F115TLRH down on power

My wife and I went fishing with my brother and saw where his F115TLRH (2001) was way down on power. It sounds like it's real lean and produces more power when you close the throttle some. I've volunteered to take a look at it, but before he brings it down here what should I be ready for? I did check the fuel filter and it had NO water in the fuel and the filter looks clean. Other than spark plugs and oil changes, I don't think anything has been done to it for quite a while.

I would focus on the fuel system. The flow of fuel is basically:

primer bulb > fuel filter > mechanical fuel pump > vapor separator chamber > through another filter at the bottom of the high pressure fuel pump > pressure regulator > fuel rail > injectors.

There's lots to check, but the fact that it gets better when you throttle down is a good observation. If it were me, I would check the mechanical fuel pump (look up Dangar Marine on YouTube.. I remember he has a good video summary of how to easily inspect these) and replace the fuel filter (even if it looks clean). If both of those things are working properly, then I would run the engine off of a small auxiliary tank that you know is filled with fresh fuel. Then see how performance is. If no improvement, then I would open up the vapor separator and change the internal filter there (or at least take a look at it to see if it's getting clogged). Also check the needle valve tip for damage. I would also send the injectors in to be cleaned/rebuilt by a shop who does that service (there are many).

Also, you can try disconnecting the vacuum sensing hose that goes from the pressure regulator to the air intake. On the F115 it might not be easily done, but that is a very easy test to see if your fuel pressure is just bit too low. If you disconnect that line, that increases the fuel pressure slightly.. sometimes enough to get you over the edge of bogging at high speeds. It should be reconnected after the test, but that would at least tell you that you do indeed have a fuel delivery issue.

Rejesterd,

Thanks for the info. It'll probably be a couple of weeks before my brother has a chance to bring it down here, but I just ordered a manual and I've got to move a few things in the workshop so the boat and my truck will fit inside. I did find and watch the video from Dangar Marine on rebuilding fuel pumps. I'll check back in when I get started on it.

Just before Christmas, my brother got it down here. I started taking it apart, and today I got it down to the vapor separator and opened that up. There is a reasonable amount of deposits in the bottom of the separator and also the filter is slightly clogged. However on the side of the filter that goes to the electric fuel pump, there is a lot of dried deposits from dried gasoline. I will inspect the mechanical fuel pump next.

Do boats typically have a fuel filter in or near the fuel tank?

Yeah, I would replace that filter inside the VST along with the fuel filter that connects to the mechanical fuel pump. And I would definitely send in the injectors to be cleaned.. if there was dried gunk going to the electric pump, then the injectors are probably clogged.

There might be another fuel/water separator filter in the bilge area, but not all boats have them.

I've run into a problem......Seeing as the valve lash has never been checked and the motor is 19 years old, I decided that it'd be a relatively easy job to check it. Per the manual, you pull the fuel pump, disconnect the breather hose, remove the spark plugs (to turn the motor over easier), remove the valve cover bolts, and then remove the valve cover. WRONG! The top of the valve cover extends underneath the camshaft pulleys, and the valve cover hits the inside of the pulleys before the locater pin (guide pin?) can slide out of the hole that it goes into. Because the top locater pin won't let the valve cover slide downward, you can't do that to remove the valve cover. It looks to me like you have to at least loosen the camshaft pulleys and slide them upward or maybe even remove them to get the valve cover off. The pulley to camshaft bolts seem to be Loctited. Are the pulleys pressed onto the cams or will they move relatively easy if I do remove or loosen the bolts? How is this usually done? Does the valve lash change enough to worry about? On my water cooled Yamaha motorcycle with what looks like a similiar setup with shims, they hardly change over a lot of miles. I've got 105,000 miles on mine and I haven't touched 15 of the 16 valves.

I have a 2002 F115 and I had a similar problem. Would only go to 4600 RPM, but seemed to run smooth at the higher RPM's. Ran a little rough at idle but not that bad. Turned out I had a bad fuel injector. You can buy new fuel injectors on EBay for $10 each. Cars use the same ones. You can determine if a cylinder is not firing by pulling off one spark plug wire at a time and seeing if it idles any different. Suggest you pull them off with the engine off. If the engine runs the same with one of the wires pulled then that cylinder is the problem. Sorry but don't have any experience with the camshaft

Update

I pulled the intake manifold and then cleaned out the vapor separator and replace the filter in the separator and also in the water separator. I also replaced the spark plugs and replaced the gas hose between the tank outlet and the 'T' where the fuel line splits for both motors. It took quite a while to get this done (not much spare time) and I also cleaned out the jets of the 9.9hp motor's carb. Last Sunday we had a warm day, so I pulled it out into the driveway and tested the motors. After I squeezed the gas bulb for the 115 what felt like a hundred times, and finally the VST was filled. I turned the key on and off a couple of times, and then the 115 started instantly and idled. However when I put it in gear and revved it up, it got rough and sometimes died when I returned to idle. I pulled one fuel injector lead at a time and I had 3 that were relatively equal and had very little effect when I pulled a connector. When I pulled #4 fuel injector lead, the motor almost died. #4 was way stronger than the others, so it appeared that I had 3 partially clogged injectors. Mills, you were right, fuel injectors are cheap on Ebay, so I purchased 4 and installed them. I haven't test run it yet. I've also replaced the trim angle rheostat so now Mark can see how his boat is trimmed without turning his head around. I'm not sure that I'd spend the $160 for the new rheostat or not, but he wanted it working. I can't bring the motor all the way down because it hits the roller on the trailer, so I'll have to fine tune this when I get it in the water or rig up a way to slide the boat back on the trailer.

When I went to start the 9.9 there was a fuel leak out of the bottom of the carb from what looks like an accelerator pump. I've got a new diaphram ordered.

With all the rain we've had, the lakes are almost all the way back up to normal pool, so in the next couple of weeks I should get to test it properly.

Now that I've got both engines on my brother's boat running good, I decided that my brother has ignored some other preventative maintenance that needs to be done. I greased the fittings on the 115 and found that the steering pivot was completely dry. It took a lot of pumping on the grease gun to finally show some grease at the ends. I also decided to replace the 90W on the lower unit. It was low---when I pulled the top plug, I reached down with a piece of wire and it came back out dry. In the owner's manual, it says that it should hold about 25.7 fluid oz. I pumped over 30 ounces in the bottom plug before any came out of the top plug. It does say to pump it full with the motor vertical, which is what I did. Is this normal?