeller selection boat repower (http://www.psychosnail.com/SailingArticles_BoatRepower.aspx)

[thudpucker]

This thing about changing Carbs and magically you get more HP keeps coming up.
Mostly its a rumor.
You can find out for sure by looking at part numbers.
The rule for Gas engines is 14.2:1, or A lot more air than fuel.
Just changing a fuel supply device is not going to change that. If you have a 15 it will use a lot more fuel than a 9.

[Frosty]

No,-- some engines are de tuned to reduce power such as 100% sure of the 2.5 and the 3.5. I always buy the 2,5 and open the carb slide restricter. The last Merc 2.5 I bought had gone a little more technical by making the slide longer so it could never open fully. This was a bit more technical to detune but is possible , just saw the slide down.

This is restricting air/ fuel supply not 14:1 mixture

Outboards do not have governors.

[thudpucker]

I was going to say that he might have a 15 with a 9.9 Carb and window dressing. I didnt have the eloquence to get that techincal with him.

I do believe in EFI and its ability to deliver the Same HP as a Carb with much better fuel economy because it knows when the engine dont need so much fuel.

[Frosty]

I don't want EFI on a engine that will be out in the rain or dumped in the sea occasionally.

Theres a place for EFI, like on your car,-- it aint on my outboard.

I like the 2.5/3.5 I can put crank bearings in In 1 hour,--done it.

I often have to clean the carb with their tiny jets . Water is like treacle to them. No sir I wont have EFI.

[gonzo]

<many motors just have a stop in the throttle to keep them from producing the HP. Others have different carburators. On outboards with mechanical spark advance it can get a bit tricky to set it up properly. You have to modify the slide too.

[ryanonthebeach]

Thought it was a simple question, guess I was wrong, let me try again.

Assuming this modification works (air and fuel) at a given RPM’s (say 3000, 5hp used) with all else being equal will the 15 use more fuel than the 9.9?

I’ve gone though all the engines parts and all parts save for a handful on the carb are EXACTLY the same for the 9.9 and 15.
Except for a diaphragm assembly which is not on the 9.9 but on the 15 but fits on the carb. Main nozzle, main jet and associated caps and seals (see pics).

The parts that are different on the 15 are:
5-13, 22, 23, 25 34

9.9 Carb
http://www.imageurlhost.com/images/dkbblzexq6ialg1kdz.gif
15 Carb
http://www.imageurlhost.com/images/ki86px6yr22s5ymrs8n6.gif

[Frosty]

It shouldnt use more fuel. the rule of thumb is 5 GPH per 100HP used
for diesel 7 Gph for petrol and 10GPH for 2 strokes.

So if you doing the same speed even with a bigger engine mathematically it should not use more fuel.

Some cars with bigger engines use less than the smaller version.

Has anyone had a 9.9 on a dyno? or a 15. Who knows what they are.

[gonzo]

0n a dyno they are, by law, what they claiim

[gonzo]

The only difference seems to be the size of the jets.

[ryanonthebeach]

Thanks!
Do you know what that diaphragm thingy that's hanging off the bottom on the 15 carb is for?

[Frosty]

I don't think anyone would be too upset if their 9.9 turned out 12HP.

Using bigger jets cant do anything but richen it, less power and more fuel consumption. cant be just that.
The carb body is different and could have a larger venturi.

[PAR]

All engine manufactures use a "base" engine and up grade it for a few different models. How they upgrade is going to vary with each package and manufacture. Some use stops, others jets, others timing, etc.

The easiest way to find out what you have is to look at the engine specs and part numbers.

An example might have a 9.9 with the same stroke and bore of a 15 HP. This is a good indication the 9.9 is the base for the 12 and 15 HP models of this product line. Check the carb rebuild kits. Typically they'll have two or three different high speed jets in the kit and a quick check of the numbers on the jet suggest which is the "big" one, though you can usually see the difference in orifice diameters. Also check the tune up specs, which may show different timing for the larger models.

The difference between the 20 HP Johnson I have and the 25 and 30 HP models of the same base engine, is two jet sizes and 3 degrees of timing. My 20 is now a 30 HP with a new jet and some timing advance. This is commonly done.

[gonzo]

I think that diafragm thing is an enricher or accelerator pump. What does the parts list call it?

[ryanonthebeach]

Gonzo
The part names are as follows:
10 - DIAPHRAGM ASSY 1
9 -COVER, DIAPHRAGM
This is attached to a
34 - CHECK VALVE ASSY

Thanks PAR, I'll take a look into the timing to make sure.

[ondarvr]

Like Frosty said, changing to larger jets does not increase power, the jets are changed because other things have been done to increase power.

Smaller jets can make it run hot, resulting in holed pistons or seizing, larger jets make it run richer and dirtier, resulting in fouled plugs, carbon build up, broken rings and/or pistons.

This is one of the biggest misconceptions about outboards and increasing their HP.

[ryanonthebeach]

Ok We have established that,

There 15's carb must be delivering more AIR AND FUEL. So in addition to the larger jets there is this diaphragm thingy on the larger carb (see pics), could this be what's delivering more air?

Take a look at the pictures, the 15 has these extra parts:
10 - DIAPHRAGM ASSY 1
9 -COVER, DIAPHRAGM
This is attached to a
34 - CHECK VALVE ASSY
15HP Carb
http://www.psychosnail.com/ChartImages/15HPcarb.gif
9.9 Carb
http://www.psychosnail.com/ChartImages/9.9HPcarb.gif

Boat Speed (http://www.psychosnail.com/BoatSpeedCalculator.aspx)

[ondarvr]

No, to deliver more air the carb body needs to be different, the throat needs to be larger. I don't see a number for the carb body, is it listed someplace else?

I only mentioned the jets because its so common.

[ryanonthebeach]

By George, I think you've got it!
that makes sense now, thanks.... there isn't a part number for the body alone, part 1 is the CARBURETOR ASSY, that's the hole damn carb... guess they don't let you just buy the body for a reason.
So I guess it's $218 for the entire carb and i have a 15HP
or $100 for the correct size prop for the 9.9

Anything else I might need to change? as I mentioned all other parts are the same, have gone through all of them in excel. According to the manuals, Also the same is : valve clearance, bore, stroke, displacement, plug gaps

[Frosty]

Ign timing will be different.

[ryanonthebeach]

Thanks guys, for all the advice, much appreciated.
I'm going to order the carb. I'll let you know how it turns out and I'll update the boat speed and propeller selection (http://www.psychosnail.com/SailingArticles_BoatRepower.aspx) article with the results.