[VF84Sluggo]

Folks,

I ran my engine yesterday for the first time after installing my rehabbed fuel servo (Bendix RSA-5A) and divider. I also replaced the UMA mechanical fuel pressure gauge while the servo was being worked on (old one sprang an internal leak.) The servo came back with a new "Experimental" placard on it instead of the original Bendix RSA-5AD1 placard (thought this was odd)

When I turned on the boost pump I noticed I got fuel pressure with the mixture at idle cutoff. Moving the mixture to full rich made no change in the fuel pressure reading (20-25 psi.) Is this normal?

Can't remember if this was the case before all of this work, but I seem to recall moving the mixture to full rich for priming caused fuel pressure to rise/indicate. That rise and counting three or four seconds was my priming routine. That's what got my attention: fuel pressure was already showing, got no change when I went to prime for start, and no drop when going back to idle-cutoff.

Also, now after pulling the mixture to idle-cutoff for shutdown, fuel pressure remains indicated (trapped fuel pressure?). I think the fuel pressure went to zero before when the mixture was pulled to idle-cutoff. Fuel pressure will drop if I go to full rich after shutdown (engine off/boost pump off). Also will bleed down over time (like all night).

Anyway, just wondering if maybe after the overhaul, now when the mixture is at idle-cutoff in the cockpit, fuel might not be fully cutoff in the servo. Or maybe all this is normal, and I just didn't pay this close attention before

Thoughts?

 

[Taltruda]

Where do you pick up the fuel pressure reading? If it’s before the servo, then that explains why you have pressure regardless of where the mixture is.

 

[VF84Sluggo]

True. I thought I had pics that would show the fuel line connections, but I don't. I'll get some.

 

[lr172]

Folks,

I ran my engine yesterday for the first time after installing my rehabbed fuel servo (Bendix RSA-5A) and divider. I also replaced the UMA mechanical fuel pressure gauge while the servo was being worked on (old one sprang an internal leak.) The servo came back with a new "Experimental" placard on it instead of the original Bendix RSA-5AD1 placard (thought this was odd)

When I turned on the boost pump I noticed I got fuel pressure with the mixture at idle cutoff. Moving the mixture to full rich made no change in the fuel pressure reading (20-25 psi.) Is this normal?

Can't remember if this was the case before all of this work, but I seem to recall moving the mixture to full rich for priming caused fuel pressure to rise/indicate. That rise and counting three or four seconds was my priming routine. That's what got my attention: fuel pressure was already showing, got no change when I went to prime for start, and no drop when going back to idle-cutoff.

Also, now after pulling the mixture to idle-cutoff for shutdown, fuel pressure remains indicated (trapped fuel pressure?). I think the fuel pressure went to zero before when the mixture was pulled to idle-cutoff. Fuel pressure will drop if I go to full rich after shutdown (engine off/boost pump off). Also will bleed down over time (like all night).

Anyway, just wondering if maybe after the overhaul, now when the mixture is at idle-cutoff in the cockpit, fuel might not be fully cutoff in the servo. Or maybe all this is normal, and I just didn't pay this close attention before

Thoughts?

Sounds right. Full rich is not full flow. The servo uses airflow as the primary means to meter fuel. With the boost pump on, I would NOT expect pressures below 25 when full rich without the engine running.

 

[RV7A Flyer]

In fact, that's how I prime my IO-360 for a cold start. Throttle open, fuel pump on, watch for full pressure, then mixture rich until ~3 gph, then fuel pump off/throttle open 1/4" and ICO, and start. Fires up every time.