# merc 20 hp 4 stroke carb issue



## marlattrr (Apr 19, 2021)

I am working on my 2011 Mercury 20 HP 4 stroke w/ carb.

I took the carb off and cleaned it good but did not replace the gaskets, everything looked good.

I put it all together and now the carb is flooding and gas is coming out of the overflow while im trying to start it. I took it apart again and check the float in it appears to be functioning and the need valve looks good to me. 

Could the plunger be the cause of this?


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## marlattrr (Apr 19, 2021)

I did replace a bad prime bulb during the carb cleaning. Not sure if related.


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## nccatfisher (Apr 19, 2021)

Sounds like you are going to have to take it back off. I would pay particular attention to the needle and seat.


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## marlattrr (Apr 19, 2021)

I looked at it and it looked fine and seemed to seal. Im going to just buy the rebuild kit and change all the gaskets and see what happens. 

Could the plunger seal cause this? Thats the only seal that looks suspect. Not ripped but seems to be flat.


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## marlattrr (Apr 20, 2021)

Replaced carb gasket kit and float/ needle valve. Same thing... im at a loss.


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## RaisedByWolves (Apr 21, 2021)

What led you to tear it down to begin with?

Symptoms?

Is your tank vented properly?

You can check the needle and seat by removing and emptying the carb and letting it dry, then with the carb inverted and with a clean piece of hose lightly pull a vacuum on the hose and see if it leaks air.

Or alternately blow into the hose while lifting and lowering the float to see of air stops passing when the float comes to a rest.

Double check the bottom of the float is level with the carb body when held upside-down.


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## marlattrr (Apr 22, 2021)

Thanks I'll try this tonight.


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## marlattrr (Apr 26, 2021)

Still not working. Tried new gas. When i crank the motor fuel starts coming out of the over flow. If i spray carb cleaner in the intake i can eventually get it going with choke halfway on and some throttle but then had no water coming out of the water pump. 

I just replaced the water pump impeller. 

I just replaced the carb gaskets, minus the plunger.

Replaced both spark plugs.

I was going to give up and let the dealer fix it but they are back up 4-5 weeks. :x


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## RaisedByWolves (Apr 26, 2021)

What plunger are you referring to?


Are you running the motor on muffs or in a bucket?


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## marlattrr (Apr 26, 2021)

Running it with muffs. 

The plunger for the prime on the bottom of the float bowl.


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## RaisedByWolves (Apr 28, 2021)

Failing hard


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## RaisedByWolves (Apr 28, 2021)

RaisedByWolves said:


> marlattrr said:
> 
> 
> > Running it with muffs.
> ...


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## marlattrr (Apr 28, 2021)

I'm not even pumping the ball.

I will take the advice and throw away the muffs. 

Everything i see online shows that the only cause for overflowing would be the float needle not seating. It looks like its seating, the needle is brand new and the float is set correctly.

I'm going to try to set the valve clearance. I havent done that since i owned the motor. Maybe that is contributing to the start issues and then fuel is just dumping out of the overflow because its not igniting?

The mercury 20 hp is actually made by tohatsu. So its a tohatsu 20hp.


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## RaisedByWolves (Apr 28, 2021)

I doubt your valve issues have anything to do with it.

By ball I meant the one on the fuel hose.


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## marlattrr (Apr 28, 2021)

Yeah the one on the fuel hose is always stiff like there's already pressure in the line. I barely ever have to squeeze it.


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## marlattrr (Apr 28, 2021)

carb


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## RaisedByWolves (Apr 28, 2021)

marlattrr said:


> Yeah the one *on the fuel hose is always stiff* like there's already pressure in the line. I barely ever have to squeeze it.



Can you try a different tank/hose?

That seems off.


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## marlattrr (Apr 28, 2021)

Does my gas line have a valve that keeps it pressurized? It always seems primed whem i put it on. I don’t usually even squeeze the pump.


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## turbotodd (Apr 29, 2021)

leaky bowl gasket can cause overflowing.


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## marlattrr (Apr 29, 2021)

Thanks. I did replace that.


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## ClemsonAlum (Apr 29, 2021)

1. Motor not peeing: Check to make sure the pee hole is clear. Dirt dobblers and other bugs will stop it up in no time flat. Run a wire or stick up it and spray a little water up in it.

2. Only starts on choke with carb cleaner: I have only worked on my 2 stroke, but if it was me, I would take the crab apart and clean again to make 110% sure the idle jet passage was clear. Soak with carb cleaner, run thin wire through all passages, clean again with carb cleaner spray (spraying the OPPOSITE direction of fuel flow) , and use compressed air. 

