Engine vibration at low RPM after hot restart

[rcpilot]

So we had just landed and during the rollout the engine quit. The pilot restarted it but there was a horrible vibration like the thing was going to fly apart. After taxing back to the hanger, we shutdown and restarted but the vibration was there at anything under 3000 RPM. Rotax 912 ULS with about 505 hours. Any idea what might cause that?

[FastEddieB]

First thing I would check is the throttle and "choke" cables. Have someone watch the carbs as you move the throttle and the choke levers through full range of motion.

The second thing I would check is the float bowls and main jets for crud/blockage.

The third thing I would check is the plugs. Sometimes the easiest thing is to just replace them.

I think there's also a crossover tube that can come adrift, so take a close look at all the connections.

Finally, I had a BMW twin that showed a similar symptom it suddenly would idle or run with lots of power, but shook like crazy inbetween. Culprit was one of the screws holding the diaphragm to the slide had backed out all the way.

Good luck - hopefully it's so something simple.

[Nomore767]

There's a note on my RV12 checklist (I'm away from home so I can't double check) but if I remember correctly a Rotax can diesel at shutdown especially on a hot day.
I believe it says to switch both ignition modules back on and apply choke. Engage ignition for 3 seconds and shutdown.

I believe the problem is due to post shutdown dieseling. Not sure if this is exactly your issue since you said it just shut down.

I once flew a Rotax 912 powered LSA which was running a bit rough but shut down twice after landing and I was told later it was due to bad carb imbalance on an engine running solely on 100LL with no lead scavenging additive.

[rcpilot]

Hmm. Well, when we pre-flighted the plane there was some water/sediment in the right tank and since I was about 100 lbs heavier than the pilot we were running on the right tank for the flight(about 20 minutes) so maybe it could be crude. The owner was going to check it again when it's cold and have his mechanic check it out.

[designrs]

I once flew a Rotax 912 powered LSA which was running a bit rough but shut down twice after landing and I was told later it was due to bad carb imbalance on an engine running solely on 100LL with no lead scavenging additive.

I'd buy the carb imbalance issue, but would seriously doubt any 100LL or additive "required" suggestion.

[designrs]

if I remember correctly a Rotax can diesel at shutdown especially on a hot day.
I believe it says to switch both ignition modules back on and apply choke. Engage ignition for 3 seconds and shutdown.

Never heard of this. What's "diesel" referring to?
Comments from our Rotax experts?

[Nomore767]

Basically the engine is turning over and firing from compression rather than the spark firing. If it sparks on a couple of cylinders and compression on others , or none at all it can cause lots of vibration, noise and consternation to the pilot

[FastEddieB]

Neglected to ask...

Any sort of engine monitoring? CHT and EGT could tell a tale.

[roger lee]

This can happen, but it's not what many think.
Dieseling on a Rotax usually happens with a poor ground and or ignition switch. A bad carb sync shouldn't add to the problem because the plugs won't have any spark if the ignition shutdown is properly working. Even if one cylinder had a hot spot it wouldn't be enough to turn over the other 3 cylinders.


My money is on a bad or loose wire or ignition switch and or its grounding

[rcpilot]

I spoke with the mechanic who regularly services the plane. He hasn't had a look at it yet but based on my description of the events he thinks the carbs probably got out of sync.

[FastEddieB]

I spoke with the mechanic who regularly services the plane. He hasn't had a look at it yet but based on my description of the events he thinks the carbs probably got out of sync.

Very possibly.

But small amounts of cable stretching and wear should not generally create a sudden big change in vibration, or an engine shutting down.

Elsewhere I posted about my Sky Arrow running rough after startup, and thinking it was related to a recent ignition module swap. Turns out a broken elbow on the choke cable on one carb had fractured and was keeping the choke partially on until it gradually vibrated off. One other time a choke cable end was hitting the carb, again keeping it from fully turning off.

In any case, I stand by my first suggestion: check the throttles and chokes through full range of control motion. Only then maybe worry about carb synch - though that's never a bad thing to check, regardless.

[3Dreaming]

What year is the engine and how many hours does it have?

[roger lee]

Hi rcpilot,

"The pilot restarted it but there was a horrible vibration like the thing was going to fly apart."

This is either fuel supply issue to or in the carbs and not just out of sync.
It can't just be strictly out of sync carbs unless you bent the idle stop screw on one and not the other carb (not likely) and since it is under 3000 rpm it is in the idle circuit and not affecting the main jet side of the throttle. Put a set of gauges on the carbs and see which one is pulling the biggest vacuum. My guess one is way worse than the other. The one that has the big vacuum has something in the idle jet which has a very tiny hole. It is so small I strip a 16 ga. wire and only use one strand from that to rod down through the idle jet. Might as well pull both carb bowls and just do a double check for debris.


Side note*
Out of sync carbs will cause rough running not dieseling at shutdown. If the wires are connected and not bad or loose then once the power is shut off to the plugs it can't run because there is no more spark.

I have delt with this before with a few others. Carbs is never the problem. It is always wiring and usually at the ignition switch(s).

[rcpilot]

Ok so the problem was there was an air leak on one of the carbs.

[FastEddieB]

Ok so the problem was there was an air leak on one of the carbs.

Yay! Should be an easy fix!

Can you be more specific so we can know what to look for?