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Old 06-04-2009, 09:13   #1
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DIY Basic Outboard Repairs

Hope to add a thread on doing basic repairs on our dinghy outboards while in the field and/or have little or no access to an expert. The first topic here is a fishing attempt for the advice of more experienced folks on the idle speed of a 15hp 2-stroke Johnson. Though it's a year 2000, it's been stored and is just getting over its break-in period, so it's like new.
Because of its age (and who knows how long fuel was sitting in the carb), I changed all the gaskets in the fuel pump and cleaned/rebuilt the carb. It's getting clean fuel and spark seems to be fine. It just doesn't want to run at idle speed and just stalls out. Is great at higher speeds. I don't have the experience to really sniff out the problem and am not sure which way to fiddle with the slow speed idle mixture screw. If anybody has any like experience with these older Johnsons, would LOVE to hear about it. Hope to save a trip to the mechanic- I'm so cheap these days!
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Old 06-04-2009, 09:38   #2
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Most probably a clogged idle jet. Remove, soak, blow out with compressed air. Compressed air is needed. The idle screw is part of this pathway, so when you remove it, first tighten it counting the turns. When you replace it you bottom it out (ever so gently) and then back it out that number. That does not need to be adjusted very often if ever.

If you cleaned the carb properly you had to remove the idle screw also. If you did not that is why I name the idle circuit as the most probable cause. Don't ask me how I know..............OK, I'll tell you anyway................ I cleaned a carb three times - it wasn't till the third time that I used lots of "sea foam", syringes to inject through the orifices and compressed air to get to "WHALA!!"



Next is leak from carb to intake manifold.
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Old 06-04-2009, 10:17   #3
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I agree with Therapy that it is probably the low speed jet and the mixture is probably lean. Since you have indicated that you rebuilt the carb, I would first assume that the mixture is improperly adjusted before resorting to cleaning it again. Therapy's proceedure for cleaning it is essentially correct, but if you have not adjusted the mixture properly counting the turns cleaning it and opening it the same number of turns will simply put you back where you are now. As suggested close the needle valve very gently (turning clockwise) and note the number of turns. Then open the valve 1.5 turns. Most outboards will at least run on this setting. If the engine runs smoothly for a few seconds and the quits or quits when you put it in gear it is most likely lean. In this case continue to turn the screw counter clockwise 1/8 of a turn at a time until it idles smoothly both in and out of gear. If the engine runs but spits and sputters and is generally rough it is most likely rich. Turn the screw clockwise until it runs smoothly, again 1/8 of a turn at a time. If this doesn't do it then I would try the cleaning procedure suggested by therapy. If that doesn't work you most likely have a small air leak around the Carb manifold gasket. A thin coating of silicone RTV sealant on that gasket should take care of any air leaks, but make it very thin. You don't want it sqeezing out into the manifold when you tighten down the carb.
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Old 06-04-2009, 10:20   #4
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Captain Bill,

Agreed.

What we don't know is if the idle jet screw was moved/removed etc during the clean.

Important info.
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Old 06-04-2009, 14:32   #5
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The latest UK Practical Boat Owner has an article on serviceing a small outboard - you might find that of some use.
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Old 06-04-2009, 14:38   #6
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Here's a link to site I have always found handy.

Johnson Outboard Motor Repair
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Old 06-04-2009, 15:39   #7
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and if that don't work try the fuel pump diaphragm. it has prob gotten weak form sitting so long with fuel in it (pump). had same prob with a 6hp Evinrude that sat for 12 yrs. rebuilt carb i think 4 times flushed all the same as said above. put new pump kit on motor and it ran so slowly you could watch the prop turn so slowly the boat would not move. you could put in gear out what ever and it just kept on purring. ran like that for about 8 yrs using it about 6 hrs aday 4-5 days a waeek before i had to put a new pump in again.
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Old 06-04-2009, 15:51   #8
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If it was the fuel pump how would it run at higher speeds?
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Old 09-04-2009, 08:14   #9
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Just rebuilt Yamaha 9.9 carb last night - idle screw and the needle. 4.75 turns!

