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I have a 90s model 115 mercury out board that wants cut out…

I have a 90s...

I have a 90s model 115 mercury out board that wants cut out at 3/4 to full throttle

Mechanic's Assistant: Sometimes things that you think will be really complicated end up being easy to fix. The Marine Mechanic I'm going to connect you with knows all the tricks and shortcuts. Tell me a bit more about what's going on so the Marine Mechanic can help you best.

When try to accelerate to full throttle the engine wants to spit and sputter I've replaced the ignition coils and rebuilt the fuel pump the cylinder compression is 130 across the board

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Answered in 7 minutes by:
6/30/2018
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 18,231
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
Verified

Hello my name is ***** ***** am I speaking with? I read what you wrote. Stalling at full throttle is a sign of fuel starvation. Have you check the fuel pumps pressure or done any troubleshooting on the pump or supply side of things?

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Customer reply replied 1 month ago
my name is ***** ***** I have not checked the fuel pump pressure

HI Jim. there are a few different things that need to be checked. Give me 10 minutes or so to get you an answer typed up.

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Stalling at high rpm means the engine isn't getting fuel. That can be because of bad/weak fuel pump or it could also be a problem with the boats fuel tank. If the fuel tank is not venting properly it can build up a vacuum and restrict how much fuel goes to the engine. So the first step is to open up the fill cap to manually vent the tank and see what happens. If it runs better with the fill cap open, the vent is clogged. If it makes no change, As simple as this sounds next is to run it off of a different fuel tank, like a small plastic portable tank. Half the time there isn't anything wrong the motor itself, but rather something wrong with the boats fuel system. This can be a pinched or collapsing fuel line, bad or old gas, or a bad anti siphon valve. This is a pass or fail test, and the engine is either going to run better or it is not. If it does run better, on this portable tank, first change the anti siphon valve and retest with the engine hooked back up to the boats fuel tank. If it goes back to running poor on the boats fuel tank, then the fuel pumps output pressure needs to be checked. Fuel pump pressure needs to be at least 3 PSI at all speeds. If fuel pressure drops before the engine stalls, the fuel pump is bad. On the tank side of things, bad anti siphon valves are a common problem, when they go back they create a blockage and restrict the amount of fuel that flows to the engine. If the engine does not get enough fuel it obviously will not make full power. Click this link, this explains more on the anti siphon valve and why they go bad. http://www.sterndrives.com/anti-syphon-valve.html. On this anti siphon valve there is an inlet side and an outlet side. The inlet side faces the fuel tank, the outlet side faces the fuel line. When the engine runs, the fuel pump creates suction. This suction pulls the check ball off of it's seat and fuel is allowed to flow around it. The problem with this is over time the spring in the valve will get weak or simply break. When that happens the check ball will be free to jump to the outlet side of the valve and create a blockage/restriction on the outlet side of the valve. If the spring is simply weak and not broken, this will more or less happen at random. Click this link for a picture, it will make more sense once you see it http://imgur.com/a/tpq4H. The anti siphon valve itself is located at the fuel tank, where the fuel line connects. If you pull the fuel line off the tank you will see a hose barb, that hose barb is the anti siphon valve. Also, you do want to replace all fuel filters on the boat and engine. If you have a clogged filter that will also cause the engine to starve for fuel. Does that all make sense to you. I do want to make sure you are happy with my service before you go. If you had a further or more specific question on the issue by all means feel free to ask. If not, Just in case you do not understand the way the website works (and some folks do not). You do have to put forth a positive rating in order for it to credit me for helping you. When ratings are not done, the website simply keeps your deposit and they will not credit me. The ratings box is located at the top of the screen. To rate, you must select the star you wish and also confirm it. Please let me know if you run into any problems or errors when trying to do it. If you do have a problem, or if you can not see the ratings box which is at the top of the screen. Please reply back "I rate Jason's service _______" and fill in the blank.

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Thank you
Jason

Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 18,231
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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