fuel screen

jpcody66

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May 10, 2005
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Mercruiser 120 bogs when throttle is opened up. I have read about a screen that could be clogged which restricts the flow of fuel to the carburetor. The engine idles perfectly but bogs severely. Just rebuilt the carburetor and replaced all electronics and fuel filters. I didn't look into the fuel tank as it is buried under the floor. I can cut a "door" into the floor and place a hinge on it to provide access to the tank. My question is, is there a screen in the fuel tank that could be clogged which is restricting flow to the fuel line? If so, where would the screen be and how can I clean it?
 

Haut Medoc

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Re: fuel screen

Yes, there is a screen at the bottom of the pick up. Most boats have an access, but if you don't you could cut one as a last resort. But first there are other things to look at.....For instance, will it bog with the gas cap off? Have you checked the anti-siphon valve? Completely draining the fuel tank might help. How old is the tank?...And lastly, who did the re-build on the carb?......JK ... P.S. I would check all the other options before I cut into the deck...IMHO
 

jpcody66

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Re: fuel screen

The screen is on the "inside" of the tank correct? The boat is a 1978 and I imagine that the tank has never been removed or cleaned out. I rebuilt the carburetor. I installed the floor also, so pulling it up or cutting a hatch into it doesn't concern me all that much. I didn't check if it bogs with the cap off but it would be another trip to the lake to do that now. It doesn't bog with the muffs on it, just when I have it in the water. When I was putting gas into the tank I had to pour it from the pump really slow. Otherwise the pump would click off or gas would shoot out the fill hose. That's why I'm thinking it's a clog in the tank. Does this sound logical?
 

Haut Medoc

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Re: fuel screen

Could be...& yes, the pickup screen is inside the tank....An easy trick would be to run it off of a portable gas can & see what that does for you. If it runs good that would at least rule out a carb problem & narrow the problem down to the vent, pick-up screen, anti-siphon valve ....JK
 

jpcody66

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Re: fuel screen

If the screen inside the tank is dirty how do I clean it out? I'm sure it involves draining the tankbut then what can I do? Is there room to work with or does the tank need to be completely removed? Complete removal would be a big job since it's mainly under fiberglass except for about a 1'x1' hole. By the way, my gas gauge reads empty on a full tank of gas then rises to 1/4 after about 5 gallons are used then back down to empty after a couple more gallons are used. Strange!
 

Haut Medoc

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Re: fuel screen

The pick-up can be removed. There should be no need to remove the tank. Follow the fuel line from the transom...Just unbolt it....The sender may need to be replaced too, that is weird....Remember that you are playing with gas ....It is good practice to dis-connect the negative battery cable to prevent any sparks from occuring....Make sure you keep the work-space ventilated....JK
 

Haut Medoc

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Re: fuel screen

What you use to clean the screen will be determined by its composition.....JK
 

vipzach

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Re: fuel screen

Originally posted by J.kopec:<br /> Yes, there is a screen at the bottom of the pick up. Most boats have an access, but if you don't you could cut one as a last resort. But first there are other things to look at.....For instance, will it bog with the gas cap off? Have you checked the anti-siphon valve? Completely draining the fuel tank might help. How old is the tank?...And lastly, who did the re-build on the carb?......JK ... P.S. I would check all the other options before I cut into the deck...IMHO
JK, he can't check the anti-syphon valve, it is under the floor, attached to the pickup tube. ;) :D The other thing to check before cutting the floor is to check the timing, he said he replaced all of the electronics, but it the timing correct, is it advancing? Just a thought!
 

Haut Medoc

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Re: fuel screen

"V", Right you are, not sure what I was thinking.... :p I have never owned a boat that didn't have a deck fitting for the pick-up tube...I assumed that all of the things electric were done properly = timing, wires, etc... I guess the real question is what were the symptoms that lead to the carb being re-built in the first place....And the portable gas tank test should eliminate some stuff...One way or the other ;) ....JK
 

jpcody66

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Re: fuel screen

The carburetor was very bad. The jets were caked with crud and the float was rotted from water contaimination. It barely idled before sputtering and coughing and stalling. I replaced the coil, wires, plugs, points, condensor, rotor and cap. I've played with the dwell, points gap and timing and also replaced all the fuel filters and water separator. I'm thinking I either have a dirty fuel screen inside the tank or a bad fuel pump. I'll give it the portable gas can trick before tackling the tougher stuff.
 

vipzach

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Re: fuel screen

Originally posted by JCody:<br /> The carburetor was very bad. The jets were caked with crud and the float was rotted from water contaimination. It barely idled before sputtering and coughing and stalling. I replaced the coil, wires, plugs, points, condensor, rotor and cap. I've played with the dwell, points gap and timing and also replaced all the fuel filters and water separator. I'm thinking I either have a dirty fuel screen inside the tank or a bad fuel pump. I'll give it the portable gas can trick before tackling the tougher stuff.
Did you set the dwell with a meter or just play with it? If you don't have a fuel/water seperator I would install one.
 

jpcody66

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Re: fuel screen

I just played with the dwell. My meter broke when I was going to use it. How sensitive is this Dwell? <br />I did install a water separator filter.
 

Haut Medoc

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Re: fuel screen

Sensitive enough! Get yourself a new meter, & do it right.....Gap, .022.....Dwell 31*-34*. You didn't post the year of your motor, but those #'s should be right....I would check my water separator now, dump it out in a clear glass container & see what you have....If your carb was in such bad condition, what about the gas? It will go bad......If you don't know how old it is....You should probably drain it any way....JK
 

jpcody66

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Re: fuel screen

The gap and dwell measurement that you listed looks correct. The boat is a 1978 Four Winns and the motor is a 120 Mercruiser. I had previously drained the tank, changed the water separator filter and put new fuel in with dry gas. The old gas did have water in it. I'll get a new meter too.<br />Thank you for your help and suggestions!
 

vipzach

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Re: fuel screen

Originally posted by JCody:<br /> I just played with the dwell. My meter broke when I was going to use it. How sensitive is this Dwell? <br />I did install a water separator filter.
The merc 4 cylinders are real picky when it comes to timing, they need to be dead on, or they will be sluggish.
 
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