Jcarne

Finally got my cowl back on and did 3 hours today with a few shutdowns. Bleeding it down seems to have solved it. I found that I don’t need to crack the throttle, just turn the mixture in barely far enough to have it bleed down. A64pilot, my theory on why just cracked and not full rich is in case anyone turned the prop. Either way it would sputter to life if a p-lead was broke but I would rather my mixture not be full. As far as hot starts I didn’t notice much difference. Still fairly easy, just following the usual mixture ico until she lights trick. p.s. I also received word that the mixture plate shuts off fuel to both metered and unmetered fuel chambers so the chances of fuel getting “stuck” on the non-metered side is pretty much mute.

Well after taking the cowl off and inspecting I'm thinking the Avstar explanation may have a bit of logic to it. You can pull on the mixture control at the servo with your hands and pull the shaft out and compress the internal spring. I would estimate that it takes about a 15-20 lb force to accomplish this. If the mixture shaft is 3/8" diameter and you do the math it will take almost exactly 200psi to start pushing that shaft out which is exactly what the Avstar tech said. I also noted that you can pull that shaft out a fair distance; in other words, I could absolutely see it coming unseated from the mixture plate based on pictures of the parts. So, I will monitor and let you guys know if I notice anything else. In the meantime I'll keep the mixture slightly cracked after shutdown to release pressure.

Tried that to no avail as well. I would be able to hear the fuel flow as well; nothing this time. I have seen fuel come out my sniffle before after flooding so I got out and looked for it; nothing. My first thought due to how cold it was was that somehow water got in and iced up right in that fuel jet but quickly realized that didn't make sense. I could definitely tell that fuel was not getting through that servo though. I guess I will just keep the fuel mixture cracked slightly after a shutdown to avoid the problem in the future. Just thought it was a weird one and surprised not many others have noticed it.

Thanks for posting the chart so I don't have to find it again. Haha While I do agree that there would likely be no pressure inside the diaphragms I do believe the Avstar explanation has some amount of merit. While there would be pressure on both sides of the spring that spring/shaft is in one chamber of fuel (yellow arrow) so there would be a net force trying to push that shaft out and compressing the spring. He specifically said this will happen around 200psi. I have a Dynon HDX and it stops sensing fuel pressure after 150 psi and it crossed that threshold. In the end it seems there are only two logical explanations. Avstars explanation or somehow fuel is getting trapped in the unmetered side of the diaphragm and forcing the ball closed. I don't see how that last one is a possibility as there is 2 ways for the two chambers to equalize. Either way it sure is an interesting phenomenon. Normally I can open the mixture control and bleed it down with no issues before starting. In the past it also doesn't matter where my throttle control is, even at idle it will bleed down. However, on this particular trip I had a long descent in some cold weather (probably coldest fuel in my tanks I have witnessed) so I don't know if that contributed to a higher than ever pressure increase from the fuel getting heat soaked. I asked him suggestions on how to reduce that pressure and he gave me a few. I asked if cracking the mixture after shutdown would work and he said "it would release the pressure, but we can't endorse that practice". haha

Yep I tried that to no avail. No combination of throttle/mixture would bleed it down until about 50-60 psi

Larry have you been flying just fine since discovering this? I had the exact same symptom of mixture control not releasing pressure once pressure builds yesterday at a fuel stop. I also talked to Avstar and a tech told me that what he thinks is happening is that pressure is building behind the mixture plate to the point where is pushes the mixture shaft out a little and compresses the spring a bit which will disengage the shaft from the mixture plate until pressure subsides.