-Kurt


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## marlattrr (Apr 29, 2021)

Okay progress made tonight. 

Water pump works great. No issues there. Did the bucket trick. 

Used a new fuel tank i had with a new line. Still overflows the carb. 

I can start it with choke on and squirting in carb cleaner. It will run if i turn the idle screw up and it runs at very high idle. If I turn the screw down to a normal idle it dies.


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## RaisedByWolves (May 1, 2021)

Are you using a tach or just going by ear?


I don’t think the carb cleaner is a good starting fluid, it actually might be bad for your engine, ie no oil.


Try to find a small squeeze bottle to dribble gas into the carb instead.

This can also help determine a lean condition via squirting fuel into the carb when it stumbles to see if it will respond.

I have never seen a tank like that, but IMO it shouldn’t be pressurized.


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## marlattrr (May 3, 2021)

Thanks for all the help. The “slow jet” was definitely clogged. I cleared it up. It runs much better but still not quite there. Will idle off of choke now and it’s restarting. 

When i roll to the throttle it bogs down and dies if i do it too fast. If i bring the throttle on slow it will stay running. Seems like i really need to open it up on the water. 

Still dumps gas while its running though. :?:


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## RaisedByWolves (May 4, 2021)

marlattrr said:


> Thanks for all the help. The “slow jet” was definitely clogged. I cleared it up. It runs much better but still not quite there. Will idle off of choke now and it’s restarting.
> 
> When i roll to the throttle it bogs down and dies if i do it too fast. If i bring the throttle on slow it will stay running. Seems like i really need to open it up on the water.
> 
> Still dumps gas while its running though. :?:



That’s a headscratcher, but the dumping fuel is why it’s stumbling when you open the throttle.

It’s loading up and there is extra fuel getting sucked out of the crankcase with the sudden intake rush.

Go back to basics, float height, needle seat, maybe find a breakdown of the carb and go through it piece by piece.


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## MrGiggles (May 4, 2021)

Quick troubleshooting tip, when it's acting up, bump the choke. If it takes off, not enough fuel, if it bogs worse, too much.


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## marlattrr (May 4, 2021)

Tried again. No luck. It has to be my float. The motor is running now but fuel still dumping. 

Is there any way I could of knocked the float level out? This one doesnt seem adjustable. The service manual says replace float if out of spec. This one and the needle are brand new. 

Theres also a brass seat where the needle valve sits. Is there a way to get that out?


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## ClemsonAlum (May 4, 2021)

Yes, that needle seat should screw out. Use a large flathead that fits, or you will strip the brass. Clean it good to be sure there is no crud keeping the needle from seating right. Look at the rubber tip on the needle. Make sure there is no groove warn into it. 

-Kurt


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## marlattrr (May 4, 2021)

The float needle seat on the right doesnt have a flat groove for a screw driver


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## RaisedByWolves (May 5, 2021)

marlattrr said:


> Tried again. No luck. It has to be my float. The motor is running now but fuel still dumping.
> 
> Is there any way I could of knocked the float level out? This one doesnt seem adjustable. The service manual says replace float if out of spec. This one and the needle are brand new.
> 
> Theres also a brass seat where the needle valve sits. Is there a way to get that out?




You may have a waterlogged float.

I don’t think that one is meant to come out, but it just might be missing the o-ring for the needle to seat on.

Double check your carb diagram.


Edit: for whatever reason the carb diagrams you posted weren’t showing on my phone.

Does not appear to use an o-ring.


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## marlattrr (May 6, 2021)

I dont see a way to adjust the float height on this thing either. 

Im stuck.


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## MrGiggles (May 6, 2021)

Try holding the carb upside down and blow into the fuel inlet. Does it seal with gravity pulling the float down? 

You can also lightly clamp the carb in a vice and gravity feed it with a bottle.

If the float was waterlogged you would be able to tell by shaking it, there should be no liquid inside. It's not on upside down is it?

That seat is not replaceable, appears to be pressed into the carb body.


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## marlattrr (May 6, 2021)

It does seal with gravity when i hold it upside down. Needle is brand new and float brand new. 

It fires right up now and will run but only with the idle up high. It runs better with the choke on. With choke on or off it dumps fuel out the overflow. 

Seems like it would run the water but id constantly be dumping fuel.


