Brantel
Well Known Member
Sticky valve or maybe not?
For the engine experts out there....
Preamble:
Leaving Osh just about passing the edge of the D, bam! Sounded like something smacked the bottom of the plane with a dead blow hammer! Scared the pooooo out of my wife! I felt nothing in the controls, no shudders, nobody around me visually, nobody around me on the ads-b, quick glance at the tail and it is all there, quick shake of the stick and it all still works, quick look at the wings for dents or blood and nothing. I thought we must have hit a bird with the gear or something. Nerves calmed down and we keep going. Due to weather had to spend the night in Danville Il. Checked the plane out real good. No bird strikes. I removed the cowl and checked everything firewall forward and found nothing. Fast forward to departure morning and we blasted off in severe clear with 1:49 to go. On the climb up to 9500 and I got another bang! Just like the one coming out of Osh. Then a few more bumps from the engine a few minutes apart but then they went away. I realized that these might be after-fire bangs from a sticky valve. Normally my engine runs like a sewing machine. Never has had morning sickness. Stayed high till we had our home field made, in the rapid decent from 9500ft, got a couple more bumps from the engine. These were more than the normal downwind pop you sometimes get from closing the throttle too fast. Nothing will prepare you for how a sticky valve will scare you when it sticks while in cruise! I am certain that my wife will need new underwear!
So now I am ready to wobble test/ream the guides on all 4 before any additional flights! I am not fooling around with this kind of stuff. A miss or after-fire bang every now and then seems like a warning sign of an imminent bent pushrod or worse!
To clarify, the after-fire events I have seen have been one cycle events. Bang and then smooth again. No rough running for consecutive cycles. When it is running smooth, it is really smooth. No time to see any changes in EGT's.
Questions:
Can a sticky exhaust (open) valve cause an after-fire thru the exhaust for one cycle? If so, how does this happen if the valve is stuck open? One would think that if it is stuck open, it would not draw in any fuel on the intake stroke or be able to make any compression on the compression stroke?
What else could cause such a thing? I am already committed to the wobble test/reaming procedure but want to ensure I am on the right track. Since I have one Pmag and one Mag, I would thing the chances of late ignition timing to be slim to none?? I checked the intake and exhaust for leaks, blown out gaskets, cracks etc. None found. I still got an RPM rise at shutdown when I pulled the mixture.
Thanks for any comments or advice. I am rounding up the stuff I need to do the wobble test/reaming. The price Lycoming wants for the official tools is ridiculous so I will be making my own. Already have the reamer required. I will be using my friend Mike Bullock's awesome thread as guidance on how to perform the reaming operation as needed.
For the engine experts out there....
Preamble:
Leaving Osh just about passing the edge of the D, bam! Sounded like something smacked the bottom of the plane with a dead blow hammer! Scared the pooooo out of my wife! I felt nothing in the controls, no shudders, nobody around me visually, nobody around me on the ads-b, quick glance at the tail and it is all there, quick shake of the stick and it all still works, quick look at the wings for dents or blood and nothing. I thought we must have hit a bird with the gear or something. Nerves calmed down and we keep going. Due to weather had to spend the night in Danville Il. Checked the plane out real good. No bird strikes. I removed the cowl and checked everything firewall forward and found nothing. Fast forward to departure morning and we blasted off in severe clear with 1:49 to go. On the climb up to 9500 and I got another bang! Just like the one coming out of Osh. Then a few more bumps from the engine a few minutes apart but then they went away. I realized that these might be after-fire bangs from a sticky valve. Normally my engine runs like a sewing machine. Never has had morning sickness. Stayed high till we had our home field made, in the rapid decent from 9500ft, got a couple more bumps from the engine. These were more than the normal downwind pop you sometimes get from closing the throttle too fast. Nothing will prepare you for how a sticky valve will scare you when it sticks while in cruise! I am certain that my wife will need new underwear!
So now I am ready to wobble test/ream the guides on all 4 before any additional flights! I am not fooling around with this kind of stuff. A miss or after-fire bang every now and then seems like a warning sign of an imminent bent pushrod or worse!
To clarify, the after-fire events I have seen have been one cycle events. Bang and then smooth again. No rough running for consecutive cycles. When it is running smooth, it is really smooth. No time to see any changes in EGT's.
Questions:
Can a sticky exhaust (open) valve cause an after-fire thru the exhaust for one cycle? If so, how does this happen if the valve is stuck open? One would think that if it is stuck open, it would not draw in any fuel on the intake stroke or be able to make any compression on the compression stroke?
What else could cause such a thing? I am already committed to the wobble test/reaming procedure but want to ensure I am on the right track. Since I have one Pmag and one Mag, I would thing the chances of late ignition timing to be slim to none?? I checked the intake and exhaust for leaks, blown out gaskets, cracks etc. None found. I still got an RPM rise at shutdown when I pulled the mixture.
Thanks for any comments or advice. I am rounding up the stuff I need to do the wobble test/reaming. The price Lycoming wants for the official tools is ridiculous so I will be making my own. Already have the reamer required. I will be using my friend Mike Bullock's awesome thread as guidance on how to perform the reaming operation as needed.
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