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Rough Engine at High Power

Michael Burbidge

Well Known Member
Today as I was climbing out my engine seemed to be running rough. It wasn't dramatic, but enough that it got my attention. I was at about 1000 AGL, and still full power when I noticed it. It didn't start then, it's just that my attention had been on takeoff.

It was subtle enough that I had to convince myself that the engine was running rougher than usual.

What finally convinced me was an anomaly in CHT temperatures. My number 4 cylinder always runs colder that the others, but it seemed unusually cool.

I stayed close to the airport and leveled off at about 2500 MSL, and pulled the power back some. When I pulled the power back, the engine seemed to smooth out and the CHT on number 4 gradually climbed while the temp of the other 3 cylinders decreased. This seemed odd to me.

I stayed level at 2500 MSL and after a few minutes went to WOT again. The CHT of 1,2 and 3 climbed, and the CHT of 4 decreased. This definitely doesn't seem normal.

My Engine is a carburated O-320-D3G, one p-mag, one slick.

I don't have much experience with engines, so I'm not sure how to diagnose this. I didn't have the right tools at the hangar to be able to de-cowl and inspect the spark plugs. I will do that tomorrow.

But I'm just wondering if this is normal and what are the possible causes?

Attached is graph of CHT/EGT/MAP. At about 12:10 minutes into the flight is where I was leveled off at 2500 MSL and applied full power. I pulled the power back again at about 13:30, and again applied full power at about 15:18, reduced power again at 17:42.
 

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Primer?

A very easy thing to check…do you have an old school plunger type of primer? If so make sure it is closed all the way.
 
Just a wag, but spark plugs can and do fail under higher pressures, simple and worth a look. Do you know someone that has a spark plug tester?
 
I don't have a primer line.

I'm thinking spark plug also. Today I'll get the aviation plug tested and just replace the auto plug.
 
My understanding is that an induction leak would cause EGT to rise. Both EGT and CHT were falling at full power.

I'll do those inflight tests today.
 
I don't have a primer line.

I'm thinking spark plug also. Today I'll get the aviation plug tested and just replace the auto plug.

If one plug failed, the EGT on that Cyl should rise (~150*) and CHTs drop a bit. If both plugs failed, both would drop like a rock.
 
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Mag Check

Have you done an inflight mag check? I have dual P-Mags and had a coil issue on one P-Mag that affected one cylinder.

Search for Martin Pauly's youtube video: The in-flight Lean-Of-Peak Magneto Check explained
 
My understanding is that an induction leak would cause EGT to rise. Both EGT and CHT were falling at full power.

I'll do those inflight tests today.

With a carb'ed engine, induction leaks should be barely detectable at WOT and VERY noticeable at idle, with a smooth transition between them. Your low RPM numbers look good, so induction leaks seem like an unlikely candidate. Either way, when well ROP, small induction leaks should raise EGT and large ones drop EGT; It all depends upon size of the leak relative to current mixture delivery.
 
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My first guess would be the beginning stages of morning sickness (i.e. partially sticking exhaust valve in #4). If the situation gets better after 10 minutes of flight, that reinforces the guess.

How many hours on the engine?

Larry
 
I 2nd the mag check. You need to see if this behavior is there independent of which ignition is on.
 
I had similar experience

A couple years ago I had similar roughness at full throttle and climb on my carburetor O320 RV4. I have old school engine monitoring that does not show separate cylinder conditions. After landing and clearing the plugs with a lean ground run up, I took off and shortly the roughness re-occured. As an A&P, I knew I needed to take a closer look before further flight. I found a plug with a cracked and loose insulator that was moving around and likely shorting the plug. Easy fix. Hopefully you find the smoking gun soon.
 
LOP Mag Check

Here's my LOP Mag Check, first left, which is my p-mag, then right which is my slick.
 

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Induction Leak Test

And here is my induction leak test. Something is going on with #4 in this test.
 

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One other note

At the beginning of my test flight I still noted the rough engine at high power and decreasing CHT on #4.

The tests took me about an hour to conduct. At the end of the flight I descended to 1500 MSL and tried to reproduce, but the engine ran smooth at high power and CHT on #4 rose as I applied full power, like the other cylinders.
 
sounds like

At the beginning of my test flight I still noted the rough engine at high power and decreasing CHT on #4.

The tests took me about an hour to conduct. At the end of the flight I descended to 1500 MSL and tried to reproduce, but the engine ran smooth at high power and CHT on #4 rose as I applied full power, like the other cylinders.

I am no expert, but I did stay at a Holiday inn Express last night. Sure sounds like plug wires/ magneto cap is bad. JMHO
 
Jerry when you had your coil issue, did it show up in the LOP Mag Check?

Yes, the EGT dropped to nothing on cylinder #1. I knew it was either the P-mag, plug wire, or plug. Replaced the wire and plug with no change. Ordered a new coil after visiting with Brad and that corrected it.

