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4.3 merc wont idle boggs down when you try to rev it up wont…

Customer Question
4.3 merc wont...

4.3 merc wont idle boggs down when you try to rev it up wont stay running fuel filter?

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.

Boat ran great put fuel in it started running like crap didn't want to start wont idle bogs down when you try to rev it up seems like it is starving for fuel

Mechanic's Assistant: Is there anything else the Marine Mechanic should be aware of?

First time it happened it ran good at high rpm now it wont stay running when it stalls you give it a minute then it will start but wont idle up

Submitted: 8 months ago.Category: Boat
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Answered in 1 minute by:
10/3/2017
Marine Mechanic: Jason, Marine Mechanic replied 8 months ago
Jason
Jason, Marine Mechanic
Category: Boat
Satisfied Customers: 17,482
Experience: Degree in Marine Technology. Gas and diesel marine mechanic.
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Hello my name is ***** ***** am I speaking with? I read what you wrote. Mercruiser engines are cataloged by serial number. Do you have that number handy so I can see exactly what version of the 4.3 you have? If you do not have the serial number. What year is the engine, and is the engine carbureted, throttle body injected, or multi port injected?

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Customer reply replied 8 months ago
1990 with a Rochester carb
Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

Got it. What is the history on the engine, have you ever personally seen it run correctly? Have you done anything on your own yet to troubleshoot the issue?

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Customer reply replied 8 months ago
It's a prepaid card
Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

I'm sorry I don't understand you. What is the history on the engine, have you ever personally seen it run correctly? Have you done anything on your own yet to troubleshoot the issue?

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Customer reply replied 8 months ago
1990 4.3 with a Rochester carb the boat ran great until I put gas in it
Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

Have you done anything on your own yet to troubleshoot the issue?

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Customer reply replied 8 months ago
I have done nothing to the motor
Customer reply replied 8 months ago
the boat ran great then started acting up after I topped it up with fuel that is why I think it is a filter
Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

I don't know what you may or may not know about boat engines, I also don't know what you have for tools or abilities. Did you have any specific questions that you wanted me to answer or would you prefer If I explained how we would troubleshoot this in the shop if you were to bring it in?

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Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

Okay give me about 10 minutes to get you an answer typed up.

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Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

1. As simple as this sounds you always start by running 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. Marine engines are very sensitive as to the quality of the fuel, and if any part of it is over 90 days, the fuel itself is always going to be suspect. 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, that tank will need to be pumped out. 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. As part of this test you also want to change all fuel filters on the engine. There can be up to 3 depending on your engines specific serial number.

2. If no change, then compression testing the engine is next. Your results need be about 100 psi or better on that engine, all cylinders should be within 10-15% of eachother. If you get below 100, then the engine is worn out or has mechanical problems, and you might as well stop your troubleshooting here, as there is nothing you can do to an engine with low compression to make it run better. From there you would perform a valve adjustment and see if the compression comes back up or not.

3. Spark test. You want to change all spark plugs before doing a spark test. After the plugs are changed, get a timing light and run the engine and hook the timing light up to each plug wire one at a time and see if the timing light flashes or not. Flash means the engine is sparking when running, no flash means no spark. Again this is a pass or fail test. If there is no spark on an cylinder, change the cap, rotor, plugs and plug wires. Next you want to check the timing advance. If the timing is not advancing there the engine will not make full power. If the timing is not advancing, the ignition module is going to be bad. (the module controls the timing advance)

4. Hook up a fuel pressure gauge between the pump and carb, measure fuel pressure with the boat going as fast as it can. Pressure needs to be at least 3 psi. If it's low, the fuel pump is weak. Make sure you do this after you do #1. If that anti siphon valve is bad or there is a bad fuel line, that will cause low fuel pressure because the fuel pump would be struggling to pull through that restriction/blockage that a bad anti siphon valve creates.

5. Lastly, if you do all of the above steps and still do not get any kind of result, the problem is going to be in the carb. They are either dirty on the inside, or one of them has a sticking float not allowing fuel into the engine. in any event if you get to this point the carb would need to come off and be rebuilt.

If you are looking to do some of this yourself but do not have the tools, places like autozone will give you free rentals on the tools. You basically pop a deposit down on a credit card, borrow the tools, and if you return them in 1 piece they give you the deposit back. The last thing is when you troubleshoot you specifically want to do it in this order. If you jump around you will miss things or make mistakes. This is the most time effective way to figure out what exactly is wrong with the engine without wasting time or spending money on parts you do not necessarily need. Does that all make sense. I do want to make sure you are happy with my service before you go. If you had a further 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.

Please note that our question sessions never close out, so if you need to come back later down the road with follow up questions to the issue, you can do so at any time, even after putting through a rating.

Thanks in advance, and good luck
Jason

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Marine Mechanic: Jason, Marine Mechanic replied 8 months ago

It looks like you read my answer and left without selecting a rating for my service, replying back, or simply taking a minute to thank me. Was there something else?

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