912uls Spark Plug Diagram

[ChrisRV12new]

Thank you for the update. I’m working today but looking at getting some flying in Sunday-Thursday. I’m located at KPLU, Pacific Northwest winter weather so VFR conditions little bit rare.

We pretty much have the same issue. I’m going to collect more data this week as we work through this issue.

I’m planning to take the Rainbow Aviation course for LSA repairmen Feb 6-7. Should get some good info there. Also have a team building an RV12 6 hangars down along with a college on field flying a Tecnam twin. Could start asking them for advice as well. Good luck, please let me know if you find anything.

-Chris

[ChrisRV12new]

Is it possible the spark plugs are fouling on the taxi out from cold engine? Mine started few weeks ago when it got cold. And it tests properly in flight. 80 RPM drop. I’m going to print off the carb section of the heavy maintenance manual to learn.

You just got yours back from Lockwood so they seem like they would be in perfect condition. Is it possible to adjust the lean/rich mixture for the taxi out?

Right now I think my plugs are fouling. But I don’t know for sure.

[Piper J3]

I still believe the rough running syndrome when doing initial ignition check at 4000 RPM is carb related and not ignition. Capacitive Discharge Ignition (CDI) is a very hot spark and will operate plugs that are in less than pristine condition.

What I think you’re looking at is carbs running rich most likely from taxing where float bowl level rises when the floats are jumping around. Carb float bowl on an airplane engine is a very violent place. This is especially true on Rotax 4-cyl where carbs are mounted at extreme distance from engine centerline. As the engine shakes the carbs mounted at the cylinder heads move significantly and float bowl level is hard to control.

All that said… When you taxi out and get ready to perform 4000 RPM ignition drop test, the float level is most likely very high and maybe even overflowing the bowl. As you run for a period of time at this higher RPM two things happen – excess fuel in float bowl is consumed establishing a correct fuel level and the engine begins to run smoother because the mixture is leaning and becoming more correct.

This also explains why RPM ignition drop test performed in the air shows no problem. Float level is easily maintained when engine is running high RPM in cruise flight. Engine runs smooth at high RPM and any burp of higher float level is consumed instantly at 5 gallon/hour flow rate.

Bottom line is everything is Hunky-Dory.

[ChrisRV12new]

That makes sense and is lining up with what I'm seeing on the line. Thank you for taking the time to write that explanation Jim.

-Cheers! Chris

[trecool44]

At this point I do not think it is carb related. I replaced the B ignition spark plugs. Right after I started it the first time with the new plugs I did a quick mag check and found the drop much larger and rougher with the A ignition off. I believe this shows that the condition exists at start up and not a fouled plug during warmup issue.

[seagull]

trecool44, How did you balance your carbs? What equipment did you use and the method?

I have my money on your issue rooted in a bad balance.
Some facts; the top plugs fire more efficiently because of their placement in the cylinder head. The front cylinders do not get the same mixture as the rear cylinders because of the longer intake manifold. I tell you this to help you understand why a carb imbalance can cause a differential mag drop. look at which plugs are responsible for the lower drop.

I had a very similar problem at 2500 RPM, a vibration and the left ignition had a much bigger drop than the right. This means the right ignition, coils, or plugs are the problem. After replacing the spark plugs, cutting back plug wires, swapping the ignition modules inputs & output plugs. It was traced to the only common denominator was the 2 coils and their related spark plugs.

Because of an unrelated issue I had to pull the prop and upon replacing it I did a dynamic balance and changed the floats to Marvel Schebler MS80-430, (got tired of the Rotax game). The last thing to do was a carb balance (which I was sure was fine). I ran the engine first to warm up and the problem still existed. After balancing the carbs it was all gone, vibration and difference in ignition drops.

MANY "experts" told me they thought my problem was fuel related, I was insistent it was electrical, I almost bought a coil pack, glad I didn't.

[seagull]

I found this it may be if some help;

-I have a bad mag drop, what do I do?
First determine if you have a carburetor problem or an ignition problem;
(WHAT?? Yes, the Rotax 9 series dual carb system can confuse the owner as a carb problem may show up as a high mag drop on one ignition system) Procedure:
Run the engine at 4000rpm (approx 1700 prop rpm) and perform a mag check. Record the numbers.
Slowly pull the choke out half way, the RPM should rise about 200 RPM.
Try the mag drop and record numbers.
If the problem is worse or better the carbs are suspect.
If there is no change the carbs are not suspect, you likely have an ignition
problem.

Source;
https://www.rotax-owner.com/pdf/ROAN%20FAQs.pdf#page4

[ChrisRV12new]

Great, thank you everyone. Weather looks good tomorrow for KPLU flight. Going to try that rotax info with the choke to test if it's carbs or ignition. Thanks for posting it.

[ChrisRV12new]

Taxi and flight test today were a success. I kept the power much higher during cold engine warm up. Included a photo because of the temperatures. Minus -20 circling around Mt Rainier. Brrrr! 😂

[Piper J3]

Originally Posted by ChrisRV12new

Taxi and flight test today were a success. I kept the power much higher during cold engine warm up.

Correct. Idle warm-up at higher speed is beneficial. I bring idle speed to ~ 2800 RPM as soon as engine starts.

• Cold engine runs smoother when speed is increased.
• Better for gearbox when engine runs smooth – no chattering.
• Smoother running is better for carb float level.

Let engine run ~3500 RPM for a minute or so prior to the 4000 RPM ignition drop test. This will normalize float levels that would otherwise contribute to large RPM drop.

Also, best to run fresh fuel. High-octane Mogas isn’t a big seller at gas stations, so it’s a little like a **** shoot when buying. I buy 93E10 for engine requiring 91 octane, so not much of a cushion there. Ethanol in the fuel is very volatile. The vented fuel tank allows air exchange. Alcohol will evaporate causing a reduction in octane, and because alcohol is hydroscopic, it will absorb moisture from the air which isn’t good either. If possible, use a Top Tier gas supplier.