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I need help diagnosing problems with my 1999 Yamaha T9.9 outboard. There have been different issues and problems through the summer. Here's a summary of the current symptom (a list of actions previously taken follows at the bottom).
1) engine has trouble starting and won't run at idle speed. If I try to start the engine, it will try to start but won't turn over. I can see the starter motor engage, the water cooling pump is working as the tell-tale is visible. If I spray carb fluid directly into the air intake it will start and run briefly but then die/stall.
2) if I manually engage the throttle link connector at the carb to full throttle it will start right up.
3) if I release the throttle link connector and let the speed drop to idle, the engine will stall/stop almost immediately
4) with the engine in neutral, if I set the remote throttle control to medium/high speed, the engine will start and continue to run (no need to touch the throttle link on the carb). I have not tried to put the engine "in gear".
5) if I lower the remote throttle control - but not even all the way back to idle - then the engine will stall/die
5a) as the engine starts to stall/die when I lower the throttle, if I increase the throttle right away, the engine will continue running at high throttle.

I've had similar symptoms through the summer, with the engine not turning over and starting. It sounded like it wasn't getting fuel. I have taken the following steps already and still have the set of symptoms listed above
a) replaced fuel line and pump bulb from external fuel tank
b) replaced fuel in gas tank, fuel line, engine fuel hosing with new gas with added stabilizer and dry gas
c) replaced fuel filter
d) earlier in the summer, when I was having problems, I took the engine to a dealer/marina and they rebuilt the carburetor. After that, it started smoothly the first time I used it. But after being out of town for two weeks, these symptoms started up again
 

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I need help diagnosing problems with my 1999 Yamaha T9.9 outboard. There have been different issues and problems through the summer. Here's a summary of the current symptom (a list of actions previously taken follows at the bottom).
1) engine has trouble starting and won't run at idle speed. If I try to start the engine, it will try to start but won't turn over. I can see the starter motor engage, the water cooling pump is working as the tell-tale is visible. If I spray carb fluid directly into the air intake it will start and run briefly but then die/stall.
2) if I manually engage the throttle link connector at the carb to full throttle it will start right up.
3) if I release the throttle link connector and let the speed drop to idle, the engine will stall/stop almost immediately
4) with the engine in neutral, if I set the remote throttle control to medium/high speed, the engine will start and continue to run (no need to touch the throttle link on the carb). I have not tried to put the engine "in gear".
5) if I lower the remote throttle control - but not even all the way back to idle - then the engine will stall/die
5a) as the engine starts to stall/die when I lower the throttle, if I increase the throttle right away, the engine will continue running at high throttle.

I've had similar symptoms through the summer, with the engine not turning over and starting. It sounded like it wasn't getting fuel. I have taken the following steps already and still have the set of symptoms listed above
a) replaced fuel line and pump bulb from external fuel tank
b) replaced fuel in gas tank, fuel line, engine fuel hosing with new gas with added stabilizer and dry gas
c) replaced fuel filter
d) earlier in the summer, when I was having problems, I took the engine to a dealer/marina and they rebuilt the carburetor. After that, it started smoothly the first time I used it. But after being out of town for two weeks, these symptoms started up again
If you haven't drained the float bowl after using it for a week or so, it is likely there is debris in the low speed circuit. Sounds like you have covered everything else, hoses, filter, etc. Unless there is a lot of debris in the tank, it is probably forming in the float bowl when the gas evaporates.

Time for another carb cleaning, then suggest you drain the carb, small screw on the bottom of the bowl, after using the motor, unless you are going to run it again in a few days.

Paul T
 

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Yes it really is essential to run the carb dry after each use, or better still to drain it.

Also essential is to keep the gas fresh. How old is yours? After a couple of months use, I pour mine into the car and buy fresh. Adding Stabil helps.

It really sounds like your carb keeps gumming up, or the jets block up via some other mechanism. Dirt in the fuel system?

When I replaced the carb on my Yamaha 8, I noted with surprise that there is no real fuel filter, so I added one before the carb.

All I can say is, with the above good practices, mine just started on the second pull after a year, and idled purring like a kitten.
 

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I have 2 Yamaha 9.9 on the same boat (cat)! Easy problem. The low speed circuit is clogged. Removing the carb and cleaning is not difficult (DIY).

a. Draining the carb every time is urban legend. That instruction is for long-term storage and is not relevant to this motor. In fact, the PO did that and had all manner of trouble with the carbs. Since running the engine dry merely ensures that the carb will dry (unless you open the drain it is not truly dry when it stops) and form varnish, better to leave it full for a few weeks.

b. Pull the carb again and make certain the low speed circuit is REALLY clean. Blast every crany and every passage. remove and reinstall the jets if possible. IF it is an older model where the idle mix is adjustable, remove the screw, clean the hole, and reinstall (count the turns on the way out).

c. Look for pitting in the carb bowl (stains and pits on the aluminum, in the bottom). With e-10 these are common. Stabil is not a highly rated corrosion inhibitor and will not help with this. In stead, try biobor EB or Merc Strore-n-start. Also Seafoam.

d. Clean the tank. Also check the filter and replace. Adding a secondary filter (raycor) is a good idea. And vent driers help. Anything to keep the fuel clean and dry.

I haven't had a carb problem in years. Had plenty before.

Sail Delmarva: Gasoline Additive Corrosion Testing

Sail Delmarva: Gasoline Filtration... and Vent Filtration Too?
 

