Yamaha 9.9 stalls at idle and low throttle - please advise

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RomanDesign

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Last season I had a problem with my 1989 Yamaha 9.9 4-stroke I bought with my Mac 26D 3 years ago. It became very unstable at idle and low throttle - sputtering and stalling unless I give it a little gas. Sometimes after starting and running for a while, then sailing, it would not want to start again, requiring a few tries and extra throttle until it started. It's used only a few hours in a season. It was fully serviced before I bought it and insides look like new for the most part. I replaced the impeller when I bought it. Yesterday when I got it out of my basement where I stored it for the winter, I changed oil in upper and lower units, changed spark plugs, that didn't help.

I bought carb cleaner spray and tried to clean carburetor that way - so the short version of my question is: is it good enough based on how it's running in the second video below (around 9 minute mark), or should I disassemble and attempt to clean the carburetor manually?

Please look at the video: you can see uneven detonation - it is shuddering neurotically, then after I put it to forward gear it becomes unstable and eventually stalls. This is before I sprayed any carb cleaner:

I bought carb cleaner spray and tried to clean carburetor that way - see video below, it was difficult not to let the engine stall and I could not do it continuously like instruction said, or the engine would stall for sure. I did it several times, using about half or more of the can. Look towards the end of the video (around 9 minute mark) - I still see some uneven shuddering but I let it run on lowest throttle with forward gear engaged for 10 minutes and it didn't stall. So it looks a it's better now, but I'm still not 100% sure if it's completely fine. Still vibrates kind of unevenly, but didn't stall at least.


So my question is: does it look normal to you (as of the end of the second video)? Or should I do something else? Should I try to adjust idle with the screw? Disassemble carburetor and manually clean it? Do anything else? Or is it OK as is?

My worry is that if I adjust idle to higher throttle - it would put a larger load on gears when switching to forward or reverse from neutral. Also when hoisting sail or reefing under motor or docking I want as slow speed as possible, so increasing lowest throttle is not desirable. But I'm worried if while docking or maneuvering in tight situations the motor quits on me at the worst moment - I'm gonna be in trouble.
 
If you are not using fresh fuel change it.

If the carb needs cleaning then its the fuel passages and jets which will need cleaning which means a disassembly, clean and rebuilding. Spraying carb cleaner into the air intake is not likely to achieve anything worthwhile. There may be a carb overhaul kit available.

The idle speed does seem low, maybe a small increase will be all that is needed but you are idling it in gear in the video

There is a workshop manual which covers the model at http://162.144.28.33/lib/yamaha/manuals/1984-1996yamaha.html#/0
 
Hi
I’m afraid carb cleaner doesn’t clean the internals of the carb. Only the inlet and throttle body.
Pretty certain you will need to take the carb off and clean the chamber and jet(s). Easily done and well worth it
 
my 15 hp 4 stroke yamaha started to run a bit ragged---took it to an outboard engineer for a sonic carb clean--------ran like new when it came back
 
Another possibility is the fuel pump failing. I had similar symptoms when this happened on my Yam 9.9. The rubber diaphragm inside had a tiny split in it. The engine ran fine above idle but wasn't happy at idle as not enough fuel was getting through at low speed. The check is at low speed to hand pump by squeezing the priming bulb. If this solves the rough running there is your problem. If this is the problem the quick solution is buy a complete new fuel pump - it's only 2 bolts and 2 pipe clips to change it. A cheaper solution is to buy just the diaphragm and replace it by dismantling the pump.
 
AngusMcDoon said:
Another possibility is the fuel pump failing. I had similar symptoms when this happened on my Yam 9.9. The rubber diaphragm inside had a tiny split in it. The engine ran fine above idle but wasn't happy at idle as not enough fuel was getting through at low speed. The check is at low speed to hand pump by squeezing the priming bulb. If this solves the rough running there is your problem. If this is the problem the quick solution is buy a complete new fuel pump - it's only 2 bolts and 2 pipe clips to change it. A cheaper solution is to buy just the diaphragm and replace it by dismantling the pump.
Click to expand...

