Occasional Rough Running Engine

[Pete G]

My RV-6 powered by a LyCon IO360-EXP (w CS Prop/P Mags/NGK Auto Plugs) occasionally runs rough at power settings above cruise, eg, takeoff. In several occurrences I have observed the #3 EGT to plummet from ~1300 degs to below 700 degs while the other cylinders were steady around 1300 degs. Reducing power resolves the issue with the #3 EGT recovering to normal. Last occurrence required power reduction to 16 inches MP. I have done repeated ground runs and test flights and have yet to duplicate the problem. Have done full power ground runs on L only, R only, and both PMAGs, all satisfactory. Have replaced plugs only to have problem return. Checked fuel nozzle and spider, no problem found. Have not done Lycoming SB 388C (valve and valve guide insp), may be my next step. Any suggestions? Thanks in advance for your ideas

[1600vw]

Well that egt tells you you have a cylinder going dead. Something is causing this. You have two spark plugs so one should be able to keep this cylinder hot without the other. So its not a plug. You then have the valves. if it was a valve I doubt it would regain power with the reduction of the throttle.

What would cause one cylinder to go dead that would be my question. But the answer I do not have.

Hope you get this worked out and sorry I cannot help anymore then this. if this was even any help. I kinda doubt it. Again good luck.

Tony

[Dana]

If it wasn't dual ignition I'd say a spark problem. Could it be a bad hydraulic tappet?

[1600vw]

Dana I was wondering this myself.

[cub builder]

My RV-6 powered by a LyCon IO360-EXP (w CS Prop/P Mags/NGK Auto Plugs) occasionally runs rough at power settings above cruise, eg, takeoff. In several occurrences I have observed the #3 EGT to plummet from ~1300 degs to below 700 degs while the other cylinders were steady around 1300 degs. Reducing power resolves the issue with the #3 EGT recovering to normal. Last occurrence required power reduction to 16 inches MP. I have done repeated ground runs and test flights and have yet to duplicate the problem. Have done full power ground runs on L only, R only, and both PMAGs, all satisfactory. Have replaced plugs only to have problem return. Checked fuel nozzle and spider, no problem found. Have not done Lycoming SB 388C (valve and valve guide insp), may be my next step. Any suggestions? Thanks in advance for your ideas

Swap your fuel injector between #3 and any other cylinder and see if the problem follows the injector. Ignition isn't going to be your problem, so the issue is likely going to be either the injector, injection line or the distribution block. A broken valve spring is a remote possibility if the cylinder drops off at higher rpms and recovers at lower rpms. Easy to verify by pulling the valve cover on #3 and leaning against each valve. If you have a broken or failing spring, you'll have a spongy valve that will push open by hand.

-Cub Builder

[Mike M]

Sticking exhaust valve is very common cause of the symptoms described. I've found that pulling the prop through by hand (warning done wrong this can kill you!) and comparing the compression between cylinders may help diagnose it. Better method is to pull each exhaust rocker arm and try moving each exhaust valve in and out by hand. If they don't move freely, one or more may be sticking when operating the engine.

Also, one may not be completely seating each time due to accumulated crud buildup. Pressurize each cylinder with a compression tester fitting (this can be done at BOTTOM dead center because you're not doing a compression check) to find out. One may want to thunk each exhaust valve with a plastic mallet to "pop" it a few times to see if it seats better. If that makes a difference one may consider cleaning the valve and guide. My limited experience has taught me if one is sticking, clean the others even if they seem to be working ok.

If that sounds kind of like Lycoming SB 388C (valve and valve guide insp) well, gosh, I guess it does. But don't let it get you down. What I described is easy, cheap, and a possible fix. You can do the SB exactly as Lyc says if you find a discrepancy or if you prefer. Much more rare to have intake valve stick (I am told) but one could do the same check to intake valves also. Your mileage may vary.