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Rough engine - any ideas?

Ed_Wischmeyer

Well Known Member
This was my second post-COVID flight to see if I was really recovered enough to fly, but that turned out not to be the issue.

Runup in the RV-9A was nominal, but wanting to beat traffic and not remembering that oil didn't warm up quickly in 50° weather, I took off before the oil was warm and oil pressure was high. We turned downwind to the crossing runway and as I retarded the throttle all the way on base, there were a few seconds of what my A&P/CFI in the right seat diagnosed as afterfire (fuel air mixture igniting in the exhaust system) and he heard what he thought was something coming loose. Mixture was probably leaned a little bit. I added a touch of power and all was fine again. Premature end of flight, but at least the plane was back in the hangar.

O-320, mags, carburetor adjusted to run rich for summertime cooling. Constant speed prop.

Hmmm:
* We added a gas two weeks before at another airport, but the plane ran fine on the flight back home from there, and it ran fine except for those few seconds.
* Mag check after we landed was nominal.
* Visual inspection of the intake and exhaust manifolds showed nothing.
* No evidence of spark plug blow by, as from a loose plug.

Did not inspect the carburetor linkages, and will do that tomorrow. Also, will inspect the exhaust system (crossover, no mufflers) for anything that might be hiding there.

Any ideas? Hypotheses? Seen it before?

Thanks!

Ed
 
If your mag checked out ok, I have no idea. When I started reading I was sure it was fouled plugs from running too rich. I just went through that. But yours checked out. I did learn that the plug media blaster from HF works great!

Good luck on figuring it out. I will follow this with great interest.
 
Can you download the engine data to Savvy and link it here? The visualizations of the parameters may help diagnose this.
 
….O-320, mags, carburetor adjusted to run rich for summertime cooling. …
Ed

I believe that’s the recipe for what your CFI called “after-burn”.

Cooler/denser air makes it too rich, leaving unburned fuel to cook off in the exhaust. Try flying around a little while before doing touch and goes, and don’t pull the throttle off too quick.

But, what do I know. Someone will be along to correct me momentarily. ;-)
 
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Just me

With oil not being warm I suspect it is just raw fuel burning. It is good you are checking things, but I suspect if you try it again, it will be different. I cant see how linkages would change from just sitting.
 
Its not oil temp. I take off all the time with OT around 70*. As long as OP stays under red line, all is good.

After fires occur for several reasons, but the most common is misfires. When one or more cycles don't fire, it sends an air fuel mixture into the exhaust pipe that is ripe for combusting. Once a successful combustion event occurs on that cylinder, the hot exhaust entering the pipe is enough to combust that stagnant mixture in the pipe and that is the bang or explosion you heard.

The misfire is likely an ignition issue, but could also be from a mixture that is too rich or too lean to fire on that cylinder. A compression issue, such as a sticking exh valve, can prevent the plug from lighting the charge and dumping that raw air / fuel mix into the exhaust tube. A spark will not ignite the air fuel mixture if it not compressed to a certain level - however, 1600* exhaust will.

After fires that occur immediately after closing the throttle plate are often to due to being too lean or too rich on the idle circuit (the former more common). Leaks in the exhaust flange can create popping sounds on deceleration, but not explosion sounds. If a slower throttle closure eliminates the problem, that points to the idle circuit.

My first step would be to properly adjust your idle mixture and see if the problem returns.

Larry
 
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I believe that’s the recipe for what your CFI called “after-burn”.

Cooler/denser air makes it too rich, leaving unburned fuel to cook off in the exhaust. Try flying around a little while before doing touch and goes, and don’t pull the throttle off too quick.

But, what do I know. Someone will be along to correct me momentarily. ;-)

Cold dense air results in a leaner mixture for a given fuel glow.
 
I believe that’s the recipe for what your CFI called “after-burn”.

Cooler/denser air makes it too rich, leaving unburned fuel to cook off in the exhaust. Try flying around a little while before doing touch and goes, and don’t pull the throttle off too quick.

