Intermittent Engine Shuddering

[SLOWR426]

We have an intermittent rough engine issue in our ‘67 F.  Wondering if anyone else has experienced this, or has ideas as to the cause.

After leveling off into cruise and letting speed build up, adjusting prop RPM down, when I go to lean the engine I get a rough “shuddering” vibration.  Feels like the prop is out of balance or cylinders are fighting each other.  When I re-enrichen the mixture the roughness goes away.  No other control change causes the shuddering (throttle, prop control, mag).

we also are having problems fouling plugs.  Both before and after a recent top-end rebuild.  It’s like we can’t lean it enough to stop fouling plugs because the engine will likewise run rough/misfire  on the ground at low RPM’s.

[PT20J]

Are the plugs lead fouled or oil fouled?

How much oil is it burning?

Can you lean to peak EGT with the engine operating smoothly?

[DCarlton]

First I am not a mechanic or expert.  I do own a 67 F and experienced a very rough running engine condition several years ago that lead to a no power landing.  Fortunately I made it to the pattern before it quit making any power.  We chased every practical possible problem.  Had it running smooth and it happened a second time after an hour of flying.  Ended up overhauling the engine.  The one thing we didn’t investigate or clean or overhaul before the second failure was the fuel servo.  I still suspect that may have been root cause.  It was overhauled with the engine.  I haven’t had any problems since.  Runs like a top.  Even hot starts are easy now.  Good luck.  Stay safe.  It could be signs of something serious.  

[kortopates]

Maybe your just experiencing normal roughness you’d expect from leaning to “roughness”. If you have an engine monitor the data can tell you the source of the roughness. As an example it could be from onset of ignition misfire or it could be to imbalance cylinder power due to a large gami spread or even cylinder anomaly which is unlikely.


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[Yetti]

The last two engines that I know of that had this symptom were stuck/broken oil control ring. One of the cylinders ended up dumping lots of oil out.   Walnut shell blasted the cylinder clean to get the coking off.   New rings. Kept flying on that cylinder.  

[air cooled dad]

When mine shuddered it was from a sticky valve.  Opened up the cover and found a broken spring.  
 

Jason

’75F

[Shadrach]

To me this reads like a high power ignition problem. Years ago I had a plug that would check out about 2000 RPM. My partner scrubbed a flight because he could not lean the the engine in any meaningful way without roughness.  The problem did not show up in a normal run up mag check. I test flew the airplane I noticed the EGT on number one had climbed to ~ 1600. I leveled and did an mag check at full rich. The bottom plug on #1 barely qualified as intermittent. With the #1 cylinder running on a single ignition source, the combustion events between it and the other three cylinders was so mismatch that any leaning would cause roughness.

[SLOWR426]

16 hours ago, PT20J said:

Are the plugs lead fouled or oil fouled?

How much oil is it burning?

Can you lean to peak EGT with the engine operating smoothly?

PT20J:  The plugs are oil fouled.  It’s burning a quart every 3.2hrs (mineral oil).  We can lean to peak EGT smoothly at run-up but not at cruise.

[Shadrach]

6 minutes ago, SLOWR426 said:

PT20J:  The plugs are oil fouled.  It’s burning a quart every 3.2hrs (mineral oil).  We can lean to peak EGT smoothly at run-up but not at cruise.

That's sub optimal oil consumption but I would not assume the rough running and the oil usage to be related (though they may be).  Do you have an engine monitor?  EGT spikes are indicative of a cylinder running on a single ignition source. A high power mag check will pinpoint an ignition issue immediately.

[Fly Boomer]

1 hour ago, SLOWR426 said:

PT20J:  The plugs are oil fouled.  It’s burning a quart every 3.2hrs (mineral oil).  We can lean to peak EGT smoothly at run-up but not at cruise.

To let PT20J know you have responded to his post, you need to use his name like this:  @PT20J

[jaylw314]

15 minutes ago, Fly Boomer said:

To let PT20J know you have responded to his post, you need to use his name like this:  @PT20J

Quoting someone automatically sends a notification too, so @PT20J got pinged already   Sorry for all the pings @PT20J

[Shadrach]

1 minute ago, jaylw314 said:

Quoting someone automatically sends a notification too, so @PT20J got pinged already   Sorry for all the pings @PT20J

You guys sure are blowing up Skip's @PT20Jnotifications.  @PT20J is surely going to think something is weird when he opens up his notifications and sees that @PT20Jhas been mentioned so many times... I know that if I were @PT20J, I would be surprised.

