The International Cessna 170 Association

Check your induction clamps

Compressions are good indicating valves are not stuck, fuel is flowing (although an internal carb failure could have happened and should be checked out), carb heat flapper/air box should be checked, but the statement that the engine lost 100rpm and ran rough all at once, sounds like ignition to me.

Induction leaks typically cause roughness at idle/low rpm and goes away at cruise rpm.

Just to add complication to the puzzle.....A broken or weak valve spring will cause sudden onset of roughness but may still demonstrate good compression-checks on the ground.

gahorn wrote:Just to add complication to the puzzle.....A broken or weak valve spring will cause sudden onset of roughness but may still demonstrate good compression-checks on the ground.

Good point. Broken springs are easy to visually check by removing the valve covers. I don’t think a weak valve spring would produce an immediate rpm drop and roughness as described in the original post, but a broken spring might.

hilltop170 wrote:Compressions are good indicating valves are not stuck, fuel is flowing (although an internal carb failure could have happened and should be checked out), carb heat flapper/air box should be checked, but the statement that the engine lost 100rpm and ran rough all at once, sounds like ignition to me.

Induction leaks typically cause roughness at idle/low rpm and goes away at cruise rpm.

Sometimes valves do not remain stuck, I have had them both ways.

And the winner is... George and Richard, sort of. The #4 exhaust valve guide is wrecked. Apparently the valve was still seating well enough to show good compression and a warm cylinder during earlier tests. Today it ran way worse and the cold cylinder was obvious. The cylinder is off to the engine shop.

brianm wrote:And the winner is... George and Richard, sort of. The #4 exhaust valve guide is wrecked. Apparently the valve was still seating well enough to show good compression and a warm cylinder during earlier tests. Today it ran way worse and the cold cylinder was obvious. The cylinder is off to the engine shop.

Exactly the same as occurred to me with my C-206....a cracked valve guide. It allowed the valve to seat MOST of the time... but every once in a while for several minutes the engine was rough. It was difficult to duplicate on the ground so it went on for several flights.
I discovered the problem during a valve-cover-gasket replacement when the exhaust-valve- tip could be "wiggled/wobbled" when in an open condition. The reason I even attempted to wiggle it was because I thought a hairline crack was visible near the top/exposed end of the valve guide. A boroscope peek and pulling the cylinder confirmed it. Part of the guide was missing and presumeably exited the exhaust at some point several operating hours earlier.

Brian,
Is your airplane at Schaumberg? Had lunch there yesterday and there was a 170 in the shop hangar.
Johneb

johneeb wrote:Brian,
Is your airplane at Schaumberg? Had lunch there yesterday and there was a 170 in the shop hangar.
Johneb

That's me. Based out of Waukegan, ended up landing at 06C on my way to Clow last weekend.

Brian,
Have you been able to repair and replace you lame cylinder?

Unfortunately not. The cylinder that came off the plane couldn't be repaired. My normal mechanic had a good condition slightly used Millennium cylinder that I dropped off at Poplar Grove this week. Crossing my fingers that I'll be up and flying for New Years.

brianm wrote:Unfortunately not. The cylinder that came off the plane couldn't be repaired. My normal mechanic had a good condition slightly used Millennium cylinder that I dropped off at Poplar Grove this week. Crossing my fingers that I'll be up and flying for New Years.

I can't recall ever meeting a "normal" aircraft mechanic.

brianm wrote:Unfortunately not. The cylinder that came off the plane couldn't be repaired. My normal mechanic had a good condition slightly used Millennium cylinder that I dropped off at Poplar Grove this week. Crossing my fingers that I'll be up and flying for New Years.

Brian, tell me if you need a airplane ride out to Poplar Grove to pick up your cylinder I will take you out there, that should help speed up the process of getting back in the air.

gahorn wrote:

brianm wrote:Unfortunately not. The cylinder that came off the plane couldn't be repaired. My normal mechanic had a good condition slightly used Millennium cylinder that I dropped off at Poplar Grove this week. Crossing my fingers that I'll be up and flying for New Years.

I can't recall ever meeting a "normal" aircraft mechanic.

I represent that remark !