# Where is my idle mixture screw?



## m32825

I have a 2013 Mercury 20hp four stroke; the shop manual for it as well. I have cleaned the carb a couple times. Removed a Welch plug to get at the idle mixture screw, so I know where it is on the carburetor, but it's *not* on any of the exploded drawings in the shop manual. What is up with that? Is there a
shop manual supplement dealers get that shows all the "secret parts"?

Maybe I'm just not seeing it in the diagram below. The screw is on the same side of the carb as the linkage. Ideas?

-- Carl


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## Brett

Memory says you have no idle mix adjustment.
What you have is a drilled passageway that uses intake vacuum
to pull fuel through it. The small diameter hole
controls the amount of fuel used at very low rpms.
If the hole clogs or gets varnish build up
that's when you remove the plug to clean with a solvent
and a fine gauge wire.


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## Guest

EPA carb, I believe Brett is correct


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## m32825

Here's the picture I should have posted last night. The screw I'm talking about is in the upper left corner, in the "v" formed by the left end of the choke linkage.

That screw was originally under a Welch plug. When you take the screw out you end up with an adjustable needle/screw, spring, and o-ring. I'm not sure what you call it, but I know it affects idle because if it is clogged up there are idle issues.

I can't find any of those parts on the shop manual diagram posted and I'm trying to understand why. I'd like a diagram of my carburetor, that's one of the reasons I bought the shop manual. Now I'm questioning whether I've got the right thing. Thoughts?

-- Carl


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## Brett

I wonder if that Merc is a rebranded Tohatsu?

Checked the diagram for the 20 hp 2013 hatsu,
it does have a low speed jet.


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## devrep

sounds like an air adjustment screw. Don't lean it out too much.


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## m32825

Brett said:


> I wonder if that Merc is a rebranded Tohatsu?
> 
> Checked the diagram for the 20 hp 2013 hatsu,
> it does have a low speed jet.


I've heard the small Nissan, Tohatsu, and Mercury outboards are all the same, just different packaging. 

Can you post a link to the diagram you found?

-- Carl


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## m32825

Pics of outboard model number and number stamped on float bowl.

-- Carl


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## Brett

https://www.boats.net/catalog/tohatsu/2013/mfs20c-4-stroke-tohatsu/carburetor


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## Guest

I’m not seeing the idle air screw either for this one. I’m thinking epa carb, welch plug, screw not available?


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## RunningOnEmpty

That screw is a part of the carb assembly and not intended to be tampered with. Good ole EPA to screw everything up. You might find the screw part info for a older model that's non EPA


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## Brett

Is that the Merc oem factory manual
or a Clymer - Chilton?

The factory oem manual is better detailed.


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## m32825

I have the service manual, $75 from Mercury Publications.

-- Carl


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## Guest

Just curious, did you damage yours or something? I’ve looked and can not locate it either which leads me to believe it’s an epa thing. You might be able to match up a yamaha idle air screw. I looked back to 08’ for the hatsu, and nissan as well. All the carbs appear to be the same, not air screw.


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## RunningOnEmpty

Yamaha shows the epa carbs with the air screw and the plug. All sold separately


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## m32825

Mine isn't damaged or anything. Just seems to me that when you buy a outboard, and shell out for the manufacturer's shop manual, that the exploded diagram for the carb should show *all* the parts, not just *most* of them. Am I being unreasonable?

I like the "EPA interference" theory. Maybe I ought take my pictures down to the local Mercury place and see what the parts guy can tell me...

-- Carl


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## m32825

Ok, Guy Harvey fans, the rest of the story... Oh, wait, that's Paul Harvey!

Dropped by my Mercury place and talked with the parts guy. He's been doing this for decades, and I've never been able to stump him. He looked through everything he has access to and it's not on any of his diagrams either. If it's not on a diagram, there's no part number, so you can't order it.

He said it's an EPA part. They don't want owners to play with the idle air control because it alters emissions. Stricter emissions standards were phased in starting in the late 90's.

From reading up on it, looks like manufacturers get a certain emissions budget to work with. They have to factor in a certain amount of change over time. By locking down owner adjustments they can argue that those will not change, and that gives them more budget to spend elsewhere.

The parts guy said that sometimes they get an outboard that needs the idle air adjusted to run well. They have to take steps to restore the tamper protection, whether it was there or not when the motor was brought in. If they don't, there can be big fines.

