Yamaha 9.9 engine trouble - looking for advice

Max123

Max123

Well-Known Member
So, I picked up an older Yamaha 9.9 4-stroke for basically nothing at the beginning of the summer to take up to our family cabin. Looks clean, but won't run properly and had been sitting for a little while when I got it. The engine is a 2007 model F9.9MSHF

My initial assumption was that the carb was gummed up, so without any further diagnosis I took the carbs apart, clean out the jets and replaced the float. I also replaced the automatic enrichment solenoid (sort of an auto-choke system that opens an additional passage in the carb to run rich, and then closes after warm up - this was my initial diagnosis, but proved to be wrong). Fresh gas in the tank, with a healthy dose of Seafoam.

After all this, the engine started, but died almost immediately.

Further investigation shows that the diaphragm in the accelerator pump has a small tear in it, so replace this as well (fuel dripping out the bottom of the carb).

Now the engine starts, hold idle and was ready for a test run in the lake. I get it out on the water, engine starts, will go into gear without stalling, but it immediately bogs down and stalls if I give it any throttle to accelerate. Here's the weird part: if I feather the throttle, full on-off, the boat will accelerate up to full speed (I got it up to about 25km/hr), but if I hold the throttle open the engine will die. I tried squeezing the primer bulb while running, and this made no difference. I ran the boat around the lake like this for a good 1/2hr, hoping that fresh fuel through the system would help knock something loose - but the problem didn't go away.

I'd like to hear if anyone has any experience troubleshooting.

Here's my current working theory, but I'd like to hear any opinions:

I think the fuel pump is likely on its last legs, or is extremely clogged up. Fuel is getting to the carb, but the pump can't maintain any pressure. The reason it works when I feather the throttle is that the accelerator pump is actually pumping gas into the carb, not the fuel pump. I may be totally wrong on this though, because I tried to maintain pressure in the system with the primer bulb, and this had no effect. Not sure if pressure from the primer buld will pass through the fuel pump into the carb, or if there is a regulator. Any ideas?

Keep in mind that I'm an amateur, and there is always the possibility I f'd up the reassembly process...

I brought the carb home with me, and will dis-assemble / clean the entire thing a second time
 
One note on Yamaha parts.

Crowley is a great source, but the Yamaha kits seem like the strangest / random assortment of parts.

When I bought the carb re-build kit, it came with a bunch of random machine screws for the carb (not exactly replacement parts, and a new air adjustment screw (which is actually a sealed port on my carb and not a replacement part). What would have been nice, is if the kits just provided new jets, and if the accelerator pump diaphragm was included as well (a $50 piece of plastic)
 
Sounds like a fuel delivery issue to me. Did you take the fuel pump apart and inspect it? Did you run a thin piece of soft wire into all the ports and jets and then blow them out with compressed air?
 
I haven't touched the fuel pump yet. I'm going to re-check the jets - however I don't think that's the issue given that I could get it up to full speed by feathering the throttle.
 
You say the idle screw is covered up? Do you have a picture? In alot of cases, they put a punch cover ( aluminum or brass) overtop of that screw. It's for their EPA regulations. They can be easily knocked out or drilled out. I've had to do this a million times.

What ends up happening in this case is the engine is set from the factory so lean that once the valves set the engine will not run properly. That adjustment screw solves it everytime. That and in relation to the idle position screw.
 
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ive had terrible luck with small engines (tohatsu, honda, merc). recommend you switch to brushless electric drives. the small engines are a huge pain and ive had shops work on them for ages without any success. everything has to be just right - fuel flow, air flow, spark - for them to light and run properly and because of the size its extremely difficult to get everything right.
 
zurk said:
ive had terrible luck with small engines (tohatsu, honda, merc). recommend you switch to brushless electric drives. the small engines are a huge pain and ive had shops work on them for ages without any success. everything has to be just right - fuel flow, air flow, spark - for them to light and run properly and because of the size its extremely difficult to get everything right.
Click to expand...
skill issue
 
ship happens said:
You say the idle screw is covered up? Do you have a picture? In alot of cases, they put a punch cover ( aluminum or brass) overtop of that screw. It's for their EPA regulations. They can be easily knocked out or drilled out. I've had to do this a million times.

What ends up happening in this case is the engine is set from the factory so lean that once the valves set the engine will not run properly. That adjustment screw solves it everytime. That and in relation to the idle position screw.
Click to expand...
I'd heard the air adjustment screw is covered as an emissions thing as well. I assumed it was on of those 'less than ideal' EPA requirements but that it should still run. I'm assuming this is a 'make it better' adjustment, and is unlikely the cause of my current issue? In either case, thanks for pointing this out, I really appreciate any insights people have on getting these older outboards to run right.

Any thoughts on the fuel delivery theory? If I pump the fuel bulb while the engine is running, would that rule out a fuel pump issue?
 
Max123 said:
I'd heard the air adjustment screw is covered as an emissions thing as well. I assumed it was on of those 'less than ideal' EPA requirements but that it should still run. I'm assuming this is a 'make it better' adjustment, and is unlikely the cause of my current issue? In either case, thanks for pointing this out, I really appreciate any insights people have on getting these older outboards to run right.

