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Old 26-06-2015, 05:44   #1
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Older Johnson 15 2-stroke

So, I have this venerable engine - old enough that it qualifies for Medicare.

Not really - but I don't have the number in my head or hand. I =think= it's a 1989. And the serial number IS that of a 15.

But when the carb has been off, I note that the hole in the block is bigger than the hole in the carb.

That is, the bottom plate of the carb has this discolored circle on it, around the intake (well, exit of the carb).

Which makes me wonder if someone couldn't find a replacement 15HP carb during a previous repair/rebuild/something job, and, knowing that the engines are identical save carb and cover, used a 9.9 carb.

No place I've looked on line tells me the hole sizes.

Does anyone know: does the bottom of a 15HP carb plate match the intake to the block, or is the hole smaller for some reason?

As I'd love the extra horses - and MAYBE it would be easier to cold start - but a new carb is big bux, and not easy to find, I'd love to know the answer for sure before going further.

Thanks for any assistance.

L8R

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Old 26-06-2015, 06:26   #2
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Re: Older Johnson 15 2-stroke

... Late 1987 and newer production carburetors up thru 1992 ...
... Apparently the bodies used the same castings but had different throat size for different HP motors from 5 to 20hp, but the covers were interchangeable ...
Johnson_carb
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Old 26-06-2015, 07:00   #3
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Re: Older Johnson 15 2-stroke

THANKS,Gord,

From the fuel pump pix, I have a 74-87 model.

From your quote, I still don't know - and in the mad dash to work some more on the boat, the admiral feeling that every second spent in front of a computer shortening your life by a day or so, I haven't found a corresponding quote regarding my machine.

There's so much info there that I'm not surprised that the mech I took it to my first time to the Bahamas was pretty uncertain about the issues involved - but had the right rebuild kit in stock.

I also had to replace the plastic bowl, which had cracked, at a different joint in the Bahamas.

The engine runs well, but I suspect not as powerful as it should be, as other 15s spank it pretty badly - and it's a bugger to start, requiring 6 very hard (I'm a big guy) yanks. When it's warm, it goes right off. But the admiral can't start it otherwise, limiting our travels, where she'd be using it, to our 6HP, otherwise used for the PortaBote.

Thanks for the article. I'm sure I've seen it before, but the throttle starting position info might be all I need to remedy the hard start (I have been putting it all the way open, choked all the way; it takes some time to get warm enough to push in the choke)...

L8R

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Old 26-06-2015, 07:22   #4
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Re: Older Johnson 15 2-stroke

The inner throat diameter is like 5/8" on the 9.9 and 7/8" on the 15. The diameter on the mounting flange is the same.
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Old 26-06-2015, 07:27   #5
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Re: Older Johnson 15 2-stroke

Make sure you pump the primer bulb on a cold start. With full choke and hard bulb should start on first pull unless compression is low.
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Old 26-06-2015, 07:38   #6
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Re: Older Johnson 15 2-stroke

Thanks, BG.


Compression was checked when my water pump went out and it bound up. Still good - the mech characterized these as bulletproof.

I didn't measure the diameter, but now that I know it's different, if I take it off again some time, I'll check.

Thanks for those tidbits. I'll try harder on the bulb to see if there's any difference.

Do you know about the start 'setting' on the throttle? Can there be too much, when the throttle already can't advance very far with the gear in neutral?

L8R

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Old 26-06-2015, 08:36   #7
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Re: Older Johnson 15 2-stroke

When was the last time you had the carb rebuilt or at least cleaned?
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Old 26-06-2015, 09:43   #8
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Re: Older Johnson 15 2-stroke

Also have Medicare eligible Johnson. Love it BUT I do have to use Gumout sprayed into carb before yearly start up. Also find the little BB ball in the fuel line at the engine end gets stuck with some frequency Otherwise No probs It is a keeper. Russ
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Old 26-06-2015, 10:09   #9
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Re: Older Johnson 15 2-stroke

Skip,apparently,after 79, OMC used the word 'INTRODUCES' as their manufacturing code,where I=1, N=2,T=3,R=4 etc. The last 3 letters of the model code are the year built and revision code. So, J10SELCTC would indicate, Johnson 10hp Sailmaster Electricstart Longshaft 83 revisionC. I came across this when trying to date my 'geriatric johnson 'It may help you in identifying things.
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Old 26-06-2015, 10:46   #10
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Re: Older Johnson 15 2-stroke

So by the sounds of things you haven't yet cleaned/rebuilt the carb... Needs to be done. I 'm in the process of rehabilitating a 1986 8hp johnson. when I got it , it wouldnt start and you couldnt shift into reverse. now it starts on the first pull (but still needs some tuning, I forgot to clean out the idle jet)

the carb kit ( gaskets, needle, seat, etc.) should be around $20-25
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Old 26-06-2015, 10:59   #11
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Re: Older Johnson 15 2-stroke

Check the plugs.

Clean the carb.

Check the kill switch if you have one.

Those are the problems I've had in the past 35 years.

Good luck.
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Old 26-06-2015, 16:14   #12
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Re: Older Johnson 15 2-stroke

Quite right about cleaning. Last rebuild was before our refit. Next to no hours on it since, and I had it serviced (including tuned up) before we launched in 2013. Minimal hours on it since then.

But there were no quantum leaps in performance after either the rebuild or tuneup.

That doesn't mean I shouldn't clean the carb

L8R

Skip, who read the code to get the year of the engine long ago, but isn't on the boat to look it up...
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Old 26-06-2015, 20:09   #13
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Re: Older Johnson 15 2-stroke

Generally you need very little throttle when cold starting & using full choke. I would would recommend idle or a little above as this seems to work best in dragging the fuel in to create the rich mixture you are after. Hot starting an old 2 stroke I go for full throttle. Try it & see. Hope it helps
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Old 27-06-2015, 05:47   #14
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Re: Older Johnson 15 2-stroke

Quote:
Originally Posted by Compass790 View Post
Generally you need very little throttle when cold starting & using full choke. I would would recommend idle or a little above as this seems to work best in dragging the fuel in to create the rich mixture you are after. Hot starting an old 2 stroke I go for full throttle. Try it & see. Hope it helps
Hm. Counter-intuitive for making it run; I'll give it a try. This one actually idles better than the 6; I have to shut it off after I do my hard-turn acceleration to the dock/down to nothing throttle routine which has my dink sidle up to the dock full-on sideways, whereas the 6 dies on full-stop idle after the goose.

Thanks for the thought!

L8R

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Old 27-06-2015, 06:40   #15
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Re: Older Johnson 15 2-stroke

I have a 1992 15 horse. With the throttle at idle it will start in three pUlls when cold
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