Fuel Pressure regulators

[99yam40]

Yes I did read it and I thought I understood it but I guess I'm confused with the following statement.

As the manifold pressure changes with the throttle opening being changed, the fuel pressure changes. Throttle more open = more manifold pressure = more fuel rail pressure. Throttle less open = less manifold pressure = less fuel rail pressure.

he also wrote "The fuel pressure is stabilized as long as the manifold pressure is stabilized. The manifold pressure is stabilized as long as the throttle opening and engine RPM remain constant."
So once you open up the throttle and the RPMs stabilize, pressures stabilize also

[boscoe99]

Boscoe99 Thank you for the ref material. It makes more sense seeing the higher pressure at WOT.
Question 1 - Using that same ref, is it safe to say the 47.9 psi is a momentary measure of the high pressure pump output?
Question 2 - Without the use of other special load tools, is running the engine in the water up to WOT with the fuel pressure gauge installed on the fuel rail with the cowling off, the only way for me to check the pressure?

Q1 - 47.9 psi should be the pressure seen on the fuel pressure gauge when the fuel pump is running but the engine is not running. Keep in mind that the fuel pump only runs for about 5 seconds when the key is just turned to the on position. That is why after about 5 seconds the fuel pressure may start to diminish as the pressure bleeds off since the pump stops running.

Q2 - Running the engine is running the engine. Does not need to be in the water but it helps to have a load on the engine. Motor could have a test wheel on it be running in a tank or the motor could have a propeller on it and be pushing a boat.

Here is what is going on.

Air pressure in the intake manifold is at its highest point when (a) either the motor is not running or (b) the motor is running and the throttle is wide open.

Air pressure in the intake manifold is at its lowest point when the motor is idling.

As the throttle is opened and closed with the motor running the air pressure in the intake manifold rises and falls. The regulator is plumbed to the intake manifold pressure so that as the intake air pressure rises and falls the fuel pressure rises and falls accordingly.

[boscoe99]

I'm inclined -as I frequently am - to throw a stone at the Yamaha "technical writer"/translator.

what is going on - and why - is pretty clear.

Describing it as they did: not so much.




"The pressure regulator changes the fuel pressure according to the vacuum pressure"

seems IMO a more "helpful" description.

The weak link with respect to Yamaha outboard motors IMO is the data. Poor, poor, poor. Or less. Maybe it is the translation. Maybe they just don't give a damn. I don't know.