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  #11  
Old 09-26-2022, 01:35 PM
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Dan 57 Dan 57 is offline
 
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Steve, are u fuel injected?
I wonder if there‘s a correlation here…
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  #12  
Old 09-26-2022, 02:21 PM
SuperCubDriver SuperCubDriver is offline
 
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Some more data points:

Dan, yes I found the exhaust valve stuck open. The following day I did the rope thing on that cylinder and tapped the valve through the guide into the cylinder. Reamed the guide and there was this black buildup on the greasy reamer. The valve then went in smooth as silk and the flight home was normal without any issues.

Steves landing sounds exactly like my first try. I did slip and the aircraft kept floating but "my" rwy was only 2500 ft long. I think I would have made it on a 4000 ft rwy. And 1200 RPM idle RPM is just the same what I saw.

I will try to run the engine the next days with one spark plug removed and see if I can reproduce the behavior and report back.
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  #13  
Old 09-26-2022, 03:59 PM
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rv8ch rv8ch is offline
 
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Quote:
Originally Posted by SuperCubDriver View Post
...
I will try to run the engine the next days with one spark plug removed and see if I can reproduce the behavior and report back.
Not sure I'd do that - I'm as curious as everyone about what caused this, but this seems pretty risky. Perhaps check with your engine builder to see what they say, or Joe Godfrey of Savvy Aviation.
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  #14  
Old 09-26-2022, 05:23 PM
SuperCubDriver SuperCubDriver is offline
 
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I don´t see issues except the electronic ignition, will have to connect and ground the removed spark plugs though.
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  #15  
Old 09-27-2022, 01:08 PM
Steve Watkins Steve Watkins is offline
 
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Dan 57,
I do have the standard Fuel injection system on the IO-360M1-B from vans.
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  #16  
Old 09-27-2022, 01:22 PM
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Dan 57 Dan 57 is offline
 
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Thanks Steve.

This increase in RPM is interesting, and I wonder if there is any correlation between a stuck exhaust valve, and injection, though I may be totally wrong here...
In the 4 cases I've experienced, I had none of this idle RPM increase, and all engines were carbed... with no connection to the MP, therefore my question(s).
The valve sequence overlapping in the 4 cycle is, AFAIK, happening on all Lycos and Contis...
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  #17  
Old 09-27-2022, 02:29 PM
SuperCubDriver SuperCubDriver is offline
 
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I might have an explanation for this:
On a carbed engine during idle the low MAP is pulling fuel through the idle circuit, so when the MAP is increased due to an open exhaust valve and not with an opening throttle plate I think the vacuum signal pulling the fuel is weaker. On an injected engine the three healthy cylinders are just getting more air and so are burning the excess fuel which in turn is producing more power.
I might be wrong but this makes sense to me.
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  #18  
Old 09-28-2022, 02:44 AM
Ingo Weise Ingo Weise is offline
 
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Default Possible solution:

With the exhaust stuck open, you have the exhaust pressure from the other cylinders working against any fresh air flowing though the dead cylinder. The throttle plate is in the idle position and the three running cylinders make enough low pressure to suck in the same amount of air. Fuel mass depends only on throttle position and is the same too. On the same fuel and airflow three cylinders make the same power as four would make, but without or with reduced pumping losses from the dead cylinder the engine speed goes up.
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  #19  
Old 09-28-2022, 03:00 AM
SuperCubDriver SuperCubDriver is offline
 
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Good point with the reduced pumping losses, this is fact. But on the other hand this cylinder doesn´t produce power at all. And - there is still the increased MAP which I believe is the key to this problem.

Bill, exhaust is 4-2.
#1 and #2 going into one pipe.
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  #20  
Old 09-29-2022, 04:10 AM
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Daida Daida is offline
 
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Default MAP/RPM reason….maybe…

Herman,
i am glad to hear about the outcome.

My ideas about increased rpm.

With a stuck-open exhaust valve you completely miss a combustion stroke and the necessary work to do it. (I think those are the “pumping losses” you already talked about).
Even so this cylinder ist dead and produces “Zero” power, the inertia of the rest of the engine is big enough to keep it running (proofen!).
I think the inertia of the entire engine, 3 power strokes (75%), makes up for the missing cylinder. In addition to that, the engine has to do 1 combustion stroke less due to the stuck open valve, thus not using any of that inertia energy produced. Furthermore, with a firing order 1-3-2-4, if #3 is on power stroke, #2 would be on combustion stroke. But due to the missing combustion the engine spools up more than normal as there is no counterforce, resulting in higher rpm as #3 has its standard power output, resulting in more inertia (stored gyroscopic energy)………..and we start from the beginning…

This does only come in place in the low-power, low-speed regime, when the power for turning the propeller is (almost) neglectable. As soon as the prop requires serious power to be turned (as when producing thrust), this result is no longer current as the high pitched propeller blade uses all of the inertia (and produced) energy from the engine to produce thrust/overcome drag. Afterall, your engine is missing a cylinder, and therefore power.

The c/s prop is supporting the whole thing while beeing in fine pitch-stop at those rpm, keeping the power required to turn the prop at a minimum.

A FP prop, due to higher pitch, resulting in higher drag (need/use more of this inertia) would probably cover the symptomes above. Even so they are still present, but not as dominant.


My ideas about increased MAP.
As your prop is at fine-pitch-stop, and nowhere near the control range, we are technically speaking about a FP prop (with minimum pitch, probably even smaller than a climb-prop).
But, now MAP and RPM are direct proportional (let’s not talk about windmilling effects now). Higher RPM means higher MAP, period.

And the higher RPMs come from the reasons above.

If you look at your EGTs right after start, there are running steady at 1100 F-ish. I would say steady.
At 03:15 #2 “departs”.
After landing, around 06:45, your engine is running as stated above with EGTs around 1300-1350 F-ish. Same fuelflow as upon initial startup, higher RPM (more air), engine is running leaner than upon initial start, therefore higher EGT on the remaining cylinders.
Higher RPM, higher MAP.

Your engine doesn’t run more efficient or anything in this direction, most probably producing less torque and overall-power-output.
But, In a low speed, steady-state environment, those are probably the results that we see……at least to my understanding.

It still seems a bit weird to me too, but those are my conclusions.

How did the testrun with the removed sparkplug went?

See you around
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