Occasional Rough Running Engine

I have an RV-6 with an IO-360 EXP, dual P Mags, using NGK auto plugs. My engine randomly runs rough at full power, just after liftoff on three occasions, I have observed the #3 cylinder EGT drop to below 700 degs while the other three cylinders are 1200-1300 degs. Reducing power resolved the roughness in each case and the #3 EGT went back to the 1200-1300 deg range. After today's event I landed and did a high (not full) power run-up at 2400 RPM. I checked each P MAG individually and found no problem. I did one trip around the pattern with no roughness. When this happened a few months ago I installed new plugs in my #3 cylinder on my A/P's advice. The problem returned today. This problem is persistent but random. Has anybody else encountered this circumstance? Any suggestions?

Rough engine with Electronic Ignition

I have an RV-8 equipped with IO-360-M1B with Slick Mag left and ElectroAir EI right side. I recently had a severe rough running engine after takeoff and landed. Long story short, it turned out to be the EI switch. One screw holding battery power wire ring terminal to bottom lug of switch had loosened causing ring terminal to float, causing intermittent contact, and caused the rough run. A good tightening allowed engine to again run smooth.

How many hours on the engine?

Any particular time of your flights that this happens----i e taxi, take off, cruise etc?

Have you heard the term "morning sickness" as it relates to these engines?

Pete G said:

I have an RV-6 with an IO-360 EXP, dual P Mags, using NGK auto plugs. My engine randomly runs rough at full power, just after liftoff on three occasions, I have observed the #3 cylinder EGT drop to below 700 degs while the other three cylinders are 1200-1300 degs. Reducing power resolved the roughness in each case and the #3 EGT went back to the 1200-1300 deg range. Has anybody else encountered this circumstance? Any suggestions?

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I will be watching closely to see what you find.

I have almost the same thing happen to me randomly on the IO-540 in our -10.

In my case #1 cyl egt drops while engine runs rough a few seconds.

reducing RPM to ~2550 makes it smooth out and run fine again and then it will not repeat the problem sometimes for a long time.

I have checked the valve stems and guides for sticking valves per Lycoming SB no less than 3 times and have run a reamer through the guide with no joy.

Cyl and valves look great with bore scope.

Changed push-rod length to put valve lash closer to the middle of the published range.

Changed spark plugs.

Checked for water in the fuel (although it is always #1 cyl so it should not be a water problem)

Checked injector orfice for trash.

The way the cyl goes cold I dont think it is spark related because I have 2 independent igniton systems (lighspeed and Bendix)

Its killing me

I once had a piece of trash in the fuel distribution spider that caused a rough running engine. It was a curl of black rubber in one of the ports, only visible when I pulled the piston out of the valve body.

Might be worth a look.

I just recently reamed my #2 exhaust valve because of a similar sounding issue with occasional rough running, cold egt that would last for a very few seconds. Based on how tight the valve stem was in the guide, I am confident this was the problem.

I would look at the injector and fuel spider. Small tiny things can do this and pass through.

Could also be a hydraulic lifter unit collapsing.

To answer Mike's questions:

The motor has 600 hours since being overhauled by LyCon in 1999. I have put about 200 hours on it in the last two years. It has very good compressions, normal oil consumption, and runs strong 99.9% of the time.

The problem has occurred three times shortly following liftoff with a thoroughly warmed up engine, eg, second or third leg of a local area tour. It occurred once while leveling off from a descent at 9k'.

To all others, thanks for your thoughts and ideas.

RV10inOz said:

I would look at the injector and fuel spider. Small tiny things can do this and pass through.

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+1
Excluding a stuck valve, your symptoms point to something floating around in that cylinders fuel delivery system. The immediate clearing on reduction of power points me there. I would pull off the spider and clean it, as well as flush out the line to affected cylinder.

Larry

Check your PM's.

Intermittent rough

Agree that if moving the throttle changes or fixes the problem it points to fuel delivery. Saw similar issue before on a Aztec with parallel valve IO-540's. Every 5-10 hrs right engine would run rough, sometimes on startup or in cruise but usually right after takeoff. Reducing power would reduce the roughness but not eliminate it. Pilot would land, shut down and on next start problem was gone and engines would run fine for a few hrs. One time it happened and before restarting I took off the valve covers and found a flat lifter. In the hyd unit was a tiny piece of clear plastic. It looked like same material as the tubes that spark plugs come in. It was too big to exit the lifter so it just floated around in there and occasionally would interfere with the check ball and let the lifter go flat. This is short version of story that took a lot of troubleshooting time.

Don Broussard

RV 9 Rebuild in Progress