Brief roughness in flight

YellowJacket RV9 · #1 08-09-2016, 06:38 PM

Hi all,
Today I had my first minor scare after about 90 trouble free hours. I am hoping to get some advice on how to track down the issue, so here is the background. Lycoming O-320-E3D with apr 750 total and 90 since new cylinders. Short story: I thought my high FP was flooding carb resulting in 5 seconds of roughness, but engine data indicates a 100 degree EGT drop on #1 only, along with 100 RPM. No other symptoms and all returned to normal after event. Long story:

Quick flight after installing nose gear fairing and wheel pant. Normal taxi and run-up. Very small, (<1 sec) hiccup on application of takeoff power, but was in the air and climbing well immediately after, so I continued the climbout and thought little of it, but stayed close and high.

Flight was uneventful for apr. 20 minutes, although I noticed fuel pressure a bit higher than usual. It was in the 5.5-6.8 range, when usually it hovers between 5-6. As I was descending to land, at about 55-60% power, there was a sudden and brief engine roughness and loss of power. I immediately traded airspeed for extra altitude, added carb heat, hit the boost pump, switched fuel tanks and confirmed mixture full rich. By the time I was done doing this the engine seemed to be making decent power again. I was babying it and due to nerves I really can't judge whether it was running rougher than normal from there on. If it was, it wasn't extremely rough. Made a straight-in high approach and landed without incident.

My initial thought was that the high fuel pressure flooded the carb, and possibly also did so during the takeoff run. I also considered carb icing (80 degrees OAT, 93% humidity). I usually apply carb heat below 55% power. I didn't notice any increased roughness after adding carb heat so I don't think that is a strong possibility.

I came home resolved to install a new fuel pump (I have heard that they can indeed fail high), as well as pull and inspect carb and float. But when I uploaded my engine logs to Savvy, it is not what I expected. At the time of roughness, there is about 100 degree EGT drop on cylinder #1 and 100RPM drop. No other changes. The fuel pressure immediately preceding the event was 6.0, so not extremely high. After the event, all parameters return to normal, although fuel pressure creeps as high as 6.9 again.

I also noticed when reviewing the logs for the pre-takeoff period that right after runup my fuel pressure was as high as 7.8, when began to drop to around 7 on application of takeoff power. This is definitely too high.

So I am thinking I have a fuel pressure problem for sure, which I will fix with a new pump hopefully. But I'm also thinking that didn't cause this specific issue as it was only an EGT drop on cylinder 1. I guess I will pull plugs next and inspect them.

I apologize for the long post but hopefully I provided enough information for the experts to weigh in. I do not intend to fly again until I am confident I have addressed however many issues need to be addressed.

This is the data log if it helps: https://www.savvyanalysis.com/flight...e-c63e6e033094

Thanks!

Chris

bullojm1 · #2 08-09-2016, 06:41 PM

Hi Chris,

How many hours are on your engine?

flightlogic · #3 08-09-2016, 06:47 PM

Has it been parked out in any rain lately? My gas caps have leaked. That got my attention!!

YellowJacket RV9 · #4 08-09-2016, 06:55 PM

Engine is about 750 SMO, but only 90 since new cylinders. Hangared and never in the rain. I will check gascolator and fuel tanks just in case however. Good thought.

Chris

Snowflake · #5 08-09-2016, 09:57 PM

Sticky exhaust valve?

Vlad · #6 08-09-2016, 10:07 PM

Were you over the water? Sorry couldn't resist

A fouled spark plug?

YellowJacket RV9 · #7 08-09-2016, 10:10 PM

I am starting to seriously consider the sticking exhaust valve theory. Even though I have less than 100 hours on the new cylinders, I have been running quite rich most of the time for break-in and low-level cruising around, and I hear the low-compression engines are even more susceptible to getting gummed up with 100LL.

I have still ordered a new fuel pump in an attempt to lower fuel pressure, but am going to check the exhaust valves as well.

Chris

YellowJacket RV9 · #8 08-09-2016, 10:12 PM

Quote:

Originally Posted by Vlad

Were you over the water? Sorry couldn't resist

A fouled spark plug?

No water but a pretty inhospitable swamp! Will check plugs though, too...

Chris

Snowflake · #9 08-09-2016, 10:15 PM

Quote:

Originally Posted by YellowJacket RV9

I am starting to seriously consider the sticking exhaust valve theory. Even though I have less than 100 hours on the new cylinders...

It's not unheard of for an engine with less than 100 hours on it (from new, even) to have a sticking valve.

DanH · #10 08-10-2016, 09:23 AM

Quote:

Originally Posted by YellowJacket RV9

As I was descending to land, at about 55-60% power, there was a sudden and brief engine roughness and loss of power. I immediately traded airspeed for extra altitude, added carb heat, hit the boost pump, switched fuel tanks and confirmed mixture full rich.

It's a curious plot. Mind a few questions?

How much altitude did you gain?

Confirmed mixture was already full rich, or did you move it to full rich?

Is your red cube located before or after the engine driven pump?

What are you logging on Aux3?

CS or fixed pitch?

Quote:

I came home resolved to install a new fuel pump (I have heard that they can indeed fail high), as well as pull and inspect carb and float

Calibrate the fuel pressure gauge first. I too have heard an engine driven pump can fail high, but based on principle of operation, I cannot determine how. On the other hand, pressure senders are notorious.

Not sayin' it has anything to do with this problem, but it sounds like a used engine from another application, so it certainly can't hurt to make sure your carb is up to date:
http://msacarbs.com/technical-data/s...bulletins.html