Very Odd WOT Speed Problem Mercury Sportjet 120 1995

[yesizmatt]

I had the boat out today and it doesnt hold a high rpm at wot. This is exactly what happens:
-From idle to wot boat speeds up all the way and has good rpm
-a few seconds later the boat slows down and looses some rpm
-occasionally it will pick up and go and then go back down.

This leads me to believe it is a fuel problem. But this is where it gets more odd. With barely any fuel in the tank, every single time when I would make a right turn while at wot, the boat would increase in speed and rpm. I thought maybe since there was not much fuel it could just be moving around and not getting a constant supply. So I fill up the tank all the way, and then took it back to the water. Now when I make a right turn at wot it would loose rpm and speed. Any ideas on what would cause this?

The engine is a 1995 mercury sportjet 120hp and has good compression and spark.

 

[Texasmark]

Since you changed the fuel level in the tank and nothing else, one has to eliminate the engine as the culprit. However, improper venting of the fuel tank causes similar problems. Not sure how a personal water craft vents it's tank but that is where you need to be looking. I assume you have a squeeze bulb on the fuel line used as a starting aid to prime the carburetors. With that said, when you experience the falling rpm, look at the bulb. If collapsed, your tank is not venting properly. Finding the source of that problem should fix it.

 

[yesizmatt]

Since you changed the fuel level in the tank and nothing else, one has to eliminate the engine as the culprit. However, improper venting of the fuel tank causes similar problems. Not sure how a personal water craft vents it's tank but that is where you need to be looking. I assume you have a squeeze bulb on the fuel line used as a starting aid to prime the carburetors. With that said, when you experience the falling rpm, look at the bulb. If collapsed, your tank is not venting properly. Finding the source of that problem should fix it.

Thanks for the reply, I will have to take a look later to see if it is venting properly and am hoping that this is the issue.

 

[yesizmatt]

I took it out again today and it still has the same problem. I looked at the vent and everything and opened where you fill the tank with gas to make sure it would be vented properly, and it still ran the same. I have no idea what the problem could be!

 

[Texasmark]

Was the bulb collapsed when you ran and had the problem? Did you try to pump the bulb to force fuel into the engine and if so what happened?

 

[yesizmatt]

Was the bulb collapsed when you ran and had the problem? Did you try to pump the bulb to force fuel into the engine and if so what happened?

Unfortunately, since the motor is inside the boat, their are regulations that due not allow you to have a fuel bulb installed. If i had the gas tank cap open wouldnt that outrule the problem being improper ventalation?

 

[Texasmark]

Unfortunately, since the motor is inside the boat, their are regulations that due not allow you to have a fuel bulb installed. If i had the gas tank cap open wouldnt that outrule the problem being improper ventalation?

Not necessarily. Problem addressed is engine fuel pump vacuum not providing fuel to engine or other problem. No primer bulb! Huh! What is your starting procedure.....just curious?

Let's eliminate that as a problem. Barring a check valve somewhere in the line from the tank to the engine, open the fuel line at the inlet to the fuel pump. Just follow the hoses from the carbs back to the fuel inlet line and you can find what you want. The fuel pump will be located probably on the Starboard side of the engine back along the side of one of the cylinders. About 2" square/rectangular, 3/4" thick, 2 hoses, one in, one out. It uses combustion gasses to provide energy for the pumping action. and easiest place to mount it is adjacent to the source.

With the cap off the fuel tank, see if you can blow through the hose making bubbles in the tank. If so, problem is elsewhere. If a check valve is somewhere in the line you won't be able to do that. Next would be a hole in the diaphragm or leaking check valve in the pump. Do a pump rebuild.....kits for sale on site....just follow the directions explicitly......gotta get the check valves in correctly.

If that doesn't get it, rebuild your carbs. You could have some crud in one that works it's way around to your high speed jet. Since HSJ is located in the bottom of the bowl (was on mine) it's easy to fall victim to carb bowl crud. Kits for sale on here too.

That will get you through some potential problems. Internal fuel filter (between the fuel pump and carb feed) is also a consideration so while you are in there, might as well do it too. Ensure you install the new one with the arrow pointing the same direction as the one you remove. Wouldn't hurt to subscribe to the Seloc service manuals. Great help. They sell them or rent www time on here too. Not a site advocate. This company sells marine products and operates this Forum to help us solve our marine problems.........and, oh yes, make a living in the process. I have purchased from them numerous times and have not been disappointed.

 

[yesizmatt]

After taking a closer look at all the fuel lines I have noticed that the line from the fuel pump to the carbs looked like it had a small inside diameter. The manual says the minimum inside diameter is 5/16", and the hose installed is only 1/4". Would this difference be causing the engine to starve from fuel at wot.

 

[Faztbullet]

It has a electric fuel pump and diaphragm pump Tex.....the electric is for priming.

 

[Texasmark]

It has a electric fuel pump and diaphragm pump Tex.....the electric is for priming.

Thanks. So now the puzzle gets more complicated.

 

[yesizmatt]

I'm going to buy the correct diameter hose today and then try it out. I'll post later today if that was the problem

 

[yesizmatt]

So I installed the correct diameter fuel line and also installed a clear piece of tubing in between the fuel pump and carbs so I could see if there was air in the line. When running at wot, their was a very noticeable amount of air going through the line. I checked all hose clamps and they seem to be tight, so my guess would be something in the fuel pump is letting air into the line. Ill most likely buy a rebuild kit and then see what happens after I install it. Do you think this will fix the problem?

 

[Texasmark]

Interesting point. With what you said about hose, I looked back at your year model and 1995 is 20 years old. My OEM hose failed at 10 years. Hose was gray in color and appeared to be made of silicon, real slick feeling. Reason why I changed it was flaking. I found semi circular tan flakes of crud, from the inner diameter of the hose, in the fuel delivery system and they had clogged things up. This sort of thing would surely cause a problem of the sort.

You know, you are there and we are where we are. Hard to second guess this and that to help in problem solutions. Answers depend on brainstorming, personal experiences, and training for the professional guys. We only know what we are told and a lot of times that's just not enough.

When you go to replace it, look around for what I said or something similar. If you find some, no telling where more is located and you have to include all the components in your fuel delivery system. My problem included the hose to the tank, squeeze bulb, internal engine hoses, fuel pump, and inline engine fuel filter. Since you have the extra parts that Faztbullet mentioned, you need to ensure that you include all of that. Good hunting!