-- Carl


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## MooreMiller

m32825 said:


> Ok, Guy Harvey fans, the rest of the story... Oh, wait, that's Paul Harvey!
> 
> Dropped by my Mercury place and talked with the parts guy. He's been doing this for decades, and I've never been able to stump him. He looked through everything he has access to and it's not on any of his diagrams either. If it's not on a diagram, there's no part number, so you can't order it.
> 
> He said it's an EPA part. They don't want owners to play with the idle air control because it alters emissions. Stricter emissions standards were phased in starting in the late 90's.
> 
> From reading up on it, looks like manufacturers get a certain emissions budget to work with. They have to factor in a certain amount of change over time. By locking down owner adjustments they can argue that those will not change, and that gives them more budget to spend elsewhere.
> 
> The parts guy said that sometimes they get an outboard that needs the idle air adjusted to run well. They have to take steps to restore the tamper protection, whether it was there or not when the motor was brought in. If they don't, there can be big fines.
> 
> -- Carl


Good info! Who’s your Merc parts guy?


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## Guest

Long version of what I was sayin lol! Glad ya got it figured out.


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## m32825

MooreMiller said:


> Good info! Who’s your Merc parts guy?


Roy, at Tracker Marine in Orlando, off of International Drive.


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## Guest

m32825 said:


> Roy, at Tracker Marine in Orlando, off of International Drive.


I like it when someone has something positive to say about us marine mechanics! Seems all I ever hear is bad!


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## m32825

Boatbrains said:


> Long version of what I was sayin lol! Glad ya got it figured out.


Yep, you were dead on. I get a little OCD about things sometimes. I'd apologize, but I know I'm in good company here from seeing some of the bragging spot posts!


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## Guest

No apology needed here, I just like bustin chops sometimes!


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## jimsmicro

If you're having idle mixture problems make sure you cleaned the low speed jet very well. It sometimes takes PHYSICAL removal. Spraying or soaking with carb cleaner isn't enough. It only takes a tiny bit of varnish in the jet to give you idling problems. The mixture screws are usually behind the welch plug to prevent tampering by the user. On my Yamaha it's called a "Pilot Screw" and it's usually towards the top of the carb. It looks like it could be part 6 or 7 on your diagram? Anyway, make sure all of these orifices in the top plate of the carb are SPOTLESS and run some heavy monofilament fishing line through any and all orifices on the carb body, otherwise you will struggle with these issues constantly. Change the gaskets too, you don't want any air leaks.


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## Sroberts

T


m32825 said:


> I have a 2013 Mercury 20hp four stroke; the shop manual for it as well. I have cleaned the carb a couple times. Removed a Welch plug to get at the idle mixture screw, so I know where it is on the carburetor, but it's *not* on any of the exploded drawings in the shop manual. What is up with that? Is there a
> shop manual supplement dealers get that shows all the "secret parts"?
> 
> Maybe I'm just not seeing it in the diagram below. The screw is on the same side of the carb as the linkage. Ideas?
> 
> -- Carl
> View attachment 35424


They were blocked off for EPA ratings


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## DennisVaz

m32825 said:


> Here's the picture I should have posted last night. The screw I'm talking about is in the upper left corner, in the "v" formed by the left end of the choke linkage.
> 
> That screw was originally under a Welch plug. When you take the screw out you end up with an adjustable needle/screw, spring, and o-ring. I'm not sure what you call it, but I know it affects idle because if it is clogged up there are idle issues.
> 
> I can't find any of those parts on the shop manual diagram posted and I'm trying to understand why. I'd like a diagram of my carburetor, that's one of the reasons I bought the shop manual. Now I'm questioning whether I've got the right thing. Thoughts?
> 
> -- Carl
> 
> View attachment 35556





m32825 said:


> Roy, at Tracker Marine in Orlando, off of International Drive.


I have a ? for you unrelated to screw image. The rubber bushing in the center of the photo I know is covering a valve which gas goes thru
when throttle is turned Mine is leaking at that point. Do you know if this valve can be replaced ?


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## m32825

DennisVaz said:


> I have a ? for you unrelated to screw image. The rubber bushing in the center of the photo I know is covering a valve which gas goes thru
> when throttle is turned Mine is leaking at that point. Do you know if this valve can be replaced ?


This part?


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## DennisVaz

m32825 said:


> This part?
> 
> View attachment 184618


Yes, where the rubber bushing is.


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## m32825

The rubber sleeve and what's below it are the accelerator pump. Parts should be available to service it, there's probably a kit that contains the diaphragm and all the parts that could wear out. Where is the fuel leaking and under what conditions?


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## DennisVaz

If the rubber cover is removed, It's leaking from what's the middle of where the rod goes up and down. Looks like s small bushing between the brass and the rod. Gas comes out from the middle of the brass and the rod when I turn the throttle.


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