The engine is a used O-360-A3A, removed from a Robin DR400 tow plane and is at about 400 hours SMOH.
While still installed in that plane, there was a permanent oil leak at the cyl. #4 rocker box gasket. Repeatedly replacing the gasket and even the cover itself did not stop the leak.
A subsequent leak test showed low compression and a leaky exhaust valve on that cylinder. A borescope inspection of all cylinders revealed cracks at the upper spark plug holes of cylinders #3 and #4. The cylinders were pulled and sent to an engine shop for inspection. Apart from the cracks, they found discolouration of the piston pins due to high temperatures and badly worn exhaust valve guides (-0.02"). Both cylinders were replaced with refurbished cylinder kits.
The CHT gauge was found to indicate about 50?F low. In a tow plane it is not uncommon to operate the engine near red line temperatures. With a false CHT indication it is not surprising that the engine was damaged.
Currently, we are troubleshooting a new problem with this engine. Since the installation of the overhauled cylinders the engine exhibited a subtle engine stutter which at first was hard to notice. Since then it has slowly become more obvious to the point where we are no longer flying it.
- at irregular intervals, it feels like the engine misses a few ignitions. The interval between the engine running fine and periods of "stuttering" is very short, below 1 second. You can feel the vibrations through the airframe.
- this occurs at all RPM, irregardless whether the engine is hot or cold.
- mixture setting, switching on the electrical fuel boost pump or activating carb. heat have no effect.
- operating on a single magneto does not stop the stutter.
Here is what we have done so far:
Ignition:
- Sent off both magnetos and ignition harnesses to a 500 h inspection. No findings, only mandatory parts were exchanged.
- Replaced all spark plugs with new ones
No change. Therefore, ignition ruled out as cause.
Cyl. #4:
After 30 hours of operation, this cylinder again showed poor compression (exhaust valve leakage).
- valve train repaired at no charge by engine shop
- shop told us to stop leaning at power > 75% (which we don't)
Therefore suspect engine running lean
Fuel System:
- Replaced carburetor with factory new Marvel-Schebler MA-4-5 10-3878
- Verified sufficient fuel delivery to carb. with a fuel flow test
No change. Old carburetor's venturi and throttle valve covered with a black residue. The intake pipes also have this residue (see picture).
Cyl. #1 + #2:
Since the other cylinders and accessories have been changed, we now suspect the problem to be caused by the front cylinders.
- Performed "wobble test" according to SB388C, even though hi-chrome valve guides are installed -> play at valve stem is near upper limits but still within spec, therefore ruled out sticking valves.
- confirmed stroke of camshaft intake lobe for cyl. #1 and #2 is sufficient
- checked valve springs and found none broken
- exhaust appears not to be restricted
We then pulled cylinder #2 for a more detailed inspection (see pictures).
To rule out intake leaks, we removed all intake pipes and connecting hoses and inspected them for cracks. We also boreskoped the intake passages in the oil sump assembly and found strange holes near the cyl. #1 and #2 ports. Both holes are located right below a stud (see pictures).
Does anybody have any suggestions? Do the pictures look right? What should we check next? We're out of ideas at this point.
Strange O-360 problem, lots of pictures
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