But, what do I know. Someone will be along to correct me momentarily. ;-)

+1 .
 
Rough engine

0 320's with the Marvel carb are notorious for that. Reducing power slowly seems to avoid it.
This was discussed in detail many years ago on the Lycoming forum. I don't know if that forum still exists.
When you say your mixture is adjusted rich for summertime temps how are you accomplishing that? There is no method to richen the mixture except for changing or drilling main jet. Idle mixture adjustment is not likely to change this behavior in flight.
 
Cold dense air results in a leaner mixture for a given fuel glow.

+1

colder air is more dense and therefore contains more O2, in the same volume, than hotter, less dense air. Colder temps require more fuel than warmer, just like lower altitudes do.

If your idle mixture was lean for the summer, then the cold weather could have put it over the edge on the lean side. If you were already quite rich, then this may not be the issue. I set my idle mixture for optimum at 55-60* I fly as hot as 100* and as cold as 0* Never had a need to re-adjust for ambients.

As mentioned, best place to start is to get the idle mixture right. After fires on rapid deceleration is usually idle mixture improperly adjusted and usually too lean. Also, intake leaks are most problematic at idle and they create a lean idle condition even though there are no symptoms at full power. THe higher the vacuum, the greater the leak.

Larry
 
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Sometimes a take off on a cold engine may result in a valve sticking. You will know it, because it causes a shot of cold urine straight to the heart! By the time the the pilot tries to troubleshoot, all is well again. Tight valve guides are not uncommon. Many factory cylinders are a bit tight. This, exacerbated with running too rich often cause the carbon to promote valve sticking for a few seconds after take off. The cure is to use compressed air in the cylinder to aid in removing the valve springs, drop the valve down into the cylinder, use the proper reamer for the valve guide, clean everything out, and then use a "fishing" tool and a magnet through the intake or exhaust port to feed the valve back in to the pre-lubricated guide.
Then, if carbon was the culprit, research proper leaning techniques. I see lots of engines where operators think they are "babying" their engines, just to realize they are doing the opposite. Too rich causes valves to stick, exhaust valves unable to transfer heat to the seat, rings to stick, premature oil becoming dirty, and washing lubricants off the cylinder walls. Take a look at Avweb.com articles from John Deakin titled "Pelicans Perch". He uses real science, not conjecture, to determine the best way to operate our engines.
Regards,

DAR Gary
 
Retarding the throttle quickly with short exhaust stacks or with exhaust that has a defect close to a cylinder will make an after fire in the exhaust. Blown exhaust or a big crack could do it. I would look at exhaust joints gaskets etc.
Good Luck,
Mahlon
 
0 320's with the Marvel carb are notorious for that. Reducing power slowly seems to avoid it.
This was discussed in detail many years ago on the Lycoming forum. I don't know if that forum still exists.
When you say your mixture is adjusted rich for summertime temps how are you accomplishing that? There is no method to richen the mixture except for changing or drilling main jet. Idle mixture adjustment is not likely to change this behavior in flight.

I talked with the factory and they STRONGLY recommend against drilling out a jet. Instead, I got a different model carburetor, MA-4SPA 10-3678-32, that runs richer, and the factory adjusted it to run at the rich end. This is a different MA-4SPA than the factory carb that Van's provides.

Interesting that this was the first time I've ever seen that phenomenon with this plane after five years or so...
 
I talked with the factory and they STRONGLY recommend against drilling out a jet. Instead, I got a different model carburetor, MA-4SPA 10-3678-32, that runs richer, and the factory adjusted it to run at the rich end. This is a different MA-4SPA than the factory carb that Van's provides.

Interesting that this was the first time I've ever seen that phenomenon with this plane after five years or so...

Strangely, I sent my carb to the factory and they opined that it *needed* to be drilled out for my application. According to them, I had the "leanest" of the O-320 carbs - the one used in the O-320 D2j.

Of course, it is too rich now that they drilled it. ;-) Fortunately, that can be managed with the mixture stop and/or the mixture lever.
 
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