Engine suddenly running very rough

[Northernliving]

Originally Posted by F1R

Annual inspection ... maybe have a look at your spark plug to wire end connections.

High probability of a human hands induced problem.
Check the B nuts or any joint in the fuel system that was inspected or worked on.

+1 for a look at your plugs and wires. Check the torque. I know someone who had a similar problem that turned out to be ignition related caused by the A&P during the conditional inspection.

[N941WR]

I had this happen one time, declared an emergency, and then fixed it before turning Base.

Turns out I pulled the mixture at the same time I reduced the throttle. Not enough to make the engine stop but enough that it was running really rough. Duh!

[lr172]

Originally Posted by Northernliving

+1 for a look at your plugs and wires. Check the torque. I know someone who had a similar problem that turned out to be ignition related caused by the A&P during the conditional inspection.

We haven't seen data yet, but severe shaking/imbalance usually points to one cylinder dropping off (failing to combust). If that was the case, ignition is improbable, as both plugs would have to fail at the same time for the cyl to drop out. One plug failing on one cylinder won't even be noticed by the average pilot without looking at the EGT gauge. Wiping out one whole ignition system, like a mag, also results in only slght changes to engine sound/feel; Though most will usually notice it as it occurs but may not notice if they missed the occurrence. Definitely no shaking / vibration issues.

Not saying ignition was not the culprit. Just not enough data to determine how likely. Never a bad idea to validate ignition health.

Larry

[Carl Froehlich]

One other thought as reading the OP reminds me of a similar incident. Back in the day working to get a ticket flying a rented spam can, this same scenario happened. The engine ran strong on take off, then when throttling back to stay in the pattern the whole plane started to shake. Put the throttle back in an the engine was smooth. Throttled back and again got the vibration.

I was over the airport so pulled the throttle out idle and landed.

Short inspection of the engine showed an intake tube with one bolt missing, the other bolt almost all the way out. It seems at full power there was enough suction to pull the intake tube tight on the cylinder. Other than full power it dropped out - as this was a carb plane that cylinder went cold. I assume with an injected engine you would still have fuel flow but not enough to burn with the excess air coming into that one cylinder.

Carl

[Phil]

I just ran down a similar issue. It turned out that I had an intake leak that would only rear it?s ugly head at certain power/mixture settings.

It took a while for me to believe I had a leak, because my engine was very new. But the age of the rubbers and gaskets were more of a contributor than the hours on the engine. It?s now on my list of things to replace every 3rd annual and it only cost about $75 and 4 hours of work to replace them.

Running great now.

Check out Vic?s article in the latest (Feb 2020) Sport Aviation magazine. It?s well written. It might wing a bell.

[lr172]

Originally Posted by Carl Froehlich

It seems at full power there was enough suction to pull the intake tube tight on the cylinder. Other than full power it dropped out - as this was a carb plane that cylinder went cold.

Carl

At WOT/full power, there is very little vacuum in the intake tube. Therefore, the leakage area doesn't introduce a lot of additional air. Air is somewhat like electricity in this regard, taking the easy path, which is the big fat intake tube. The leakage will create some leaning, but propotional to the difference in size between the tube and leak opening. Lyc's are typically quite fat at full rich that this extra air wouldn't make a significant impact. At lower power settings and especially idle, vacuum is very high and that will draw in a much large proportion of air with no fuel mixed in through the leakage area. Vacuum is caused by a restriction and the vacuum will happily pull disproportionately more air from the leak air to overcome the restriction.

Larry

[BobTurner]

Originally Posted by lr172

At WOT/full power, there is very little vacuum in the intake tube. Therefore, the leakage area doesn't introduce a lot of additional air. Air is somewhat like electricity in this regard, taking the easy path, which is the big fat intake tube. The leakage will create some leaning, but propotional to the difference in size between the tube and leak opening. Lyc's are typically quite fat at full rich that this extra air wouldn't make a significant impact. At lower power settings and especially idle, vacuum is very high and that will draw in a much large proportion of air with no fuel mixed in through the leakage area. Vacuum is caused by a restriction and the vacuum will happily pull disproportionately more air from the leak air to overcome the restriction.

Larry

+1. This is the correct explanation. A simple test for small induction leaks is to run full rich, wide open throttle, observe EGTs. Smoothly retard throttle to idle. If any cylinder shows an egt rise significantly different than the others, look for an induction leak on that cylinder’s plumbing.

[Raymo]

Another data point is idle MAP, which should be around 12" or less. If higher (15"), suspect an induction leak.