Lycoming engine problem

[navygm]

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 :)

[BigEndBob]

Have you replaced the mag switch?
Try running with the wires disconnected.

[CloudHound]

Don't forget that with the switches out of the circuit the mags are live all the time. Just watch that prop.

[stevef]

I'd check the following:
Air filter.
Carb heat control rigging.
Primer seals for internal leakage.
Does moving the mixture control have any effect on the misfire?

[wrecker]

What fuel are you running on?
We had a similar problem using UL91 with a high compression 0-320 in temperatures above about 25C
Changed back to 100LL and all well

[Jim59]

I was also going to ask about fuel and agree with wreker above when hot. it's the fuel temp in the tank that is critcal - not the ambient temperature. Is it 'fresh'. After about 6 months fuel is starting to break down. CAA used to require avgas to be chemically tested if storage tank not refreshed with new fuel within six months before dispensed. Are aircraft fuel tank vents clear / unblocked? Fuel filters OK?

[NutLoose]

Primer seals for internal leakage.
Not just the seals but the primer lines as well, also check the induction elbows are not leaking, another on the induction side it loosen the rubber joint between the elbow and the pipes that come out of the sump and ensure those are fully swaged into the sump, IE they do not move, have known them to be loose and drawing air on Lycomings.
I take it they are the correct plugs for the engine, check.

Other possibility is a sticking valve.

https://www.lycoming.com/sites/default/files/Suggested%20Maintenance%20Procedures%20to%20Reduce%20the%20P ossibility%20of%20Valve%20Sticking.pdf

[dogsridewith]

Additional to fuel filter(s), can blow back through the fuel line to clear any socks or screens in the tank.

[stevef]

Any update on this?

[Fl1ingfrog]

A simple thing to check are the fuel caps. If only one of the filler caps is vented and on the wrong wing you can have a similar problem to that which your experiencing. If only one filler cap is vented (it is common to have both caps vented nowadays) the vented one should be on the starboard wing. The port wing is always vented of course because of having the vent pipe attached. This problem can be accentuated when both tanks are full to the brim and the overhead crossover pipe is also full of fuel.

Its worth flying with one tank only selected to see if the problem your having still persists or does so on only one of the tanks. It is normal to operate a C172 with both tanks selected. Cessna warn that an air lock is possible, although at high altitude, at the point both tank feed pipes are joined. They therefore recommend that both tanks are not selected at high altitudes. Your operating at low altitudes of course but ..........

[Big Pistons Forever]

There is an AD that requires both fuel caps be vented. No C 172 should be flying without 2 vented caps.

To the OP: I would suggest that as soon as the engine starts misfireing switch to each mag in turn, I had a similar problem in a club airplane. I isolated it to one Mag which despite having just been overhauled, turned out to have a bad coil.

[27/09]

Is it actually misfiring or is it a roughness or vibration?
Your problem may be a faulty or broken valve spring or a sticking valve.