F60 - adjustment of A/F screws

[boscoe99]

While Yamaha calls these air screws, I'd suggest they are fuel screws. I say that because turning counter clockwise makes things better - thus meaning that it lets more fuel in.

The screws are allowing a mixture of both air and gasoline to flow through. It is a very, very rich mixture. A small amount of air when compared to the gasoline.

More of that same air/gasoline mixture, when added to the rest of the air that is flowing through the carburetor, will result in a richer overall mixture that is in the combustion chamber when it is time for the spark plug to do its magic.

I like to call them idle mixture screws because that is what they change when they are changed.

[boscoe99]

Full disclosure, I had no knowledge of a carb tuning device. Did some research after reading your post and it seems pretty nifty. Might be time to add another device to my tool box.

You stated you tweaked the jets by turning counterclockwise. I'm a little confused. Are you saying you opened the brass plug for carb 1 and 4 and turned the air adjusting screw?

I looked at your engines carb schematic below so I could follow along to your detail. Did you open plug #40 and adjust screw #20?

2004 F60TLRC Yamaha Outboard CARBURETOR Diagram and Parts

A real mechanic is going to have this in his tool box. A real mechanic will possess more tools than talent. To steal a line from seahorse.

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With respect to the illustration linked to above, plug #40 is removed so as to gain access to idle mixture screw #20. One for each carburetor. The plug was added in later year models at the direction of the EPA. It is supposed to prevent backyardigans like you and me from making the engine run better.

[boscoe99]

Well, I got my Carbtune directly from the manufacturer in Ireland yesterday. Today I connected it up - and sync'd my carbs. Now they are in good balance - but I still had a bit of a lean sneeze.

But, an added bonus of the carburetor synchronizer is that it showed very clearly which cylinder (in my case cylinders) were sneezing. I slowly tweaked the jets for each (counter clockwise to richen) and got rid of the sneeze.

Here's the interesting part for me. I removed the brass plugs - so I know that part of the carbs was never touched. They were 1.5, 3+, 3.25, 1.25 respectively for cylinders 1 through 4. And yup, it was 1 and 4 that were sneezing. I can believe the difference (from the factory) between the carbs.

But, the engine now idles up smoothly with no sneezing. I'll get it out on the water soon and hope it goes right up to WOT nicely.

So, if you have a lean sneeze, a carburetor synchronizer can show you which carb(s) are lean!

What did you see on the carbtune that indicated to you that numbers 1 and 4 were lean? Lower pressure, higher pressure or...?

The idle mixture screw setting for all four F60 carburetors is 2 3/4 turns plus or minus 1/2 turn. I doubt that what you saw was the factory setting. Some backyardigan mechanic apparently has been in your motor but replaced the EPA plugs with new ones.

[exeter27]

The carbtune showed a nice even balance across the carbs as indicated by the rods. When a sneeze happened, you would see the rod for the cylinder drop. When I saw that, I'd enrichen the screw by 1/4 CCW - and then repeat until I could no longer reproduce the sneeze. Both 1 and 4 were lean and sneezed.

Based on your information, it's clear that 1 and 4 were well out of spec. And, it explains why they were sneezing. I suppose it is possible that someone removed the original brass plugs, did some messing around with the fuel mix screws - and then replaced with new brass plugs. I'm not replacing them - but no EPA police in sight in my neighborhood!

[99yam40]

Good chance a Yamaha shop messed with them and then installed new plugs.
they are required to replace the plugs after removing them right.
just does not make sense why they would have miss adjusted the 2 screws tho

[99yam40]

If it happened at the factory the dealer that installed and set up the motor would have found the problem when they ran it.

it has to have run Ok for years ,it is a 2004

[99yam40]

As I said I am fairly sure Yamaha techs are required to reinstall after cleaning to keep non authorized people from easily dinking with them.
now I guess someone else could have put new ones in for some reason we do not understand.
dealer prep and check out for warranty to be activated should catch bad running motor.
They would not take anodes out to check for gaskets tho

[Jason2tpa]

With respect to the illustration linked to above, plug #40 is removed so as to gain access to idle mixture screw #20. One for each carburetor. The plug was added in later year models at the direction of the EPA. It is supposed to prevent backyardigans like you and me from making the engine run better.

Ok, now I'm following. It had me scratching my head as to why his carb air screws (aka idle mix screw) were plugged. For moment I was thinking he was adjusting "jets", such as the pilot or main. But I had thought this wasn't possible, unless replacing jet with a jet of different size orifice.

And I agree, I must have a carb tuner. I'm kinda anxious to get one and find out how out of wack my four barrels are.

[Jason2tpa]

Jason, make sure the motor you want to use it on has a place to hook it to, before buying it.
not all motors have places to hook it up to

I was already ahead of you.. I looked at my intake schematic and couldn't figure out where the four hoses would plug into. Tho these schematics are pretty grainy. Take a look. #7?

1998 S115TLRW Yamaha Outboard INTAKE Diagram and Parts

[99yam40]

Might need to see where those hoses go to
not many 2 stroke outboards have places to hook up vacuum gauges

[Jason2tpa]

Might need to see where those hoses go to
not many 2 stroke outboards have places to hook up vacuum gauges

Did you look at the schematic? Hard to see, but with your experience you might be able to identify where the intake connection is made to the hose by the name of the part, identified on the right side of web page.

[Jason2tpa]

Boscoe must have a super duper engine schematic that details whether or not my engine intake has ability to attach vacuum hose......

[panasonic]

How about taking a couple of pics of your carbs from different sides, angles and we can use our super duper eyes to let you know...lol

Seriously a couple of pics would be good. The ports generally are a small brass nipple on each carb or intake manifold that has a rubber cap on them. Take the cap off to connect vacuum gages.

Have not seen many on two stroke engines though.

[Jason2tpa]

Something else I've been meaning to ask that is related to this thread. Is the "air adjusting screw" on my carb actually a screw that meters air or fuel? Because I'm a little confused.

I had read that if the mix screw on the carb is located on the side of the intake manifold it's a fuel adjusting screw, therefore it meters fuel. If the mix screw is located on the side of the air intake (silencer) it's an air adjusting screw, therefore it meters air.

#20 in this schematic is identified as the air adjusting screw, but it's located on the side of the intake manifold. Hope someone can clear this one up for me.

1998 S115TLRW Yamaha Outboard CARBURETOR Diagram and Parts


Here is a pic of my carb, red arrow pointing to screws in question. Intake on left, silencer on right.

[Jason2tpa]

How about taking a couple of pics of your carbs from different sides, angles and we can use our super duper eyes to let you know...lol

Seriously a couple of pics would be good. The ports generally are a small brass nipple on each carb or intake manifold that has a rubber cap on them. Take the cap off to connect vacuum gages.

Have not seen many on two stroke engines though.

Next trip out to the boat I snap a few pics.