Regarding induction issues, when you reduce the throttle, the EGT nearest to the leak will rise. Throttle back on downwind and pay attention to cylinder 4 EGT to see if it is rising. If so, the gasket and hose or connector needs replaced or tightened up.
 
Following as I'm having similar issues. C1 is low - C2 is 'normal' - and C3/4 high after take off. C2 will raise above C3/4 (they lower) with some leaning.

Went to 5000 and did mag check few days ago - when on EI, temps dropped. On Mag, temps rose (CHTs - less concerned with EGTs - but they did all rise when on one or the other).

Took Mag in for inspection/rebuild.

Checked plug wires on EI - found 2 of them reading high or intermittent (in the kohm - Mohm or Infinite ohm range). Replacing all 4 (already replaced Plugs a few weeks ago with no improvement).

Haven't ordered coil yet - but will do better ohm checks to verify no coil issue on EI.

Hope to see solution to your problem posted here soon :) Will see if I can download my Mag check data later tonight (Dynon D10.. takes a bit more technology to download).
 
And here is my induction leak test. Something is going on with #4 in this test.

Hmmm.

When you increase power, EGT 4 drops. When you reduce power, EGT 4 rises to be in family with the others. Puzzling.

Some ideas:
- Post the full URL of the Savvy Analysis browser page for this flight. I think access to the full URL lets us see the whole flight and select different parameters for exploration.
- Consider conducting and posting a lean sweep in the condition where the #4 EGT drops (full power). That might offer insight into what's going on with #4.
- Consider subscribing to Savvy Analysis for a year. They are good at figuring out engine problems based on analyzer data. $129 is probably money well spent.
- As has been mentioned, this could be the result of #4 exhaust valve sticking. Lycoming SB 388 and SI 1485 recommend the wobble check (and potentially subsequent exhaust valve reaming) at 1000 hours. It appears you have more hours than that on this engine.
-- If you don't have access to the wobble test fixture, consider just going ahead and reaming #4 exhaust.
-- A local mechanic at my airfield was an absolute genius with valve reaming. He could drop a valve (using a little long reach magnet and a wire tool he made), ream the valve guide, and pop the valve back in to position in less than an hour. I paid for his time just to watch and learn. $100 well spent.
 
Savvy URL

Here's the URL: https://apps.savvyaviation.com/flights/5171302/0f04b70c-9317-4787-9982-fc36f5ffae5a

I did a lean sweep, just didn't post it. It starts at 25:00. I need some practice doing that test. It wasn't the best. I also don't have auto pilot, so I have to fly the airplane while doing the test. Not too hard, but...

I did sign up for SavvyAnalysis Pro. Their initial response was to check the primer nozzle. I hadn't indicated that I don't have a primer line installed. So now I'm waiting for their next response.

Michael-
 
Maybe a broken valve spring on #4, causing one of the two valves in #4 not to completely close at high RPM. On the Lycomings I've worked on, there are two springs (concentric) for each valve. Either could be broken.

One needs a good quality valve compressor tool to disassemble the valve spring system(*). Seems worth a look.

That first article in bjdecker's post on page 1 of this thread describes that, and several other possible causes.

(*) A crappy generic valve spring compressor can side load the valve spring retaining plate and scratch up the side of the valve, ruining it. (**)

(**) Experience: what you get just after you need it.
 
Here's the URL: https://apps.savvyaviation.com/flights/5171302/0f04b70c-9317-4787-9982-fc36f5ffae5a

I did a lean sweep, just didn't post it. It starts at 25:00. I need some practice doing that test. It wasn't the best. I also don't have auto pilot, so I have to fly the airplane while doing the test. Not too hard, but...

I did sign up for SavvyAnalysis Pro. Their initial response was to check the primer nozzle. I hadn't indicated that I don't have a primer line installed. So now I'm waiting for their next response.

Michael-

This looks very different than the chart in the first post and shows #4 CHT similar to all others. Sticking exh valves tend to be on and off at first, though eventually it will be consistent. This further reinforces my guess of a sticking exh valve on #4. One flight it is very cool and the next flight it is normal. Still recommend doing the wobble test to confirm or reject that guess, as there are not too many things that will cause one cylinder's CHTs to be 50* cooler on one day and normal the next.

Larry
 
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Hi,

It's probably an sticking exhaust valve as other said but I don't have the knowledge to tell what are the symptoms of a sticking exhaust valve.

but you can check if the 2 halves of the carb are properly torqued.
Sometimes it is possible to have fuel leaking between the 2 halves sucked on the venturi, then one cylinder (nearest to the leak) become really rich (like with a primer) leak.

During your leaning tests the #4 didn't make any peak, it was definitly the richest.
But during take of it was fine until you reduced the throttle (and then if was the leanest, at 9'34").
Perhaps your hand pressure on the throttle lever can do something if the carb halves are not correctly screwed (the more you push, the more it leaks) ??

Herve
 
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