Engine miss and maybe dropped couple hundred RPM and Intermittent as well on 2 trips back to back a few hundred feet off the runway on takeoff. I had time to kick the boost pump back in, apply carb heat and play with the mags. Maybe 10 to 15 secs and then it would go away. The first time I thought maybe carb ice, maybe fuel pump getting weak or **** in my fine wire plugs. Then the trip home it happened again but not as strong but the harmonics in level flight were off slightly. No one else would notice it but with it being my only ride I could certainly feel something not quite right. I had a suspect cyl (#3) in mind as 15 hrs ago it tested 72/80, as I could hear it bypassing, it was not through the exhaust nor the intake. Sounded like the crank case so I was thinking rings and even used my stethoscope to confirm it so not overly concerned.
After this flight I had the suspect cyl in mind and checked it to find 62/80. Nothing out the exhaust nor the intake and sounded like the rings again. I had a friend come over and give me hand and he accidently ran his hand under the cyl and could barely feel the air leaking out while I held the prop in place. I confirmed the leak with soapy water and bubbles. Pulled the head and found a crack from the bottom plug to the exhaust valve. This also explained why I would find a fresh 1 or 2 drops of oil after each flight. Found the mysterious oil leak as well.
Impossible to see the crack with the cyl on the engine. With it off it is barely visible by the exhaust port.
This is in a RV4, the engine is a O360 A1A with a wooden prop. 580 hrs or so since rebuild in 1998. Not much history on the engine otherwise. I?m now waiting on parts to try this again. Bore scoped the other jugs and found nothing so I will change out the one cyl with a new piston and re manufactured jug. If I had found another one cracked, then I would have changed them all. The valves turned freely on this jug so they don?t appear to have been sticking. But, I will also check the other cyls while I?m in there waiting on parts.
I?m assuming the I have probably flown this with the crack for closer to 20 hrs when I think back to when the minor oil leak started that I couldn?t find and that (annoyed me). No real power loss except for the little miss. The comp test was not definitive in this case. But hearing the air by passing a little differently was a warning sign that I should have dug deeper on. And having a borescope might have helped as well. I?m probably lucky this didn?t end up being a catastrophic failure and only a minor nuisance.
Tim
After this flight I had the suspect cyl in mind and checked it to find 62/80. Nothing out the exhaust nor the intake and sounded like the rings again. I had a friend come over and give me hand and he accidently ran his hand under the cyl and could barely feel the air leaking out while I held the prop in place. I confirmed the leak with soapy water and bubbles. Pulled the head and found a crack from the bottom plug to the exhaust valve. This also explained why I would find a fresh 1 or 2 drops of oil after each flight. Found the mysterious oil leak as well.
Impossible to see the crack with the cyl on the engine. With it off it is barely visible by the exhaust port.
This is in a RV4, the engine is a O360 A1A with a wooden prop. 580 hrs or so since rebuild in 1998. Not much history on the engine otherwise. I?m now waiting on parts to try this again. Bore scoped the other jugs and found nothing so I will change out the one cyl with a new piston and re manufactured jug. If I had found another one cracked, then I would have changed them all. The valves turned freely on this jug so they don?t appear to have been sticking. But, I will also check the other cyls while I?m in there waiting on parts.
I?m assuming the I have probably flown this with the crack for closer to 20 hrs when I think back to when the minor oil leak started that I couldn?t find and that (annoyed me). No real power loss except for the little miss. The comp test was not definitive in this case. But hearing the air by passing a little differently was a warning sign that I should have dug deeper on. And having a borescope might have helped as well. I?m probably lucky this didn?t end up being a catastrophic failure and only a minor nuisance.
Tim
Replace the cylinder assembly before further flight, if air leakage and bubbles are observed on the side of the cylinder assembly, near the head-to-barrel interface.