Quote:
Originally Posted by Captain Bill View Post
I agree with Therapy that it is probably the low speed jet and the mixture is probably lean. Since you have indicated that you rebuilt the carb, I would first assume that the mixture is improperly adjusted before resorting to cleaning it again. Therapy's proceedure for cleaning it is essentially correct, but if you have not adjusted the mixture properly counting the turns cleaning it and opening it the same number of turns will simply put you back where you are now. As suggested close the needle valve very gently (turning clockwise) and note the number of turns. Then open the valve 1.5 turns. Most outboards will at least run on this setting. If the engine runs smoothly for a few seconds and the quits or quits when you put it in gear it is most likely lean. In this case continue to turn the screw counter clockwise 1/8 of a turn at a time until it idles smoothly both in and out of gear. If the engine runs but spits and sputters and is generally rough it is most likely rich. Turn the screw clockwise until it runs smoothly, again 1/8 of a turn at a time. If this doesn't do it then I would try the cleaning procedure suggested by therapy. If that doesn't work you most likely have a small air leak around the Carb manifold gasket. A thin coating of silicone RTV sealant on that gasket should take care of any air leaks, but make it very thin. You don't want it sqeezing out into the manifold when you tighten down the carb.
Yes, over 4.5 turns. I have twin engines, and they are both that far out. Go to 4 turns, she dies. But checking before disassymbly is key - glad I did!

The carb bowl was bone dry and the needle was stuck, so that was obvious. I had already tested the fuel pump pressure and volume.
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Old 09-04-2009, 13:07   #10
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Therapy! if the fuel pump is the same as the one on my Evinrude then the reason is that even when a diaphragm is weak at pumping enough fuel at low speed as the rpms increase and the exhaust pulse increases. this then causes the pump diaphragm which is (weak/stretched) to pull fuel more effectively because the exhaust pressure increases to a more even pulse flow against the diaphragm which then will compensate for the lack of pressure at low rpm. you would have to see a used/old diaphragm verses a new one to see the difference. the old will look like a baggy old stretched sail verses a new sail.
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Old 09-04-2009, 14:17   #11
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Therapy! if the fuel pump is the same as the one on my Evinrude then the reason is that even when a diaphragm is weak at pumping enough fuel at low speed as the rpms increase and the exhaust pulse increases. this then causes the pump diaphragm which is (weak/stretched) to pull fuel more effectively because the exhaust pressure increases to a more even pulse flow against the diaphragm which then will compensate for the lack of pressure at low rpm. you would have to see a used/old diaphragm verses a new one to see the difference. the old will look like a baggy old stretched sail verses a new sail.

OK.

Thanks for that explanation. I always like to learn here from someone that knows.
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Old 16-04-2009, 07:45   #12
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Hey, this is great! Like to see all this handy input. One of the things I do on the side is repair Yamahas and was new to this Johnson. Am very embarrassed to have missed this, but the idle problem was simply too low RPM thanks to one lone screw in the throttle linkage not being where it should've been. The Johnny has such a different set-up than the Yamahas that I totally missed that simple problem. (duh!)
Once that screw was adjusted, I had to fiddle with the low-speed idle screw (for small 15hp, start at 3 turns out) to get it just right. Not too lean and not too rich... Now the little outboard acts like a champ.
Cleaning a carb three times to get it right is more common than the "experts" want to admit. Glad I'm not alone in that. One little Suzuki outboard I had to repair for someone had rust in the carb(!) and that took alot of special attention. Fuel pump diapragms will stretch after a while and motor will notice poor performance at higher speeds- a handy deal is to check pump with a v-pressure gauge and see if it meets the manufacturer's values.
Rebecca now-can-idle in Florida
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Old 16-04-2009, 10:51   #13
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OUTBOARD MOTORS LEARNER’S GUIDE
http://www.spc.int/coastfish/Section...oard_lg_en.pdf

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Old 17-04-2009, 07:53   #14
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Am very embarrassed to have missed this, but the idle problem was simply too low RPM thanks to one lone screw in the throttle linkage not being where it should've been.
Cleaning a carb three times to get it right is more common than the "experts" want to admit. Glad I'm not alone in that.
Rebecca now-can-idle in Florida
LOL,

Great to hear the result.
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