This one's for Harry or anyone familiar with these engines. I'm stumped....
It's an O-170, a military A-65. 156hrs. Built in 1944. Hasn't run since 52. Wie open it up, inspected it, was very nearly perfect. New bearings, new rings, lapped the valves, honed the cylinders and put it back together. I did start measuring all the clearances and halfway through have up, realizing that this was really 150 hour engine with very little wear.
Rebuilt the Stromberg carburettor, by the book. Fuel level in bowl perfect, new stainless needle, no leaks.
Bought rebuilt mags from Fresno Air Parts. Basically all new parts inside including remagnetized rotors. New ignition harness.
Put it all back together, hung it on the airplane, and started swinging the prop. Eventually got it running, let it warm up, but every time the throttle was advanced, it would start to pop and bang. Would not go over 1200 rpm, but seems to idle okay. Pulled the carb off, thinking it was a fuel delivery problem. Cleaned and checked everything again. Put it on, same problem. Swapped the cab for an old Stromberg. Started up okay but same problem when advancing throttle.
Clearly not a carburetor problem.
Rechecked the timing twice and got another pair of eyes to double check that. Everything perfect.
Started it up again, let it idle for several minutes, and then took temperatures near the exhaust flange on all four cylinders. Number 2 and 3 were running cold, on opposite sides. Left mag fires all lower plugs, right mag fires all upper plugs so it could not be an ignition issue. Mag check, by the way was normal, barely any drop on either.
Suspecting a stuck valve or two, pulled all the rocker covers off and found precisely nothing. All valves move normally. I hear no leaks. All cylinders seem to have equal compression.
When in the correct position, I.e fully closed, each valve does appear to have a very small amount of clearance, as they should. I did read somewhere that this kind of check can be misleading since there are problems in the hydraulic lifters that only show up when running. I am not sure why or how. The manual does state that some lifters can take up to 40 minutes to settle down. But that should not prevent a cylinder from firing at all.
We have checked all the other obvious faults such as induction leaks, blocked fuel vents, inadequate fuel supply, inadequate fuel head, etc. All good.
So - Hivemind suggestions please?
It's an O-170, a military A-65. 156hrs. Built in 1944. Hasn't run since 52. Wie open it up, inspected it, was very nearly perfect. New bearings, new rings, lapped the valves, honed the cylinders and put it back together. I did start measuring all the clearances and halfway through have up, realizing that this was really 150 hour engine with very little wear.
Rebuilt the Stromberg carburettor, by the book. Fuel level in bowl perfect, new stainless needle, no leaks.
Bought rebuilt mags from Fresno Air Parts. Basically all new parts inside including remagnetized rotors. New ignition harness.
Put it all back together, hung it on the airplane, and started swinging the prop. Eventually got it running, let it warm up, but every time the throttle was advanced, it would start to pop and bang. Would not go over 1200 rpm, but seems to idle okay. Pulled the carb off, thinking it was a fuel delivery problem. Cleaned and checked everything again. Put it on, same problem. Swapped the cab for an old Stromberg. Started up okay but same problem when advancing throttle.
Clearly not a carburetor problem.
Rechecked the timing twice and got another pair of eyes to double check that. Everything perfect.
Started it up again, let it idle for several minutes, and then took temperatures near the exhaust flange on all four cylinders. Number 2 and 3 were running cold, on opposite sides. Left mag fires all lower plugs, right mag fires all upper plugs so it could not be an ignition issue. Mag check, by the way was normal, barely any drop on either.
Suspecting a stuck valve or two, pulled all the rocker covers off and found precisely nothing. All valves move normally. I hear no leaks. All cylinders seem to have equal compression.
When in the correct position, I.e fully closed, each valve does appear to have a very small amount of clearance, as they should. I did read somewhere that this kind of check can be misleading since there are problems in the hydraulic lifters that only show up when running. I am not sure why or how. The manual does state that some lifters can take up to 40 minutes to settle down. But that should not prevent a cylinder from firing at all.
We have checked all the other obvious faults such as induction leaks, blocked fuel vents, inadequate fuel supply, inadequate fuel head, etc. All good.
So - Hivemind suggestions please?