Pilot Screw

[evinrude25]

Question, On 1992 40hp (40MJHQ) does the pilot screw control air, fuel or both?

[boscoe99]

Question, On 1992 40hp (40MJHQ) does the pilot screw control air, fuel or both?

Both.

Air is introduced into the flow of fuel through the carburetor to facilitate the flow of that fuel. What crosses the idle air mixture screw is a combination of fuel with air bubbles in it.

Turning the idle mixture screw outward allows more air and fuel to flow to the motor with the net effect being that the mixture entering into the combustion chamber is richer than it otherwise would be.

Wish the illustration below was a bit more clear but hopefully it will help with your understanding of how the carburetor functions.

[rodbolt17]

neither.
the A/F ratio is fixed by the pilot air and pilot fuel jets.
all the pilot air screw does is allow more or less of the mix to pass.

[boscoe99]

neither.
the A/F ratio is fixed by the pilot air and pilot fuel jets.
all the pilot air screw does is allow more or less of the mix to pass.

Absolutely correct.

As the screw is turned outward more of the fuel and bleed air mix pass through. As the screw is turned inward less of the fuel and bleed air mix pass through.

Net result is that the engine will run richer with the screw turned further outward and leaner with the screw turned further inward.

Which is why on many motors it is referred to as the idle mixture screw. Not a pilot screw.

In an aviation motor with an idle cut off valve, the change in mixture with the change to the screw can be readily seen. Richer mixture will cause a greater rise in idle RPM before the motor starts to die. Leaner mixture will cause a lesser or no rise in idle RPM before the motor starts to die.

Our resident aviation guru Mr. Panasonic probably knows all about this. If he were paying attention in P school that is.

[99yam40]

Absolutely correct.

As the screw is turned outward more of the fuel and bleed air mix pass through. As the screw is turned inward less of the fuel and bleed air mix pass through.

Net result is that the engine will run richer with the screw turned further outward and leaner with the screw turned further inward.

Which is why on many motors it is referred to as the idle mixture screw. Not a pilot screw.

In an aviation motor with an idle cut off valve, the change in mixture with the change to the screw can be readily seen. Richer mixture will cause a greater rise in idle RPM before the motor starts to die. Leaner mixture will cause a lesser or no rise in idle RPM before the motor starts to die.

Our resident aviation guru Mr. Panasonic probably knows all about this. If he were paying attention in P school that is.

strange all of the small motors(2 strokes) tend to speed up when leaned out because they tend to idle a little rich with no load

[boscoe99]

strange all of the small motors(2 strokes) tend to speed up when leaned out because they tend to idle a little rich with no load

They are made to run a bit rich for driveability. A motor that is not a tad on the rich side with stumble and/or die if and when the throttle gets goosed from idle.

Actually, most outboard motors, and certainly Yams, run very rich throughout the entire power range. Not sure if that is part of the "making oil" issue or not.