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Old 22-05-2019, 03:47   #1
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Yamaha 15hp 2 stroke bogging

I have an older Yamaha 15hp 2 stroke which bogs down at anything over 1/3 throttle. I have replaced spark plugs, rebuilt the carb and fuel pump (new float, jets, seals, needle, etc). Filters and hoses are clean, have run thru various typed of petrol and played with the oil mix. No change. Here’s the clue: I can adjust the mix screw from all the way in to all the way out in 1/4 turns and it makes zero difference on how the engine runs. So what does that mean?
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Old 22-05-2019, 04:52   #2
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Re: Yamaha 15hp 2 stroke bogging

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Originally Posted by camerondueck View Post
I have an older Yamaha 15hp 2 stroke which bogs down at anything over 1/3 throttle.
Please describe "bogs down".

As you throttle up slowly, what happens? Does it just refuse to go faster? Does it seem to sputter, gasp or choke past a certain setting? Does it seem like it's flooding? Is there a difference between throttling up fast or slow?

Does the choke setting change anything? Tried a different tank & hose?

Is it easy or hard to start? Will it run at idle or below 1/3 indefinitely without stalling?

I'm not an expert, but I've managed to keep our 2-stroke happy for 12 years. My very rough guesses are clogged jet, bad fuel pump or some other restriction in the fuel line.
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Old 22-05-2019, 05:08   #3
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Re: Yamaha 15hp 2 stroke bogging

I have an old 20 hp and when I had this problem it was what I called spark advance( my own term). Around the fly wheel is an arm that has to advance as you increase throttle. Mine needed wd40 and lots of wriggling until it moved freely as I opened up the throttle . That solved the problem for me.
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Old 22-05-2019, 05:11   #4
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Re: Yamaha 15hp 2 stroke bogging

As I twist the throttle it does not rev up after the first 1/3, then it just sputters, but keeps running. Every now and again -- rarely -- it does rev up, and takes off, and the dinghy hits planing speed. So it appears to be a fuel issue, but I can't find the blockage. I have opened the tank, checked the filter, it is clean. I have tried pumping the bulb on the hose while running it, no difference. It starts easily, idles well. I have opened the carb four times now to clean it with carb cleaner, as well as replace the jets, etc. The fuel filter on the engine is also clean. The fact that adjusting the mix screw has no effect seems important to me, but I don't know what it says...
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Old 22-05-2019, 05:12   #5
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Re: Yamaha 15hp 2 stroke bogging

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Originally Posted by xbillaroo View Post
I have an old 20 hp and when I had this problem it was what I called spark advance( my own term). Around the fly wheel is an arm that has to advance as you increase throttle. Mine needed wd40 and lots of wriggling until it moved freely as I opened up the throttle . That solved the problem for me.

Interesting, I'll take a look at this. Thanks
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Old 22-05-2019, 05:28   #6
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Re: Yamaha 15hp 2 stroke bogging

Probably something still in the high speed jet. Also, how old is the fuel line and bulb? I just yesterday worked on a two stroke motor, and a fuel line became almost like chewing gum, probably due to the ethanol in the gas. I let it sit all winter by mistake.
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Old 22-05-2019, 08:18   #7
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Re: Yamaha 15hp 2 stroke bogging

You are much more advanced than me. My similar problem with Yamaha 2-stroke 25hp was solved by adding SeaFoam to the fuel, which i now do to every tank.
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Old 22-05-2019, 08:28   #8
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Re: Yamaha 15hp 2 stroke bogging

Remote tank air vent open?
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Old 22-05-2019, 08:32   #9
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Re: Yamaha 15hp 2 stroke bogging

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Remote tank air vent open?

yes it is
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Old 22-05-2019, 08:34   #10
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Re: Yamaha 15hp 2 stroke bogging

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Probably something still in the high speed jet. Also, how old is the fuel line and bulb? I just yesterday worked on a two stroke motor, and a fuel line became almost like chewing gum, probably due to the ethanol in the gas. I let it sit all winter by mistake.

the jets are clean, see light through the holes and i can spray carb cleaner through all the passages
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Old 22-05-2019, 08:55   #11
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Re: Yamaha 15hp 2 stroke bogging

Try the O ring and clamps in the tank and engine connections, doesn't take much to tear or gouge an old one when putting on and can cause lack of fuel at higher rpm and hard starting due to fuel drain back. Sounds like you've tried most everything else.
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Old 22-05-2019, 09:11   #12
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Re: Yamaha 15hp 2 stroke bogging

Does it rev up to WOT with no load?
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Old 22-05-2019, 09:24   #13
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Re: Yamaha 15hp 2 stroke bogging

Try running a tank or 2 of best premium gas with AMSoil 2 stroke at 50 to 1 ratio. Worked for me a DREAM
Bill ...no connection to AMSoil





Quote:
Originally Posted by camerondueck View Post
I have an older Yamaha 15hp 2 stroke which bogs down at anything over 1/3 throttle. I have replaced spark plugs, rebuilt the carb and fuel pump (new float, jets, seals, needle, etc). Filters and hoses are clean, have run thru various typed of petrol and played with the oil mix. No change. Here’s the clue: I can adjust the mix screw from all the way in to all the way out in 1/4 turns and it makes zero difference on how the engine runs. So what does that mean?
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Old 22-05-2019, 10:38   #14
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Yamaha 15hp 2 stroke bogging

Quote:
Originally Posted by camerondueck View Post
The fact that adjusting the mix screw has no effect seems important to me, but I don't know what it says...

Nothing, because it’s an idle mixture only, as soon as you begin opening the throttle you come off the idle jet and onto the high speed jet, which has no air bleed screw.
As a general rule you “tune” a two stroke to run as lean as possible at idle so it doesn’t foul plugs, but as rich as it will run smoothly in high speed so it doesn’t lean out, get hot and seize.

I’m betting you have either a spark advance problem or maybe a bad fuel pump, or something else blocking fuel flow. It may be time for a new carburetor, if I had a a carbureted outboard, I’d want a spare anyway as problematic as they seem to be now.
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Old 22-05-2019, 11:10   #15
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Re: Yamaha 15hp 2 stroke bogging

Like RBK suggested, check for an AIR leak around any/all of the fuel connections. Current fuel connectors are very suspect after even one season. We used to get many years out of them but I am finding the seals cracked to be one of the most common problems these days, even when non alcohol fuel is used. The high speed cutout is the typical symptom.

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