Low RPM Vibration on Left Mag

[designrs]

My plane just completed an annual, with Paul, so I know it was done right. Carbs did not have to be rebalanced. Plugs showed good symetry left vs. right. Motor ran great with the carb bypass closed. New plugs installed & gapped. Carb floats were weighed and checked OK. (My floats are not in the serial number recall list either.)

Problem is that the motor vibrates while running on left mag only at low RPMs (at idle when shutting down and at say 2,500). Right mag only is fine.
Runup at 4,000 RPM is fine (left, right & both including drop & difference).

The vibration is the exact same vibration feeling that I get when I pull to idle on final.

I'd really like everything to run smoother.

Thoughts?

[FastEddieB]

First thing I'd do is install new plugs.

Probably $20 for the plugs.

Many plug problems are hard to track down, so this is a good place to start.

If you want to avoid the $20 expense, you could try switching the plugs top to bottom and see if the problem "migrates".

Easy things first.

[designrs]

New plus were just installed.
Good suggestion though.
A top / bottom switch would be good troubleshooting.
We wanted to pull a few plugs for an additional inspection anyway.

[designrs]

Left would be top or bottom on Rotax?
Or does that depen on the manufacturer installation?

[drseti]

Could go either way, depending upon how the "mag switch" was wired. (And of course you know that CDIs are not really mags...)

[3Dreaming]

Could go either way, depending upon how the "mag switch" was wired. (And of course you know that CDIs are not really mags...)

Based on the definition of a magneto I think you could argue the point about a Rotax ignition.

[3Dreaming]

I have seen new plugs out of the box do this, and I have also seen an idle mixture problem do this as well.

[3Dreaming]

I also seem to remember someone else saying that to much silicone heat paste will cause problems too.

[drseti]

In my experience, rough running from a bad plug does not manifest just at a low RPM ignition check, and an idle mixture problem would show up in examining plug coloration (1 and 3 vs. 2 and 4). So, I remain baffled.

[3Dreaming]

As a general rule if there is a problem that shows up after maintenance it usually has something to do with the work that was done. My guess since it is at a lower RPM and on one mag that it is likely carb (mixture) related.

[drseti]

Perfectly logical, Tom - except I did nothing that would have changed the mixture! A slight vibration occurred when I did a runup at 3k RPM. Richard had always run up at 4k, so had not seen this before. I ran up at 4 and everything was smooth. So, maybe the problem was there all along, but that power setting had not been explored before? (i'm not opposed to rechecking idle mixture, and maybe even replacing plugs, if he brings the plane back to me.)

[FastEddieB]

Wonder why a carb issue would show up on one ignition and not the other?

[drseti]

Let's say one carb is running too rich. Assume this fouls just one plug (not all that likely, but bear with me). Now we would get an abnormal ignition check. But, that would manifest at any RPM, not just one narrow range - so I don't think that explanation is the right one. Thus, I remain confused.

[FastEddieB]

Let's say one carb is running too rich. Assume this fouls just one plug (not all that likely, but bear with me). Now we would get an abnormal ignition check. But, that would manifest at any RPM, not just one narrow range - so I don't think that explanation is the right one. Thus, I remain confused.

Think horses, not zebras!

[designrs]

Perfectly logical, Tom - except I did nothing that would have changed the mixture! A slight vibration occurred when I did a runup at 3k RPM. Richard had always run up at 4k, so had not seen this before. I ran up at 4 and everything was smooth. So, maybe the problem was there all along, but that power setting had not been explored before? (i'm not opposed to rechecking idle mixture, and maybe even replacing plugs, if he brings the plane back to me.)

As I recall the issue wasn't rough at 3,000... it was that we were not seeing a drop at 3,000 to check mags.