I will check, thanks everyone for the suggestions. Come to think of it, I remember when I got the boat, the bulb felt very firm and when it felt full it wasn't possible to squeeze it any more. Now I can easily squeeze it even when full, and I hear something wheezing. And that feeling co-existed with rough idle and stalling. So it may be fuel line or most likely the tank tube or something like that sucking air?
 
thinwater said:
No, you probably just noticed it then.

If the bulb is hard and it is still rough, there could be a fuel line problem, but it is coincidental.
Click to expand...

Hmm, not sure I got you, or maybe I wasn't clear. In the beginning I would pump with the bulb and it would become firm and not possible to squeeze anymore and no wheezing after full. Engine run fine. Around the same time the idle problems started happening, and now, when I initially squeeze the bulb - it does fill up but is much softer, it is easy to squeeze even when full and I can hear wheezing when I squeeze the full pump. When I noticed it, it felt like it's not pumping at all, but then I tested the line and it squirted fuel, so I thought it's OK then. But it can still suck some air along with fuel, I didn't think of that. I haven't tested yet if squeezing when running the engine changes the rough idle - it's a good test, I will do it. The line with bulb was new when I bought the boat, so it should be OK. But the fuel tank is old. It may be coincidence, but maybe it's sucking air from inside the tank. When disconnected on both sides, bulb is not squeezable, so there's no leak in the line and bulb.
 
RomanDesign said:
Hmm, not sure I got you, or maybe I wasn't clear. In the beginning I would pump with the bulb and it would become firm and not possible to squeeze anymore and no wheezing after full. Engine run fine. Around the same time the idle problems started happening, and now, when I initially squeeze the bulb - it does fill up but is much softer, it is easy to squeeze even when full and I can hear wheezing when I squeeze the full pump. When I noticed it, it felt like it's not pumping at all, but then I tested the line and it squirted fuel, so I thought it's OK then. But it can still suck some air along with fuel, I didn't think of that. I haven't tested yet if squeezing when running the engine changes the rough idle - it's a good test, I will do it. The line with bulb was new when I bought the boat, so it should be OK. But the fuel tank is old. It may be coincidence, but maybe it's sucking air from inside the tank. When disconnected on both sides, bulb is not squeezable, so there's no leak in the line and bulb.
Click to expand...

Yup, that's not how I understood it. You may be right. The line and bulb is cheap. I would check EVERY fitting, including those at the engine and the tank. They are all trouble prone. Also check the strainer inside the tank and the antisyphon valve at the top of the tank. I have had trouble with each and everyone of those over the years, often several at the same time.

That said, if you can get it reasonably firm it is probably not the only problem. HAving five problems at once with an outboard is not unusual; they are each minor, building up over time, until....

(BTW, the boat in my avitar, a PDQ 32, is powered by 2 Yamaha 9.9s. I've had two sets of engines over the years. Lots of practice!)
 
Hi RomanDesign, I know a few years have passed but I'm having the same issue with my yamaha 9.9 4-stroke outboard, wondering what happened in the end, if you're still on here? Initially I thought I'd fixed the issue by resolving a bad fuel connection. Then I had the motor serviced and winterized and she was running fine when I started her back up again, until I launched off the dock for my first voyage of the season and... watched the engine promptly sputter out. So now I'm not sure what the issue is. I used a fresh tank of gas with fuel stabilizer in it, and it starts up reliably, even after it cuts out... but it will unpredictably sputter out again, often when it's idling, but sometimes in gear as well. My next thought is to take out and clean the carburetor... any suggestions are very welcome! Expecting my engine to fail every time I come in to dock is getting a little tiring...
 
Hi, firstly you dont clean carbs with cleaner that way. You need to strip and clean.
Strip, clean and adjust slow idle screw and it will be fine
 
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