Fuel pressure - again...

Seeking to track down problem described as follows.

RV9a with YO-320-D1A, with carb and about 60 hours TT. Do not have fuel flow monitor, yet.

Engine not running, mixture set to "cut off". Fuel pressure = 0.4 psi. Turn on electric fuel pump and fuel pressure increases to 2.2 psi. Turn off electric electric pump, fuel pressure remains at 2.2 psi. Open mixture, and pressure drops.

Engine running, at idle, mixture rich, engine running smooth, then switch on electric fuel pump, engine stumbles and black smoke out exhaust. Switch off electric fuel pump, engine runs smooth, exhaust clears. At idle, fuel pressure without electric boost = 1.9 psi, pressure with electric boost switch on = 2.3 psi.

Engine at run up @ 1800 rpm. Fuel pressure without electric boost = 1.9 psi. Fuel pressure with electric boost = 2.2 psi. No noticeable change in engine operation or rpm.

Engine running at cruise, during flight. Fuel pressure without electric boost = 2.2 psi. Fuel pressure with electric boost = 2.2 psi. No noticeable change in engine performance or rpm.

Presume electric fuel pump is causing engine to run rich at idle.

Some potential causes of rich conditions at idle with electric fuel pump on:
1. with electric fuel pump on, fuel pressure is sufficient to overcome the resisting float pressure on the needle, thereby pumping excess fuel into the carb, and flooding the engine. Weak float?
2. with electric fuel pump on, air bubbles are being "sucked into the fuel on suction side of pump" and bubbles entrained in the fuel enter the carb bowl, cause the float to sink, and therefore allow excess fuel to enter the carb. Seems ths would impact all phases of operation.
3. electric solenoid type of primer, and possible leak in the primer system that allows excess fuel to enter cylinders when electric fuel pump is switched on.
4. Other?

Are there any other ideas? Suggestions on the most efficient way to track down the cause?

Thanks,
Wayne

Dues paid 2017.

I thought that Marvel Schebler carbs would tolerate 6+ psi without overcoming the float. It's curious that you're only seeing 2.2 psi. When I turn on my Facet boost pump, I always see 6.0 psi on the gauge. (MA-3SPA carb here, with boost pump in series with the mechanical fuel pump)

IO-360 Fuel Pressure Readings?

I bought my RV7A from the original builder almost two years ago and am now getting my "squawks list" down to just a few items. My fuel pressure readings on my EFIS/EIS have always been high. I figured it was either a bad sensor or incorrect EIS settings. The Superior Engine Owners manual says fuel pressure should stay within the range of 2 to 35 psi. I believe my fuel pressure sensor is the old style VDO 80 psi sensor. Using the GRT recommended EIS settings of AuxSF = 90 and AuxOFF = 15 my fuel pressure readings are around 75 psi. If I set my EIS settings for the 150 psi VDO sensor my fuel pressure readings double to 150 psi.

If I set my EIS settings back to the settings the original owner has set in the EIS of AuxSF = 19 and AuxOFF = 30 my fuel pressure comes down to 12 psi at idle rpm. These settings make me think the sensor is working but my issue is something else.

Question 1: what should my fuel pressure readings be at idle, in climb and cruise?
Question 2: does anyone have any suggestions as to why my EIS settings are so strange compared to published EIS settings and why the spread appears backwards?

Did you bleed the fuel pressure line?

N941WR said:

Did you bleed the fuel pressure line?

Click to expand...

No but I've flown over 100 hours with this issue so I don't think it's trapped air.

Edit: this is for WSQuare.

Off the top of my head, sounds like it could be a float and/or needle valve issue. Check for latest AD's and SB's, and for proper float installation.

Does the engine shut down properly when you pull the mixture to idle cut off? If not, it could be that when the carb was assembled, the mixture needle, which is flexible, might not be installed correctly. This would be rare.

I'm not an expert, and I didn't sleep at all last night!

Also, upon re-reading your post, make sure the fuel pump switch isn't wired to the primer valve, and that the primer valve actually works properly. Your "black smoke" comment sounds suspiciously like fuel reaching the cylinders through the primer system.

I would completely physically disconnect the primer system from the engine, and cap off all fittings, and try running the engine again without a primer system. This would tell you if the primer system is at fault.

Easiest troubleshooting is to eliminate the primer system as a first step.

The Lycoming operators manual shows the normal fuel pressure range between .5 and 8 psi. My airplane runs between 4 and 6 for the electric pump and the same for the mechanical pump.

.5 PSI would have me nervous just because there is little margin to 0, but I guess as a transient condition it would be OK.

Thanks for the good ideas. Just to be clear. The 0.5 psi was fuel pressure in the hangar, engine not running, mixture off. Thus, representing the static pressure, and not fuel pressure while engine running. I have adjusted the "range" on the fuel pressure "gauge" on my Dynon, and will be able to provide more accurate pressure numbers next flight.

Yes, engine shuts down properly when mixture pulled to idle cut-off.

Current suggestions seem to be pointing to the engine priming system.
1) check wiring to primer valve to make sure it is correct.
2) check installation of primer valve to make sure it is installed in the correct direction.
___________________
Wayne W.
Due paid 2017.

Problem identified. Primer valve was installed incorrectly - backward. Corrected fuel flow direction through primer, and all is fine now. Thank you for all of your ideas and help.

Next question. For those that use the solenoid primer, how long do you typically push the primer button to prime the engine?

Thanks again,
Wayne W.
Dues paid 2017.

wsquare said:

Problem identified. Primer valve was installed incorrectly - backward. Corrected fuel flow direction through primer, and all is fine now. Thank you for all of your ideas and help.
Next question. For those that use the solenoid primer, how long do you typically push the primer button to prime the engine?
Thanks again,
Wayne W.
Dues paid 2017.

Click to expand...

Other's results will not necessarily be relevant to your situation.
Every engine is different. You will need to experiment with your particular system. Start with no prime at all and add "seconds" of prime a little at a time. Summer time may need no prime at all. Winter will probably need more.

I'm glad I eliminated the primer system on my engine. I have the primer tubes in a box somewhere. I have dual impulse mags that ease starting.