Engine rough (miss shots) when idle in flight

[BoydBirchler]

Something leaking on the intake manifold makes sense, Intake/ induction leaks are greatest at idle when the difference between manifold pressure and ambient pressure are the greatest, and least at full power.

[rv8ch]

Originally Posted by elev666

Attachment 20467

I had kind of the same issue and I chased it for four months before we went looking inside the 0- 320 , very concerning this was occasionally jamming under the valve cover and causing a miss once every couple hours ��

Garry - any idea what caused that? Must have been quite a surprise when you saw it!

[Camillo]

As Freemasm wrote, the first thing I thaught was a vapor issue.

I may add that also during winter, when idling during flight (i.e. before stalls), engine is not very smooth. And that is why before every flight I run the engine WOT and cutting mixture to clean spark plugs. Or at least I thaught it was a sparking plug issue. Until september, when I flew in a very hot day.

The problem arose during both trips, after having idled for approx. a minute and after having pulled down flaps, turning in base, maybe in a nose up attitude. Both times with electric pump on (and immediately turned off). And after a few seconds of "up and down" RPMs (say 900-1200; however Dynon only shows a steady 900 RPMs) RPMs normalized, during the final approach and up to the flare.

So, it may be something connected with heat AND attitude/different air flow inside the cowl? When I slow down from 150 knots to 80 to lower flaps no problem. Maybe during this phase a lot of air passes. Then, when passing from 10° to 30° of flaps and speed reduces to 70 knots, with nose higher than before, maybe heat builds up inside the cowl and heats the fuel line.

There should be something which is a mix with heat and airflow.

Strange thing is that in both trips during the final approach (65/60 knots.), nose slightly up for landing the RPMs are OK (no swing). And on ground, idle speed is very smooth. Problem shows only in flight and only after more than 30'' idling.

In order to be on the safe side, I just ordered a couple of exhaust heat shields and a SCAT-4 1'' tube and flange to bring fresh air to the fuel line, as for magnetos.

Thanks.
Camillo

[elev666]

Originally Posted by rv8ch

Garry - any idea what caused that? Must have been quite a surprise when you saw it!

No real way of telling what caused the casting to break , they were first run cylinders and I had flown behind the engine 1000 hrs before it was overhauled, so 3000 hobes hours total time on the cylinders before this incident ,guessing it could have been metal fatigue , I understand Most places today that are overhauling engines are re-working cylinders because of the long backlog in new ones ,

[Camillo]

Update:
I installed 2 more shields on the exhaust, close to the fuel line.
I checked for the minimum idle setting and screwed clockwise two notches (it was only slightly leaned).
I got used not to idle the engine during the downwind, base and the first part of the final.
I only made a few flights from the issue, and not with that hot day when the issue occured. However, no more RPM fluctuation occured...
Thanks.
Camillo

[olyolson]

Camillo,

I’m no engine expert but your fuel line that attaches to the throttle body just above the throttle arm is VERY close to 2 exhaust pipes. Even though it’s fire sleeved I suspect that the fuel may be getting too hot and introducing some vapor. I might suggest rerouting that line up and over the intake tube to completely avoid the exhausts (or making a new fire sleeved fuel line to fit).

Just my two cents but you’ve ruled out the obvious intake leak and mixture.

I may be full of it and get flamed out in this forum but it wouldn't hurt and it’s certainly worth a try.

[rv8ch]

Originally Posted by olyolson

Camillo,

I’m no engine expert but your fuel line that attaches to the throttle body just above the throttle arm is VERY close to 2 exhaust pipes. Even though it’s fire sleeved I suspect that the fuel may be getting too hot and introducing some vapor. I might suggest rerouting that line up and over the intake tube to completely avoid the exhausts (or making a new fire sleeved fuel line to fit).

Just my two cents but you’ve ruled out the obvious intake leak and mixture.

I may be full of it and get flamed out in this forum but it wouldn't hurt and it’s certainly worth a try.

I agree with Oly - this looks like a vapor issue probably caused by the long runs of your fuel line near the hot exhaust. In fact, you've pretty much confirmed this since adding heat shielding to the exhaust pipes reduces the problem.

You can get hose of any size, shape, and clocking from suppliers like these guys: http://www.aircraftspecialty.com

[olyolson]

Camilo,

When I overhauled my engine I replaced all the fuel and oil hoses with new ones from TS Flightlines. They can make any custom length, size and whatever type of fitting you want. Check out the link below:

https://www.tsflightlines.com/products.html

I think it’s also recommended to have a fresh air blast tube to the mechanical fuel pump.

[Camillo]

Thank-you, all.
I will try to reroute that fuel line.
What should I inspect for the intake leak? I already checked the intake tubes. Somewhere else?
Thanks.
Camillo

[rv8ch]

Originally Posted by Camillo

Thank-you, all.
I will try to reroute that fuel line.
What should I inspect for the intake leak? I already checked the intake tubes. Somewhere else?
Thanks.
Camillo

Here is a procedure to test if you have an induction leak:

https://resources.savvyaviation.com/...t-diagnostics/

It's really hard to visually see if something is leaking. There are some other methods that don't involve flight, like using a vacuum or some pressure, but I have not tried them.