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Lycoming engine problem

We’ve got a C172 with a Lycoming O-320 engine and it’s having all sort of problems after the aircraft came from a major overhaul. The biggest issue is with the engine.. On the ground, at idle and high power all works fine, but when flying, at a certain point the engine starts misfiring at no particular time or engine thrust.

We’ve changed both magnetos, all plugs, leads, cleaned carburettor, checked compression of each piston and still is misfiring.

Just for reference, aircraft flies at about 1500ft, and the average temperature is 27degC

Anyone ever had this problem, or maybe someone knows of any solution?

Any help would be appreciate :)

LMML

Navygm wrote:

We’ve changed both magnetos, all plugs, leads, cleaned carburettor, checked compression of each piston

You did that after major overhaul?

I suppose you do not have kind of engine monitoring system?

What do you mean by “misfiring”? Is it like there is one cylinder that that misfires, or all of them? How long does it last? Does it clear by itself? What happens if you advance the throttle when this “misfiring” happens? Ditto if you play with mixture?

What was done to the engine during this “major overhaul”?

LFPT, LFPN

The problem with something like this is that you have no choice but to replace the easy bits first i.e. the accessories. It’s like this.

I wish I knew what to suggest – if indeed all those items really were changed.

Compression cannot be the reason.

Administrator
Shoreham EGKA, United Kingdom

Navygm wrote:

Just for reference, aircraft flies at about 1500ft, and the average temperature is 27degC

Do you lean the engine? If not, if you do, does it still missfire? Does it misfire also if you run it on one magneto (try left or right or both)?

What did the engine do during the overhaul of the airplane? How many hours since the last full overhaul has the engine got?

Misfiring like that may have various reasons, but it may well be an idea to check the valves. If one of them sticks intermittedly, that would do it.

LSZH, Switzerland

You will need an expert to run the full check list but misfiring on most of lycos is mostly ignition system (mags) or fuel system (carb).
Before you look to fuel/ignition, one has to rule out mixture and its control as reasons first (do you lean it on taxi? lean it on idle? lean on cruise? red lever stuck? over-rich cutoffs?)

Then have carb and mags checked, mags problems tends to be easy to solve while carb ones are more vicious, so I hope you will sort it out soon

After a major overhaul (you got new clean tight plugs?) if it misfires irrespective of power/mixture settings, I will rather bet on fuel quality (check for water in fuel, not just the visual look) otherwise it is probably carb related, so have the carburettor checked by someone who knows if you have limited knowledge on these (or if your aircraft has CoFA) as this happens only while flying, the carb diagnostic will be tricky on you side all you can do is to check those air intakes as well….

I would not fly it if it misfires on full throttle power and full rich mixture, that’s when I need it

Last Edited by Ibra at 25 Jul 23:24
ESSEX, United Kingdom

A sticking valve could show up as apparent misfiring.

But as I said I would double check that all the listed items (basically the whole ignition system) have actually been changed.

Administrator
Shoreham EGKA, United Kingdom

A sticky valve would show up when the engine is starting, it would not suddenly appear in flight, misfire or roughness at cruise would normally be fuel distribution – but to really isolate it you would need a engine analyzer – ie always one particular cylinder etc.

One curious possibility is that the wire going from the mags to the ignition switch is damaged and is rubbing on the airframe. Or even the switch itself. That would cause interruptions to the ignition.

It “should” happen on the ground too, but you never know…

Administrator
Shoreham EGKA, United Kingdom

aidanf123 wrote:

A sticky valve would show up when the engine is starting, it would not suddenly appear in flight

For having experienced a sticking valve in flight (very obvious from the downloaded engine temperatures), and researched other cases of sticking valves, it can definitely appear in flight.

Last Edited by at 26 Jul 07:55
LFPT, LFPN
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