Any thoughts on the fuel delivery theory? If I pump the fuel bulb while the engine is running, would that rule out a fuel pump issue?
Click to expand...
Drill out the cap. Adjust while on the water, in gear.

Yes if your fuel pump was an issue pumping the ball would keep it from dying. All the pump does it lift the fuel from that tank and puts positive fuel supply/pressure into the carb bowl. Not much pressure either. Just keeps a constant flow of fuel. In your case it's not the pump. Adjust the carburator
 
ship happens said:
Drill out the cap. Adjust while on the water, in gear.

Yes if your fuel pump was an issue pumping the ball would keep it from dying. All the pump does it lift the fuel from that tank and puts positive fuel supply/pressure into the carb bowl. Not much pressure either. Just keeps a constant flow of fuel. In your case it's not the pump. Adjust the carburator
Click to expand...
Thank! So, fuel pump is unlikely to be the source of my problems.

Any theory on why it will run up to full speed (in gear), when I am feathering the throttle, but will die if I hold it at a single throttle position (above idle speed). I was thinking that by continuously feathering the throttle open/close, it is pushing fuel in because the throttle linkage is tied to the accelerator pump (so its like manually priming the carb). I tried adjusting the accelerator pump linkage but it had no real effect (other than making it run worse).

I'm trying to think of what would restrict fuel that is unrelated to the accelerator pump (repaired), clogged jets (cleaned), or fuel pump (tested by priming bulb while operating, with no effect).
 
Max123 said:
Thank! So, fuel pump is unlikely to be the source of my problems.

Any theory on why it will run up to full speed (in gear), when I am feathering the throttle, but will die if I hold it at a single throttle position (above idle speed). I was thinking that by continuously feathering the throttle open/close, it is pushing fuel in because the throttle linkage is tied to the accelerator pump (so its like manually priming the carb). I tried adjusting the accelerator pump linkage but it had no real effect (other than making it run worse).

I'm trying to think of what would restrict fuel that is unrelated to the accelerator pump (repaired), clogged jets (cleaned), or fuel pump (tested by priming bulb while operating, with no effect).
Click to expand...
Its a carburated 4 stroke, and a Yamaha at that. Its all in air fuel mixture. Once its wide open the throttle plate is fully open and the idle circuit is out of the equation. The engine is gulping back fuel and air. That little transition between idle and throttle up is the hardest part on these little engines. Some have accelerator pumps some dont. These guys have changed carburator designs over the years. the reason they set it so lean is because of emissions. If it was your fuel pump you would see majority of your problems at WOT. You have a tuning issue. Drill out that stupid little plug

You can also do a valve adjustment, put some new plugs in it. However these things wont sove your issue, but they might add a bit to your solution in the end results.
 
Thanks! I've heard that this generation of Yamaha doesn't have the best carb design (no manual choke, fussy lean burn fuel ratio).

So far the main 'benefit' is that the carb is so easy to remove for me to tinker with it - two 10mm nuts, disconnect one vacuum line, one fuel line and its off.
 
Just curious, what regulates fuel pressure/delivery? For example, once the primer bulb is hard, you can't just use the primer bulb to shoot excess fuel into the carb (can you?) Can you flood an engine just with the primer bulb?
 
The carb float. If you press on the bulb too hard you can actually bend the carb float. Especially the ones with metal tabs at the pin mount. The fuel pump is a pass through diaphragm with check valves. It's got 2 little flap valves. Kinda like a Reed valve.


Once the carb bowl is full it closes the inlet. As I'm sure you already know. If the fuel level in the bowl goes past the float level the fuel will just spill through the main stem into the venturi.
 
ship happens said:
The carb float. If you press on the bulb too hard you can actually bend the carb float. Especially the ones with metal tabs at the pin mount. The fuel pump is a pass through diaphragm with check valves. It's got 2 little flap valves. Kinda like a Reed valve.


Once the carb bowl is full it closes the inlet. As I'm sure you already know. If the fuel level in the bowl goes past the float level the fuel will just spill through the main stem into the venturi.
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Thanks again,

When I was originally going through the carb with the rebuild kit, I found that the stem on the needle valve attached to the float was bent - so I replaced it along with the float (both parts included in the rebuild kit). I was a bit unclear on how much stroke the float should have. The original float had about 3/4" of movement from open to close, whereas the metal tabs on the new float allowed no movement. I matched the new float to the old, but maybe that was incorrect? First time I'm going through this, and a shop manual is impossible to find so I'm always worried that I'm missing something.

Anyway, i took a video of the engine when I was testing it on the lake which shows the problem.

 
Max123 said:
Thanks again,

When I was originally going through the carb with the rebuild kit, I found that the stem on the needle valve attached to the float was bent - so I replaced it along with the float (both parts included in the rebuild kit). I was a bit unclear on how much stroke the float should have. The original float had about 3/4" of movement from open to close, whereas the metal tabs on the new float allowed no movement. I matched the new float to the old, but maybe that was incorrect? First time I'm going through this, and a shop manual is impossible to find so I'm always worried that I'm missing something.

Anyway, i took a video of the engine when I was testing it on the lake which shows the problem.

Click to expand...
That's a pretty good video. Question, did you pull out the stem underneath the main jet when you had it apart?
 
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