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S300C Engine roughness and loss of power


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I was scheduled to fly with a student yesterday, and on the way to pick him up from his house, I got a phone call from another instructor who was flying our ship. He said he had lost approx. 50% of available power, with engine roughness and black smoke pouring out of the exhaust. They did a precautionary landing at the nearest field and I drove their with my student. Ground runs showed the left mag left mag running rough and losing a lot of power. We pulled the spark plug and it was fouled with oil. That particular cylinder has been leaking oil since it blew a seal (seal was replaced, but a small leak remained). We replaced the plug, ran the aircraft up, and everything was fine. The instructor and student flew it back to home base while I drove with my student.

 

When my student and I got to the airport, we did our preflight and my student noticed signs of water on the dipstick (white milky residue) but we determined that it was not in the actual oil sump (wiping the dipstick and re-checking revealed clean oil). We completed the preflight and started the ship. Start went perfectly, and the engine was running smoothly with normal pressures and temps, until we ran it up to 2000 RPM when it suddenly started running VERY rough and losing power gradually. When two licensed pilots both stop and look at each other with wide eyes, you know something is wrong!

 

I ran it up to 2500 and the manifold went right to 14 1/2 inches (I have only flown this particular ship twice and I think the MP gauge is out of rig) without raising any collective. I did the mag check repeatedly but could not link the roughness with a particular magneto. Both alternately would run smooth and then rough. I ran the helicopter up to operating RPM and did another mag check, which showed nothing unusual, but at operating RPM the helicopter had a high frequency vibration and when I looked back I noticed the tail stinger was a blur! We decided to shut down and not fly (who would?)

 

I am wondering if maybe there is more to the water signs on the oil dipstick as the aircraft is frequently left outside in wet weather. Or maybe the piston rings are lining up? What would foul the plugs that badly? It's worth noting that since the last engine work was done the plugs have been fouling regularly. Compression checks were done on the last 50 hour and all was good.

 

I have no intention of flying the thing until they find the problem, but I am curious if anyone has any insight.

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when you say the engine blew a seal recently you'd have to know exactly what happened and what was replaced as there aren't really any seals that would cause loss of power. An oil leak may be out of the nose seal and that is very common. It sounds like you have an ignition problem. The 300 will not hover if you have one or two bad plugs or a mag that has problems. I'd put my money there. Usually if you have a cylinder problem you can still make rated power (low compression). You also might have a stuck valve (very common on the lycoming 360 series).

#1 ignition

#2 stuck valve

 

The vibration down the tail boom is because the engine if running rough.

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when you say the engine blew a seal recently you'd have to know exactly what happened and what was replaced as there aren't really any seals that would cause loss of power. An oil leak may be out of the nose seal and that is very common. It sounds like you have an ignition problem. The 300 will not hover if you have one or two bad plugs or a mag that has problems. I'd put my money there. Usually if you have a cylinder problem you can still make rated power (low compression). You also might have a stuck valve (very common on the lycoming 360 series).

#1 ignition

#2 stuck valve

 

The vibration down the tail boom is because the engine if running rough.

 

ApiaGuy knows his 300's.....if the valve is sticking, you can often hear the difference in sound from one side of the engine to the other.....kind of like a "ticking: sound. You also may see some oil if the push rod tube is bent.

 

The water in your crankcase could just be from condensation...I'm not a mechanic, but just from experience. You can fly with a stuck valve....you shouldnt as more damage can occur....but it still develops HP...until something comes flying thru the cylinder head!

Edited by Goldy
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Well, obviously it is ignition related as I had just replaced an oil fouled plug a few hours before. I am not sure what seal they replaced as I was not involved with that. I think the piston rings lined up and were allowing oil to transfer into the combustion chamber and that was what was fouling the plugs. There may be more to it than that, but I have not been informed as to what. Apparently they are replacing the cylinder this week.

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The first thing i would focus on is the valves, sounds like at least one might be sticking. I would check the log book and see when and if a 388c was performed on the engine. We do them every 300 hours on our 300s and R44s as part of our mx regiment. If one has not been done it might be time to do so, some schools/shops never do them, for us it's just another part of our safety program as we cannot afford to lose ANY power up here. In recollection, i remember someone posting that it's more expensive to train in Denver, it might be, we have to be extremely meticulous in our mx practices as there is just no margins on power, most schools around here start at a mile high, our ramp is at 5673, so we have our magnetos adjusted and spark plugs pulled and checked a lot. Before Heli-Ops i saw stuck valves in two or three 300s and two R44s, since we opened, almost three years, i have only seen two sticking valves and they were in 300s, both of which had not had the 388c performed.

 

dp

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  • 2 months later...

Trie your advice and we got the valves cleaned, but the engine was running very rough and I was still getting high MP settings, so we decided to shift our attention to the magnetos and associated wiring. Therin lies the problem. The leads to the wires were all frayed where they go into the spark plug which was causing mis-firing. There were several other discrepancies we noted and wound up just completely re-wiring the mags. It starts and runs better than ever now. It was a rather nerve racking ordeal, but I sure have learned a lot about these engines! Thanks for the help and the suggestions!

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Trie your advice and we got the valves cleaned, but the engine was running very rough and I was still getting high MP settings, so we decided to shift our attention to the magnetos and associated wiring. Therin lies the problem. The leads to the wires were all frayed where they go into the spark plug which was causing mis-firing. There were several other discrepancies we noted and wound up just completely re-wiring the mags. It starts and runs better than ever now. It was a rather nerve racking ordeal, but I sure have learned a lot about these engines! Thanks for the help and the suggestions!

 

If the valve cleaning doesn't help the oil issue, take a look at the rings. Has the engine been overhauled? Who did it. As I have seen proven time and time again, a good overhaul is much less expensive than a cheap overhaul. Just in lost time and frustration alone.

 

As for the tail boom vibration, check the bushing inside the tail boom for wear. If you can move more than just a little bit, maintenance needs to look at it. We had a tail boom vibration in one of our 300s. Turned out it was caused by a slightly elongated bolt hole in the center attachment fitting where the boom support strut attaches. Unless you had your fingers on the right place on the fitting, you would never feel it move.

 

If you start to see MP of 15 inches or more at 2500 RPM, you will most likely have an induction leak. The first thing maintenance wants to do is lean the the idle which only makes the problems worst.

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High MP indications can also be a sign of ignition problems, like we were having, because if a spark plug is not firing, you are not getting as much power out of a burn as you normally would, so you need a higher power setting to maintain a given RPM. In our case it was a bad mag. I think there were several different issues that were contributing here. All in all, we replaced a cylinder (it was transfering oil), we cleaned the valves (they were nasty and causing sticking), and we re-wired the magnetos (the leads were all jacked up and causing mis-firing). Have not had any issues since. Thanks again for the advice. If it were MY ship and MY school, we would certainly have a better maintenance program, but it's not, so all I can do is put my foot down when something is wrong and say "this thing is not flying until it's fixed".

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