'98 115hp 2 stroke Idle Adjustment

[boscoe99]

To quote the Beach Boys

"wouldn't it be nice"

if a Yamaha SM contained detailed information as to how a specific carburetor works on a specific model outboard motor?

I found this on the inner web. Makes a confusing subject much more understandable. http://www.tillotson.ie/docs/techinfo_HS_US.pdf

NOTE: This is not indicative of how any Yam carburetor works. It is to show what Yamaha should have been doing/could have been doing to help their customers.

[Jason2tpa]

Big thanks guys for all the input. As soon as this cold weather backs out of tampa ill check and make adjustments and let yall know the results.

[boscoe99]

most Yamaha carbs use fixed jetting for air and fuel.
once its fixed its simply fixed.
the A/F screw simply allows more or less of the FIXED A/F mix into the intake.

simply study the carb design.
it aint hard.

Been studying the Yamaha carburetor as you suggested.

As you say, the idle A/F screw simply allows more or less of the FIXED A/F mix into the intake. However, the fixed air/fuel mix that is being allowed in via the idle mixture passages is very very rich. More than likely richer than .1 to one air to fuel. It is more gasoline than it is air.

So, as the idle mixture screw is turned outward the net effect is that the air/fuel ratio that the engine burns will be richer than it otherwise will be. If the screw is turned inward the air/fuel ratio that the engine burns will be leaner.

That's my story and I'm sticking to it.

[boscoe99]

Big thanks guys for all the input. As soon as this cold weather backs out of tampa ill check and make adjustments and let yall know the results.

Yes. Let us know. Thanks.

[99yam40]

Been studying the Yamaha carburetor as you suggested.

As you say, the idle A/F screw simply allows more or less of the FIXED A/F mix into the intake. However, the fixed air/fuel mix that is being allowed in via the idle mixture passages is very very rich. More than likely richer than .1 to one air to fuel. It is more gasoline than it is air.

So, as the idle mixture screw is turned outward the net effect is that the air/fuel ratio that the engine burns will be richer than it otherwise will be. If the screw is turned inward the air/fuel ratio that the engine burns will be leaner.

That's my story and I'm sticking to it.

this reminds me of the Prime start system on my C40. It sucks air in from the middle carb vent to mix with the fuel that is pulled through the needle and seat while cold. I thought that was strange that it was not solid fuel.

And I agree with you on that turning pilot screw CC making mix in combustion chamber richer,
If the only air being drawn into the cylinder during idle came from the idle air jet, then it would be different

[boscoe99]

this reminds me of the Prime start system on my C40. It sucks air in from the middle carb vent to mix with the fuel that is pulled through the needle and seat while cold. I thought that was strange that it was not solid fuel.

And I agree with you on that turning pilot screw CC making mix in combustion chamber richer,
If the only air being drawn into the cylinder during idle came from the idle air jet, then it would be different

Goes back to the physics that a bit of air added to the flow of fuel can improve the flow of fuel. Seems counter intuitive at first until the theory is understood.

People talk about carburetors being dumb and/or simple but from a physics standpoint they can be incredibly complex and quite sophisticated. Engineering marvels they be.

[rodbolt17]

think as you wish.
but even the picture shows it.
to physically change the idle A/F ratio you either have to change the pilot air jet or change the pilot fuel jet.

member the old 90*looper and crossflow jonny/rudes that used NO a/f screws?

I cant tell you how many I fixed the lean sneeze by decreasing the idle air jet.
sometimes you could figure out which cyl was lean using a tapered tooth pick in the jet.

on the jonny rudes there was no way of adding more or less, you could only change the idle air jet,the idle fuel jet was a pressed in placed crimped brass tube.

[Jason2tpa]

my idle adjustment was a success. Only had to turn it out maybe an 1/8th of a turn. As I had initially posted, I only needed to lower the rpms by 100-150. But by the looks of this thread I think I inadvertantly began a war of knowledge between you masters lol. I'll keep reading what y'all post, but the job is done. Much appreciated as always...boscoe, rodbolt, and 99yam!

Yes. Let us know. Thanks.

[99yam40]

I guess if the throttle plate was touching the walls when at idle and there was a hole in throttle plate letting only a certain amount of air in, I could under stand that.
But depending on how far you have the idle speed screw turned you are holding open the throttle plates open some amount. the more you have the screw turned in the more air you have coming by the plates.
To compensate for how far the plates are open you would need to add more of the mix with the pilot screw.

Or maybe the more you open the plates the change in pressures already compensates the amount of fuel being sucked in.
I am not an engineer and probably do not understand all of the forces involved in this.
My head hurts now, so I will drink another beer