Undocumented Feature in the Carbon Cub

[turbopilot]

Ever wanted to run your Carbon Cub on 3 cylinders versus 4? Did you now that every Carbon Cub has a control on the instrument panel to allow you to selectively "turn off" combustion in the #4 cylinder?

During a ground run up of my new Carbon Cub about after about 2 minutes at idle, the engine began to run rough. I had just installed Dynon Skyview Touch and the Engine Management System was working great. Just as the engine began to run rough I noticed that #4 CHT and EGT was falling off rapidly while #1, #2 and #3 CHT and EGT's were normal.

So #4 combustion had stopped at idle. If I increased RPM the CHT and EGT would rise slightly to about half the values of the other cylinders and the engine would run very rough.

That started a day of troubleshooting. Compression was normal, valves where moving, cylinder head looked fine and the LightSpeed ignition system tested normal. Induction system looked good. Change the spark plugs, no change.

After several calls to CubCrafters. Stan asked if the primer was closed. Coincidently I had checked the primer control just before the call and fount it fully in the "closed" position but it was not locked over the detent. So visually it looked like the primer was in the closed position but by pressing on the primer and rotating it, the control was on the wrong side of the detent in the "unlocked" position.

After properly "locking" the primer, I started the engine and all was normal. So the primer fully closed but "unlocked" will stop combustion in the #4 cylinder. The capillary tube from the primer control empties into the top of cylinder head on #4.

I have now confirmed that others have discovered this "undocumented feature". At least one person discovered the feature in flight. Had this happened to me in flight I would be looking for the first safe place to land.

Why did this start after a few minutes of a ground run? I am guessing when I attempted to close the primer originally I did not rotate the control far enough to get past the detent. So at some point during the engine run the control popped off the detent to the "open" position causing additional fuel to be introduced down the tube to #4 cylinder. It is amazing to me that such a small "leak" in the cylinder can kill combustion. My speculation, but I would guess since the primer sources fuel from the top of the fuel strainer when the primer is unlocked the lower manifold pressure in #4 pulls fuel from the strainer through the primer then out to the cylinder head of #4 causing an excessively rich mixture. When I pulled #4 spark plugs they had black soot suggesting a rich mixture.

The "Starting Engine" checklist has the following warning about the use of the Engine Primer.



But I can find no warning about what will happen if the control is not "Closed and Locked" versus just closed. The "Locked" part was very important in my situation as I confirmed the primer was fully closed, it just was not locked.

I consider this "feature" to be a safety of flight issue. Loss of combustion in one cylinder will persuade most of us to get on the ground as soon as possible, possibly making a decision to land in an off airport location.

I have never used the primer in a Carbon Cub. I am considering deactivating the primer control. It is also a source of nuisance fuel leaks in the cockpit. The system could be easily deactivated by capping the line at the primer and #4 cylinder head. Certainly deactivating the priming system for summer operation would make sense.

[1473C]

I have inadvertently failed to securely lock the primer in the winter when an extra pump encourages a coughing engine. I sit high enough that the primer detent location is hidden. I ended up making a line on the primer face from the peg location to the center. After priming, I rotate the line to 12 o'clock and pull to test. The insert groove on my plane is at 5:30. Jake

[turbopilot]

I have inadvertently failed to securely lock the primer in the winter when an extra pump encourages a coughing engine. I sit high enough that the primer detent location is hidden. I ended up making a line on the primer face from the peg location to the center. After priming, I rotate the line to 12 o'clock and pull to test. The insert groove on my plane is at 5:30. Jake

Sounds like a good approach.

The reason I posted my experience was to let folks know what can happen if the primer is not locked. Not locking the primer is pilot error. It is also one of the issues that is ripe to be repeated by other aviators which can lead to a lot of lost time trying to understand what the problem is.

I would like to see a kit from Cubcrafters to deactivate the primer system. It could be treated as a seasonal aid for those who need it in the winter time (i.e. activate it in the Fall). For most of us the system would probably be deactivated permanently.

[richphil]

Sounds like a good approach.

The reason I posted my experience was to let folks know what can happen if the primer is not locked. Not locking the primer is pilot error. It is also one of the issues that is ripe to be repeated by other aviators which can lead to a lot of lost time trying to understand what the problem is.

I would like to see a kit from Cubcrafters to deactivate the primer system. It could be treated as a seasonal aid for those who need it in the winter time (i.e. activate it in the Fall). For most of us the system would probably be deactivated permanently.

Thanks for the info Bob, this was news to me. Will make sure it is locked. BTW, your panel is awesome. Let me know if you need any info on the servo hookups.

Richard

[turbopilot]

Thanks for the info Bob, this was news to me. Will make sure it is locked. BTW, your panel is awesome. Let me know if you need any info on the servo hookups.

Richard

Thanks Richard. Will be talking to Dynon engineers at AirVenture about servo mount structure. I showed them your mounts and everyone agreed they seem to be a better solution than the TruTrak servo mounts. TruTrak mounts don't seem to be very strong to me, but I am not an autopilot engineer.

For now I just ran the servo cable down below the seats. Will work on the servo mounts after getting some input from Dynon.

[Pete D]

As installed from the factory the primer is oriented such that when the knob is in the locked position the writing on the knob is right side up and level.

I know you have a post-factory custom panel Bob, but that is one way you can easily tell if it is locked or not if the primer has been set up that way. Typically you would set the nut on the aft side of the panel to clock the writing on the knob the use the nut behind the panel to tighten it up.

I have seen this before and watched someone say "yes it's in and locked" and push on the knob to verify. But to verify it is locked you need to PULL on the knob slightly to make sure it doesn't come out.

[turbopilot]

As installed from the factory the primer is oriented such that when the knob is in the locked position the writing on the knob is right side up and level.

I know you have a post-factory custom panel Bob, but that is one way you can easily tell if it is locked or not if the primer has been set up that way. Typically you would set the nut on the aft side of the panel to clock the writing on the knob the use the nut behind the panel to tighten it up.

I have seen this before and watched someone say "yes it's in and locked" and push on the knob to verify. But to verify it is locked you need to PULL on the knob slightly to make sure it doesn't come out.

Thanks Pete. Suggest you consider amending the warning in the "Starting Engine" checklist (4.3.2.2) of the AFM about a locked primer to include the warning that an unlocked primer will result in excessively rich mixture in #4 with rough running engine. In my case the engine would start fine but run rough. The engine never "stopped" in my experience as the current language suggests.