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## RaisedByWolves (May 6, 2021)

Is the float level with the carb body when the carb is held upside down?

The method to adjust a float is to just bend the little metal pivot tab that connects the float to the pivot.


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## ClemsonAlum (May 6, 2021)

What he said ^ . Also is the float needle attached to the float with a mini cotter pin? Maybe needle is getting stuck in up position? Not sure if those carbs have the little pins?

-Kurt


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## marlattrr (May 6, 2021)

Its definitely level. I checked it. 

There is no metal tab but when upside down i can blow into the fuel inlet and cant get air through. 

Its not even a slow drip of gas when running its a constant drip.


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## marlattrr (May 6, 2021)

Im almost ready to just buy a new carb.


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## marlattrr (May 6, 2021)

Could something outside the carb like my fuel pump cause this?


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## marlattrr (May 6, 2021)

What about the O-ring on my main jet?


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## marlattrr (May 6, 2021)

Ordered a new carb on amazon for 108 bucks. At this point ive invested far too much time to keep going. :|


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## ClemsonAlum (May 6, 2021)

Hmm. Since you said the slow speed jet was clogged and you cleaned it, and it ran better, I bet the high speed jet or passage is partially clogged still. Don't know how else to explain running better with choke on at high idle, with the bowl full of fuel, with fuel getting purged out. Assuming you are 100% sure that it is leaking out of the overflow hole. Run mono fishing line in there ? Or a get a carbuerator brush set? Even if the fuel pump was somehow giving too much fuel, you would think the float/needle would seat when the bowl got full and it would run without choke?

-Kurt


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## marlattrr (May 7, 2021)

I ran a paper clip in and out. Sprayed carb cleaner and air through it. I can see through the hole of the main jet. 

Thanks for sticking with me guys!


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## RaisedByWolves (May 7, 2021)

ClemsonAlum said:


> Hmm. Since you said the slow speed jet was clogged and you cleaned it, and it ran better, I bet the high speed jet or passage is partially clogged still. Don't know how else to explain running better with choke on at high idle, with the bowl full of fuel, with fuel getting purged out. Assuming you are 100% sure that it is leaking out of the overflow hole. Run mono fishing line in there ? Or a get a carbuerator brush set? Even if the fuel pump was somehow giving too much fuel, you would think the float/needle would seat when the bowl got full and it would run without choke?
> 
> -Kurt



Something isint adding up.

Im wondering about the carb gasket or a possible manifold leak at this point.

This could easily be more than one problem.


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## marlattrr (May 7, 2021)

I replaced the gaskets.

The only thing I didn't replace is the accelerator pump assembly.


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## nccatfisher (May 7, 2021)

marlattrr said:


> I replaced the gaskets.
> 
> The only thing I didn't replace is the accelerator pump assembly.


^^^^^^^ I am assuming what you are describing here is the choke?


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## RaisedByWolves (May 7, 2021)

marlattrr said:


> I replaced the gaskets.
> 
> The only thing I didn't replace is the accelerator pump assembly.



How tight did you tighten the bolts?


A little counterintuitive, but over tightening the bolts can also cause a leak, even on a new gasket as it deforms the mounting surface deforming the gasket.


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## RaisedByWolves (May 7, 2021)

nccatfisher said:


> marlattrr said:
> 
> 
> > I replaced the gaskets.
> ...



Im thinking primer, which could cause issues he described.


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## nccatfisher (May 7, 2021)

Im thinking primer, which could cause issues he described.
[/quote]Yep, I am still old school and call them chokes but you are correct. I was wondering if that may be the root of his evil. I know if you don't fully close/shut them off they will allow air in the system and cause all kind of problems.


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## marlattrr (May 10, 2021)

how do i adjust or check that primer? i backed off the nut so that its not engaging at all and i still have the problem


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## RaisedByWolves (May 11, 2021)

marlattrr said:


> how do i adjust or check that primer? i backed off the nut so that its not engaging at all and i still have the problem



Not being familiar with that motor I couldn't hazard a guess.

You could try and disassemble it and look for a torn o-ring or diaphragm etc.

Something causing your issues should stand out.


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## marlattrr (May 11, 2021)

New carb from Amazon and shes purring like a kitten again. 

Idk whats wrong with the old carb but that did it.


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## RaisedByWolves (May 13, 2021)

Well, at least we were on the right track. :wink:


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## marlattrr (May 13, 2021)

Thanks for all the help everyone. 

By the way-wolves. Im just one county north of you in Lehigh.


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