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I have 2 Yamaha 9.9 on the same boat (cat)! Easy problem. The low speed circuit is clogged. Removing the carb and cleaning is not difficult (DIY).

a. Draining the carb every time is urban legend. That instruction is for long-term storage and is not relevant to this motor. In fact, the PO did that and had all manner of trouble with the carbs. Since running the engine dry merely ensures that the carb will dry (unless you open the drain it is not truly dry when it stops) and form varnish, better to leave it full for a few weeks.

b. Pull the carb again and make certain the low speed circuit is REALLY clean. Blast every crany and every passage. remove and reinstall the jets if possible. IF it is an older model where the idle mix is adjustable, remove the screw, clean the hole, and reinstall (count the turns on the way out).

c. Look for pitting in the carb bowl (stains and pits on the aluminum, in the bottom). With e-10 these are common. Stabil is not a highly rated corrosion inhibitor and will not help with this. In stead, try biobor EB or Merc Strore-n-start. Also Seafoam.

d. Clean the tank. Also check the filter and replace. Adding a secondary filter (raycor) is a good idea. And vent driers help. Anything to keep the fuel clean and dry.

I haven't had a carb problem in years. Had plenty before.

Sail Delmarva: Gasoline Additive Corrosion Testing

Sail Delmarva: Gasoline Filtration... and Vent Filtration Too?
Agree, draining every time may be overkill. However, I forgot to drain for over the winter storage once & it got me, what a mess. So, how long between uses before draining? I really don't know. I drain if I am not going to run it again for about a week, and have not had any fuel related problems for many years, it just takes about a minute or so, & is easy.

Your mileage may vary. :D

Paul T
 

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Agree, draining every time may be overkill. However, I forgot to drain for over the winter storage once & it got me, what a mess. So, how long between uses before draining? I really don't know. I drain if I am not going to run it again for about a week, and have not had any fuel related problems for many years, it just takes about a minute or so, & is easy.

Your mileage may vary. :D

Paul T
I make certain I sail every month :))). That seems to help everything.
 

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I make certain I sail every month :))). That seems to help everything.
Whatever works, :D as mentioned, I don't know "how long is too long?" We talked about this subject some time back. I made a couple of tests letting gas evaporate, one in a coffee can, which got to a point like about 90 weight gear oil, never truly evaporating, & the other in an old spare carburetor, which evaporated completely with absolutely no trace of anything.

I suppose the gas from the pump can vary, or some gas has more water in it, or it's age, lots of variables. From the number of "motor won't idle, or run" posts, it seems to be a fairly common problem, good business for repair shops, I would think.

Paul T
 

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Discussion Starter #8
Thanks for the comments. I'll be heading over to clean the carburetor shortly. Which components need the most attention for the low speed circuit?
 

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I looked, I have been using a Mercury fuel stabiliser, Quickcare, on the advice of a Mercury service tech. Is it the best, I can't say, but I've had no problems since I started using it.
 

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a. Draining the carb every time is urban legend. That instruction is for long-term storage and is not relevant to this motor. In fact, the PO did that and had all manner of trouble with the carbs. Since running the engine dry merely ensures that the carb will dry (unless you open the drain it is not truly dry when it stops) and form varnish, better to leave it full for a few weeks.

Sail Delmarva: Gasoline Additive Corrosion Testing

Sail Delmarva: Gasoline Filtration... and Vent Filtration Too?
I have to confirm this point. On the advice of my mechanic not to run carb dry....
I use a Mercury 6hp four stroke. I use Valvtec ethanol fuel treatment. Have not run my engine dry all season and it always starts, 2nd or 3rd pull. I use premium grade gasoline. I think the key is a good cleaning at the start of the season, quality gasoline and use of gas treatment. I haven't had a problem starting my outboard in three years using this stuff. And sometimes my motor will sit for two weeks between uses. Ps...put my boat in mid May and still no problems.
Chris
 

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I have to confirm this point. On the advice of my mechanic not to run carb dry....
I use a Mercury 6hp four stroke. I use Valvtec ethanol fuel treatment. Have not run my engine dry all season and it always starts, 2nd or 3rd pull. I use premium grade gasoline. I think the key is a good cleaning at the start of the season, quality gasoline and use of gas treatment. I haven't had a problem starting my outboard in three years using this stuff. And sometimes my motor will sit for two weeks between uses. Ps...put my boat in mid May and still no problems.
Chris
Valvetec was another that got all excellent anti-corrosion ratings in the Practical Sailor testing.

With ethanol gasoline I many are under weighting the effect of corrosion on carbs. Aluminum builds this nasty gel very quickly, and cleaners can't dissolve it. Whereas varnish used to be the main worry, with modern fuel it is less problematical. The new problem is ethanol corrosion, and these additives can stop it dead in it's tracks.

Always check a problematic carb bowl for pitting in the bottom; if you see pits, you know at least part of the problem.

The other solution is controlling tank venting (reducing water absorption), but that is another topic. Even dissolved water has been shown to dramatically increase corrosion. The gas (or diesel) does NOT have to separate.
 

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Thanks for the comments. I'll be heading over to clean the carburetor shortly. Which components need the most attention for the low speed circuit?
Any/all that can be taken apart. Suggest you lay them out from left to right with the "up" side up in sequential order of dis-assembly. Pictures & a good shop manual or at least a parts diagram would help. Compressed air or spray carb cleaner is also helpful.

No hands on experience with your motor, but there may be spring loaded check balls? Go slow & easy. :)

Paul T
 
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