2003 F90 Dasphot/Diaghragm ???

[newboater]

What is the purpose of the dashpot/diaphragm and the check valve that feeds all 4 carbs?

I thought they were supposed to provide for smooth deceleration, to keep the motor from cutting off, when you pull back quickly on the throttle.

I noticed when I snap the accelerator it sends a puff of gas up the main jet into the throat of the carburetor. That would seem to provide a little boost for acceleration. Is that normal or is there something wrong with the one way valve?

[boscoe99]

Maybe more than you wanted to know but...

The F90 carburetor assembly uses an accelerator pump to ensure that the proper amount of fuel reaches all of the carburetors during rapid throttle
openings, preventing temporary lean conditions. As the throttle is opened, the throttle lever rotates and pushes the throttle lever link rod. The throttle
lever link rod then pushes the diaphragm in the dashpot, forcing out the air. The air flows into the carburetors and mixes with the fuel from the main jets. This air/fuel mixture is then injected into the venturi, ensuring that enough fuel reaches the engine.

As the throttle is closed, the throttle lever rotates back and the throttle-lever link rod returns causing the diaphragm in the dashpot to move back. As the diaphragm moves back, suction causes the air from the venturi of carburetor #4 to flow through a one-way valve in the carburetor and back to the dashpot. The in-line, one-way valve prevents the air in the hoses attached to the carburetors from flowing back to the dashpot. This also prevents fuel from flowing back into the air hoses attached to the carburetors.

Also, the diaphragm functions as a dashpot during quick deceleration preventing the engine from stalling. A restriction in the one-way valve at the carburetor prevents the air from entering the dashpot too rapidly. This in turn slows the closing action of the throttle preventing engine stall when
decelerating.

[newboater]

Thank You boscoe99. I appreciate all the info. I'm trying to learn all I can. It's a never ending task though.

[boscoe99]

Never stop learning. The more that one knows about his motor the better that motor will treat him.

There is more being invented faster than I can learn about any of the old stuff.

[99yam40]

The old saying was if you stop learning your are dead, so it is good to learn something every day

[TownsendsFJR1300]

Maybe more than you wanted to know but...

The F90 carburetor assembly uses an accelerator pump to ensure that the proper amount of fuel reaches all of the carburetors during rapid throttle
openings, preventing temporary lean conditions. As the throttle is opened, the throttle lever rotates and pushes the throttle lever link rod. The throttle
lever link rod then pushes the diaphragm in the dashpot, forcing out the air. The air flows into the carburetors and mixes with the fuel from the main jets. This air/fuel mixture is then injected into the venturi, ensuring that enough fuel reaches the engine.

As the throttle is closed, the throttle lever rotates back and the throttle-lever link rod returns causing the diaphragm in the dashpot to move back. As the diaphragm moves back, suction causes the air from the venturi of carburetor #4 to flow through a one-way valve in the carburetor and back to the dashpot. The in-line, one-way valve prevents the air in the hoses attached to the carburetors from flowing back to the dashpot. This also prevents fuel from flowing back into the air hoses attached to the carburetors.

Also, the diaphragm functions as a dashpot during quick deceleration preventing the engine from stalling. A restriction in the one-way valve at the carburetor prevents the air from entering the dashpot too rapidly. This in turn slows the closing action of the throttle preventing engine stall when
decelerating.

Interesting (and very helpful) and as usual, one more "system" to deal with...

The older Evenrudes, most used ignition advance for the initial rise in RPM's, say 500-800 RPM of advance above idle BEFORE the carb butterflys even opened. Nice and simple, higher RPM's without additional fuel or the "advancments". (I had this set up on an old 75 HP triple, 2 stroke)

Anyone know if Yamaha ever used a system as as such?

*Bob, did you get the carb bowl(s) off again and inspect?

[rodbolt17]

almost every carbed motor with a mechanical advance used this.
its called throttle pick up timing.
its in the tune up and service guides.