[PT20J]

3 hours ago, SLOWR426 said:

PT20J:  The plugs are oil fouled.  It’s burning a quart every 3.2hrs (mineral oil).  We can lean to peak EGT smoothly at run-up but not at cruise.

Leaner mixtures are harder to ignite. It might be that the plugs are fouling and cutting out as you lean, but that's just a guess.

Something isn't right though. It shouldn't be burning that much oil and fouling the plugs. Maybe the cylinders didn't break in properly. I might do a compression test and a borescope exam. Are the cylinders chrome or steel? Steel should be mostly broken in within a few hours. Chrome takes longer and I've heard of people mistakenly installing chrome rings in chrome cylinders which pretty much never break in.

Skip

[PT20J]

1 hour ago, Shadrach said:

You guys sure are blowing up Skip's @PT20Jnotifications.  @PT20J is surely going to think something is weird when he opens up his notifications and sees that @PT20Jhas been mentioned so many times... I know that if I were @PT20J, I would be surprised.

Ross, I learned something new: No matter how many times you use the same @XXX, it only shows up in notifications once per post!

[SLOWR426]

So a correction on our situation that spawned this thread:  plugs are not fouled.  Uppers we’re fine and lowers we’re oily.

[Fly Boomer]

3 hours ago, Shadrach said:

You guys sure are blowing up Skip's PT20J notifications.

Mum's the word from now on.

[Shadrach]

2 hours ago, PT20J said:

Ross, I learned something new: No matter how many times you use the same @XXX, it only shows up in notifications once per post!

Next time I will recruit others!

[jaylw314]

23 hours ago, SLOWR426 said:

After leveling off into cruise and letting speed build up, adjusting prop RPM down, when I go to lean the engine I get a rough “shuddering” vibration.  Feels like the prop is out of balance or cylinders are fighting each other.  When I re-enrichen the mixture the roughness goes away.  No other control change causes the shuddering (throttle, prop control, mag).

Does that mean you ran on one mag at a time without the shuddering?

[Andy95W]

3 hours ago, Shadrach said:

Next time I will recruit others!

@Shadrach

@PT20J

Count me in!

[SLOWR426]

So we found the problem.  25 hours after a new top-end one cylinder has compression of 8psi, another 62, 73, 74.  My A&P has noticed that the zero compression cylinder has a pretty heavy hone pattern on chrome.  The 62psi one has no home pattern.  These were overhauled top ends from a repair shop.

Does anyone know of a document or manual that describes if honing is allowed on chrome IO-360 cylinders?

[Shadrach]

55 minutes ago, SLOWR426 said:

So we found the problem.  25 hours after a new top-end one cylinder has compression of 8psi, another 62, 73, 74.  My A&P has noticed that the zero compression cylinder has a pretty heavy hole pattern on chrome.  The 62psi one has no home pattern.  These were overhauled top ends from a repair shop.

Does anyone know of a document or manual that describes if honing is allowed on chrome IO-360 cylinders?

I’m not a cylinder expert but I would think that honing chrome cylinders would be a no no.

[Will.iam]

I would look at the other 2 cylinders and then play the game which one of these does not look like the other 3. 

[Ragsf15e]

I have had 3 chrome cylinders “overhauled” and they’ve been fine so far.  Not sure exactly what magic tgey did on them, but when I tried to have the 4th done a couple years later, I found it very difficult to find a shop that could do it.  Ended up buying a steel one.

[Jsno]

I had my cylinders nickel plated.  They initially broke in in about 40 minutes.  Ten hours later they are running great and oil usage is stable.  I would opt for nickel over chrome or steel.  You can have them replated with nickel for about 2K.  I don't think that you can hone chrome.

[EricJ]

4 hours ago, Jsno said:

I don't think that you can hone chrome.

Generally not supposed to since the chrome surface may be harder than the hone.