1980s Yamaha/Mariner carburetor adjustment

[mulla]

I've disassembled the carb on my mariner 6a1 from -89 and I'm curious how I would go about tuning it.
I've linked the carb diagram and serial number below.

What I'm wondering is if the screw marked as "Pilot Adjusting" changes the fuel mixture?
From what I can tell it just changes the lowest point of the throttle valve so it should not alter the mixture right?
I know that the needle has an e-clip that roughly changes the mixture but it says in the manual that it's not to be touched.
Anyone that has experience with these small carbs that can shed some light on this?

6A1-14301-03-00
CARBURETOR - 1989 Yamaha Outboard 2hp 2SF | Crowley Marine

 

[RichardS]

It could be a screw that alters the mixture at low revs or, as you say, it could be a screw to adjust the idle speed. You should be able to tell by looking at what it does but if all it does is hold open the throttle plate a bit more or less than, as you say, it is not a mixture screw.

I would have expected it to be called Idle Screw or Throttle Screw which is part no 4 so perhaps check what that does.

Richard

 

[VicS]

You should find the answers to your questions in the manual at http://www.boatinfo.no/lib/yamaha/manuals/1984-1996yamaha.html#/0

Refer to chapter 2 for the tuning instructions and chapter 3 for the carb overhaul instructions.

ITYWF that the screw you refer to varies the air flow, as opposed to fuel flow through an orifice. I assume from what you say that it does this by adjusting the height of the throttle valve and needle
Note the advice to record its setting ( number of turns out from gently seated) before dismantling .
I gather a new screw is supplied in the carb overhaul kit.

 

[mulla]

It could be a screw that alters the mixture at low revs or, as you say, it could be a screw to adjust the idle speed. You should be able to tell by looking at what it does but if all it does is hold open the throttle plate a bit more or less than, as you say, it is not a mixture screw.

I would have expected it to be called Idle Screw or Throttle Screw which is part no 4 so perhaps check what that does.

Richard

Yep indeed the throttle screw(part number 4) seems to do the same thing as the "pilot screw" that's why I'm confused.

You should find the answers to your questions in the manual at http://www.boatinfo.no/lib/yamaha/manuals/1984-1996yamaha.html#/0

Refer to chapter 2 for the tuning instructions and chapter 3 for the carb overhaul instructions.

ITYWF that the screw you refer to varies the air flow, as opposed to fuel flow through an orifice. I assume from what you say that it does this by adjusting the height of the throttle valve and needle
Note the advice to record its setting ( number of turns out from gently seated) before dismantling .
I gather a new screw is supplied in the carb overhaul kit.

Looking at chapter 2 I can't find anything that relates to changing the fuel mixture on the 2hp model, might be because it's late here ?
It's weird because it says on page 2-48 that "idle speed is not adjustable on these models" referring to the 2hp from 84-94, but from what i can tell it's adjustable both with the "pilot screw" and the "throttle screw" that RichardS mentioned.

I have recorded the pilot-screw setting when dismantling the engine and it ran well before so I guess it should all be good when I reassemble it, but I'm still curious about how it's supposed to be tuned.(if it there even is a way)

 

[VicS]

Yep indeed the throttle screw(part number 4) seems to do the same thing as the "pilot screw" that's why I'm confused.
Looking at chapter 2 I can't find anything that relates to changing the fuel mixture on the 2hp model, might be because it's late here ?
It's weird because it says on page 2-48 that "idle speed is not adjustable on these models" referring to the 2hp from 84-94, but from what i can tell it's adjustable both with the "pilot screw" and the "throttle screw" that RichardS mentioned.

I have recorded the pilot-screw setting when dismantling the engine and it ran well before so I guess it should all be good when I reassemble it, but I'm still curious about how it's supposed to be tuned.(if it there even is a way)

The instructions for the same carb on a 2hp Mariner badged version says to adjust the idle screw to give an idle speed of 900 to 1000 rpm, using a tachometer

I did not think #4 was an adjustment. Does it not screw down tight to hold the bracket #3

 

[mulla]

The instructions for the same carb on a 2hp Mariner badged version says to adjust the idle screw to give an idle speed of 900 to 1000 rpm, using a tachometer

Same is written on page 2-46 in the Yamaha manual, although the recommended rpm is 1000-1100, but still this only changes the idle speed and not the fuel mixture. What I'm looking for is the equivalent of the high/low adjustment screws that are present on bigger carbs. I found an old thread on another forum where one guy said that that the "pilot screw" indeed acted as a mixture screw, sadly half of the exchange was missing so I don't know if it's true and how it would work if it was true.

I did not think #4 was an adjustment. Does it not screw down tight to hold the bracket #3

Hmm depends, on my carb it was not fully seated but I guess it's more logical that is supposed to hold down the bracket now that I think about it.