Okay, I apologize since I have seen this post many times on the forum. i have searched and my issue appear to be unique as to my symptoms. I bought this boat from my dad after he had it about 5 months. The boat was running fine while he had it. However, since I bought it, it appears to be doing gremlin type things. First, I can usually get it started without to much problem for an older motor. However, when it starts it has a very low idle even with the idle bar (choke, or whatever else you want to call it) raised to full position and will still idle when I put the bar down, but it never really revs to high RPM I expect if I raise the bar all the way up.
When I put it in gear and the engine does not bog (which it sometimes will), the engine slowly picks up and will usually run at full speed with no apparent problem. Okay the story continues, randomly, could be five minutes, could be 45 minutes, could be, sometimes right when I take off from idle, the engine reduces its RPM by about 2500 and will not get on plane. Once again randomly it will pick up and go full blast and run great. I have not had the compression checked and will, but to be honest if the engine runs great sometime would it not be good on compression. I will have this checked so please assume compression is good.
Okay a couple of things I noticed this last weekend. If it is idling slow and I push key in the engine revs up. Also, the bulb is not sucked in, but usually requires me to pump it a couple of times when I restart. My analysis. My dad said he changed the bulb and lines, I am thinking he may have put the wrong size line on, second I am targeting the fuel pump, since when I push the key and it dumps fuel into the top carb, the engine revs. Also, if I get to the back and pump the bulb it will rev the engine up, but will not fix the slow idle.
You have answered your own question. When you pump the bulb and the engine speed picks up. Is a test we use to determine if there is a failure in the fuel supply. But the problem is determining if it is in the fuel pump, the supply hose, fittings, the pick-up tube in the tank.
You have to check them ALL to be sure that it is not a little bit of each, finally adding up to so big a problem that it is just now getting noticed.
A restriction in the fuel or a small air leak into the line as the pump sucks, the end result is not enough fuel which will cause the engine to run lean and burn a piston.
Rebuild the pump, take apart every fitting, trim hoses if they are long enough or replace them all, replace quick connectors with FACTORY quick connectors or run hose straight through and do away with the connectors, they are one more thing to go wrong. Pull the pick up tube out of the tank and clean it.
Mercury issued a Service Advisory (#2001-17) that provides some troubleshooting guidance on fuel starvation. All you need is a vacuum gauge, a piece of clear hose, and a t-connector to do the troubleshooting. The tests help confirm and/or narrow down the the possible common causes of fuel starvation, such as a failing fuel pump, a restriction in the fuel line upstream of the fuel pump, or a vacuum leak in the fuel line. Once you confirm (or eliminate) possible causes, you can then limit your repair effort to what is wrong and avoid spending time and money on those compnents that are OK and not causing a problem. This is how I would approach the problem anyway.
The below link discusses fuel starvation symptoms in more detail and also contains a link to the Service Advisory.
I appreciate the additional advise. I have already purchased the fuel pump rebuild and am going to do that tonight. Probably needed one anyway. I also replaced the plugs and ran a bypass tank (which proved is was not in the supply line. If the rebuild does not work, I will run test mentioned in advisory and then attack the carbs.
Quick update for those following the tail of lost power. I rebuilt the fuel pump yesterday. When I arrived at the lake, the motor started up fairly well, but would not idle at high speed with the idle bar all the way up. After I finally opened the cowling and manually revved the engine, it seemed to get a bit happier. I finally was able to get enough power to get it off the trailer and it ran like a champ. I am still skeptical as to whether or not I fixed the problem, or I the engine is just cooperated at that point, which happens. Anyone ever heard of having air trapped in the lines that might have gotten blown out when i revved the engine? I doubt that is the issue, but thought I would ask. Anyway, I think I will take it out a couple more times to see what it does and then my next step will be to rebuild the carbs.
Mercury issued a Service Advisory (#2001-17) that provides some troubleshooting guidance on fuel starvation. All you need is a vacuum gauge, a piece of clear hose, and a t-connector to do the troubleshooting. The tests help confirm and/or narrow down the the possible common causes of fuel starvation, such as a failing fuel pump, a restriction in the fuel line upstream of the fuel pump, or a vacuum leak in the fuel line. Once you confirm (or eliminate) possible causes, you can then limit your repair effort to what is wrong and avoid spending time and money on those compnents that are OK and not causing a problem. This is how I would approach the problem anyway.
The below link discusses fuel starvation symptoms in more detail and also contains a link to the Service Advisory.
Now I figured out the problem with this test. How does one get their hands on a digital vacuum guage. Those things are expensive. I could almost change out all the fuel lines and pickup tube for that price. What else can you do with these things?
Re: 1989 Mercury 115 Losing Power - Sound familiar
You do not need an expensive digital gauge. A simple cheap dial type (analog) vacuum gauge is sufficient, as you do not need a highly precise measurement of the vacuum level. A cheap guage would be accurate enough for your purposes.
Ebay or Walmart would be a source for a cheap gauge. Or search the internet with the term "vacuum gauge and fuel pump tester" to find other cheap sources. You should be able to find somthing acceptable in the $15-$25 price range. I have found that the cost of test equipment often pays itself back quickly with the time, expense, and frustration is saves by keeping me from "fixing" the wrong stuff.
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1996 Mercury 150HP V6 carbed outboard motor and Maxum 1900XR bowrider boat
Re: 1989 Mercury 115 Losing Power - Sound familiar
Okay, update I did find a vacumm pump for a good price, thanks nkeiner for the advise and for serving our country. Next, I did not have a chance to get to the lake and decided to go ahead and replace the fuel line up from the tank connection to the carbs. Took it back to the lake and no help. My next trip to the lake will involve the vacuum guage. If it checks out, I am going to tear into the carbs.
1989 Mercury 115 Losing Power - Sound familar
. First, I can usually get it started without to much problem for an older motor. However, when it starts it has a very low idle even with the idle bar (choke, or whatever else you want to call it) raised to full position and will still idle when I put the bar down, but it never really revs to high RPM I expect if I raise the bar all the way up. 
Linear Mode
