A-65 problem

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PTAirco

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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?
 

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Victor Bravo

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Incorrect (85/90/O-200) induction spider above the carburetor? Sharp burr inside the spider? Someone took a Dremel tool to hog out the spider for more power and FUBAR'ed it? No plugs or fitting screwed into the primer hole in the spider?

No air filter/airbox/airscoop bolted on, and prop blast is pulling air out of the carburetor or fouling up the inflow?

Is Harry still with us? I really hope so, but nobody seems to have heard from him in ages.

EDIT: just zoomed in on the picture. Put the airbox back on and see if there needs to be a scoop there.
 

PTAirco

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I'm 99% certain the engine is in exactly the configuration it came in 1944. I have the logs for it as soon as it left the army. The only civilian owner was an Army Air Corps mechanic who seemed to know his business.

The two non firing cylinders have me baffled. Otherwise it would seem like fuel issue or induction leak.

Carb internals are correct too. Right size jet etc.

I might pull it out this evening and run it up for 15 minutes or so and get it really warmed up and see if anything changes.

I just checked the valve clearances again. Bleeding down those lifters takes a while! The clearances are on the low end but about within tolerances. And the two non firing cylinders do not seem to differ in any way from the running ones.
 

TFF

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Worst case valve sticking open. Did you lap the seats when you had it apart? No valves warped or stems bent? That stuff checked?

Best case wires swapped. I have done it. I had a friend do it twice on airplanes he was trying to sell right when buyers showed up.

Bleed all the lifters. That’s really a Hail Mary. But you might be the one. No gaskets with gaps?
 

proppastie

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two possible errors ....if you have the old mags (Bendix .Eisemann?) labeled 1234 me and a master mechanic made the same mistake on the same engine 30 years apart....1234 refers to the firing order not the cylinders....however you should have not been able to start it.....probably not that......same engine after "overhaul" at the 30 yr mark made horrible noise but ran....turns out the lifters were rusted closed ....great overhaul.
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Dan Thomas

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The two dead cylinders have the plug wires swapped. On both mags, too. The wires at the distributor are numbered 1, 2, 3, 4. The firing order is 1, 3, 2, 4. If you connect the leads to the plugs as numbered at the mags, you get 2 and 3 swapped, and you get a half-dead engine that barks at you when you try to put it to work. The unburnt gases coming out of the dead cylinders are set off in the exhaust by the flame that starts coming out of the live ones when the throttle is opened.

Swap those two leads on both plugs in those cylinders and it will run just fine.

Or I will buy that defective engine for $100.
 

proppastie

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So. An update?
 
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