I had a similar problem with my O-540 (not fuel injected) about 5 hours after having had 6 brand new cylinders fitted. In flight it started running slightly rough and on the ground showed a 250 rpm mag drop at 2100 rpm but was fine at idle and full power. We had both mags overhauled, cleaned and checked plugs, tested the leads etc. etc.
It was only by chance when my engineer was about to remove the carb and I was looking around the new cylinders that I found the cause. Lycoming cylinders have a small grub screw to blank off the hole where the fuel injector is fitted on IO-540 engine cylinders; one of the 6 had lost it’s grub screw, the fix was a new screw costing £1.50. Red face for the engineer and a big bill to me for all the unnecessary work!
Check all the seals, there could be air getting in somewhere. I have seen this before.
Check inlet manifold gaskets, carburettor fitting and gaskets, airbox gaskets etc.
On an old TB20 I even found a pin hole in one of the aluminium fuel pipes which was drawing in air in flight but not on the ground. The owner hadn’t noticed the smell of 100LL in the cockpit as it had built up over a period of time. The only way we found it was to pressurise the fuel lines to see if they held their pressure.
LeSving wrote:
We’ve changed both magnetos, all plugs, leads, cleaned carburettor, checked compression of each pistonYou did that after major overhaul?
Because mainly the major overhaul was done to the aircraft, engine wise only minor maintenance was done.
Aviathor wrote:
I suppose you do not have kind of engine monitoring system? NoMinor things, like seals, timing, pressure check, as the engine was not due for an overhaul, it was the aircraft that was due.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?
When I flew with it, its like a car is knocking if you can understand that. It is being done only in flight randomly, no particular time or flight phase. If throttle is advanced, or mixture is varied, its the same, it keeps misfiring. *What was done to the engine during this “major overhaul”?
Mooney_Driver wrote:
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.
Regarding mixture, I replied on a previous post.
We’ve tried with 1 magneto yes, and on the left and right it still does the same.
Dont know exactly at the moment the engine hours.
Peter wrote:
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…
Infact that’s the only thing we did not check as yet… I will have a look at it in the coming days. I am suspecting that there is chaffing to ground intermittently
Appreciate all for your feedback… I will have a deep look at the wiring from the starter to the ignition switch and I’ll post updates in here so everyone will know just in case :)
A loose wire, unconnected with ignition, flapping about and shorting, but only in the airflow in flight?
(Sudden drops in oil pressure on our electric oil guage were eventually tracked to the “starter engaged” warning wire not reconnected, coming loose from its tape, and shorting. Why that only affected the one apparently unconnected guage I do not understand.)
Navygm wrote:
Appreciate all for your feedback… I will have a deep look at the wiring from the starter to the ignition switch and I’ll post updates in here so everyone will know just in case :)
Hmm, it would be very rare that this kind of problem affects both magnetos. Since you have confirmed the problem stays when switching to one magneto AND then the other then I would discard ignition issues.
(ie if one mag p-lead is being grounded intermittently , and you short it to ground with the ignition switch, then the intermittency should go away and that magneto should be completely off, leaving the other one to run the engine smoothly. This is assuming the mag switch grounds the mags properly, ie: mag check runs OK on the ground and in flight, when the engine is not acting up)
I dont think you confirmed about an engine monitor , but a problem as reported by Marchettiman or Colin could be detected by looking at individual EGT’s.
Failing that, you could look at spark plug colour post landing after an occurrence, and try to determine which is the ‘odd’ cylinder out of the four.
I am inclined to a mixture problem other than the carburetor (since mixture control actuation does not affect the problem)
It could be caused by either restrictions or leaks in airflow to one cylinder:
-All of the causes above about induction leaks
-An inlet or exhaust valve not functioning properly, either due to sticking open or to a leaky hydraulic tappet not opening the valve fully.
Good luck!
