By9468840 wrote:
Any bit of blockage in any injector would cause pressure increase and false indication of unusually high fuel flow rate overall.
For the benefit of the group and at the risk of stating the obvious, this is only true if:
a) The blockage is on the fuel path of the injector (the usual one) and not on the airflow path.
AND
b) You are using fuel flow distributor (spider) pressure to indicate fuel flow. (That is the typical mechanical indication, but a lot of modern aircraft use actual flow)
I have both indicators and multi-cylinder EGT.
I once had a leak in an injector fuel line and I could tell almost exactly where the leak was before opening the engine cowl because flowmeter fuel flow was indicating much higher than spider pressure fuel flow and ROP EGT was much higher on a certain cylinder (running lean)
Oh, well I cheated further because I knew injector work had just been done….
While not entirely disagreeing with ANTONIO the problem of baffling detaching inside a muffler is a dynamic one.
If the baffles have cracked and become detached they move around influenced by RPM and gas flow. It is quite a cheap defect to check for and is one of the basic checks to do before large amounts of money get spent changing expensive components.
There is a very interesting paragraph here:
Cleaning_Fuel_Injector_Nozzles_pdf
I bet not many people knew that.
My nozzles, from GAMI, have two markings, on adjacent hex faces. I wonder which of the two should be at the bottom?
Reading the text carefully, maybe this applies only where the injectors are actually horizontal, not angled as they are in the IO540.
This general comment is also interesting
Interesting results today.
Changed the #2 injector. I happened to have another one with the same hole, from years ago. Soaked it in solvent overnight…
Started the engine up and immediately thought “fixed”. At 1200rpm it ran very nicely. Taxied along to a loop where engine tests can be done and by the time I got there I thought it is back to being rough again.
The #2 EGT dipping is still there. But I could not get the CHT to dip by 50F as before. More like 25F now. Historically this is seen on #2 CHTs a long way back. At 1200rpm the temps look normal now, and the idle check is done for only seconds and usually without looking at the EDM.
So I went back through the EDM downloads. Only on the last flight do I see #2 EGT dipping so much during the idle check
The one before, a few hours earlier, doesn’t show the dip, and neither do any others, other than one mentioned below
One has to be ever so careful with one’s imagination. But there is something there – unless the variation in low rpm running is normal. I found one other flight, a year ago, on which #1 EGT dipped in a similar way. However there is a common factor: the issue doesn’t show unless the EGTs dip a long way (below 1000), and during idle checks they normally don’t. I found just two cases of idle checks where they dipped a long way and, as described, one was a year ago and another was the last flight.
Also, whether the #2 EGT dips so far really depends on just how far you close the throttle during the idle test. I don’t necessarily go all the way down to the stop.
Amazingly, the enroute EGTs are exactly the same on the two above flights.
Does anyone know what is inside the fuel distributor – the round bit on top of the engine? Is there any potential for a cylinder-specific issue there? Could there be floating dirt? It looks like one can just take the lid off (although there is a gasket there which may be lost)
Peter wrote:
Does anyone know what is inside the fuel distributor
On a TCM fuel spider there is a screen in there which can be cleaned. I have frightened the life out of myself with bad injectors in a Lake that stood for 8 years and I fully understand the rough running such a small item can manifest. Just an aside, you have tested the plug leads to make sure they are ok?
The issue – whatever it really is – is precisely identical on either or both mags. So I am fairly confident it isn’t the ignition system.
One thing I didn’t check is heating up EGT probe #2 and checking it shows up as #2 on the EDM 
It probably will, since #2 CHT correlates. Well, it used to until the injector was changed today. Now, CHT is in line with historical data.
Incidentally, are there any baffles in the exhaust? One reads a lot about loose baffles but I hav searched all over and see absolutely no reference to baffles in the TB exhaust. I also had the whole system in pieces 2 years ago and don’t recall seeing anything inside. It seems to be just plain inconel tubes.
Inside the spider there is one diaphragm-operated valve which shuts fuel flow when fuel pressure is below a certain value. This is to aid in an even (not rough) shutdown in all cylinders. This cannot act differentially for different cylinders.
My plan is to visually check under the #2 rocker cover: springs, hydraulic tappets, valve movement, etc. And spark plugs, and borescope the cylinder.
I cannot see a magneto failure which would be cylinder specific especially on either or both mags.
We may find nothing at all… then what does one do? The injector change has reduced the CHT drop at 1200rpm from 50F to 25F; that’s definite. Maybe there is nothing else, and the rest is a combination of imagination, and these engines being “agricultural”.
I can’t say my engine has ever idled smoothly at the very bottom end. The idle speed is also engine temperature dependent; if you close the throttle after landing after a long flight on a hot day, it can go very low, almost stopping (it did stop once or twice). There is a clear tradeoff between this, and having too high an idle which extends the landing distance.
