Low RPM Vibration on Left Mag

[SportPilot]

.......

[3Dreaming]

Maybe he's starting to lose an ignition.

Generally it is the starting circuit that fails on the Rotax ignition modules, and the failure is on or off.

[drseti]

No, we did get a mag drop at 3000 (and it was accompanied by a slight vibration). What we didn't get at 3k was a drop with carb heat (but that turned out to be related to having the upper cowling off).

[3Dreaming]

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

It has to do with which sparkplug is not firing in relation to how the fuel air mixture flows into the cylinder.

[drseti]

Then, why would that show up at one RPM and not another?

[3Dreaming]

When you are feeling the roughness you are feeling the difference in power pulses. At higher RPM the roughness in the power pulses is likely still there, but mask by the different frequency.

[drseti]

So, Tom, since we don't know which, are you suggesting we swap all 8 spark plugs?

[3Dreaming]

So, Tom, since we don't know which, are you suggesting we swap all 8 spark plugs?

If I still had the set that came out I would put them back in for a test run. As stated earlier you can also do a top to bottom swap and make a test run. When you start trying to track something like this down make sure you keep a log of each change you make, and note if the problem followed or not.

[designrs]

No, we did get a mag drop at 3000 (and it was accompanied by a slight vibration). What we didn't get at 3k was a drop with carb heat (but that turned out to be related to having the upper cowling off).

That's right. It was carb heat that we were not getting. I stand corrected.

[roger lee]

Carbs should always be synced at every annual condition / 100 hr.. They are almost always off. I see 1 out of 25 that might be still good, but usually the idle or high end need a tweak. Putting a set of gauges on the carbs is a good diagnostic tool. using an electronic sync tool doesn't tell you anything, but they aren't in sync. Unless you put a set of gauges on for diagnostics or sync the carbs trouble shooting will be a hit and miss proposition.

Plugs should be between .023 and .027 with thermal paste. It would only take one plug to have a gap that is different from all the others to cause on odd mag drop check. Make sure the thermal paste is not on the electrode or even near it. The silicone base paste will cause miss-fires. Make sure all plug wires are securely on each plug. I find some that are cocked off to the side and not really on.
Carb heat in some planes does not register a drop and in some it is a round 25 rpm.

What is the mag drop at around 3500 or where ever you do a mag drop? 3000 is a little low to get a good reading. Don't worry what the mag drop is with the carb heat on. A normal mag drop can very quite a bit depending on several factors, but normal is around 40-100 rpm. Normal differential is a 0-20 spread between mags, but could be a little more.

Don't swap wires. It really isn't necessary and not a good diagnostic tool. Follow a logical order or you may be trouble shooting for weeks until you find what should have taken you an hour.

[designrs]

Thanks Roger.
What about carb jets?
I'm only at 275 hours TT.
Could carb jets have anything to do with it?

[drseti]

Carbs should always be synced at every annual condition / 100 hr.. They are almost always off. /quote]

I've found it a good technique to block off the compensating tube. If starting and engine idle/performance aren't affected, it's a pretty good indication that carbs are in sync. In the case of Richard's plane, it appears the carbs were synched right before he bought the plane (and that was very recently).

Putting a set of gauges on the carbs is a good diagnostic tool.

I've found the CarbMate also does a rather good job.

Plugs should be between .023 and .027 with thermal paste.

Agreed. I gapped all of his to .024". And I always use the white silicon heat sink paste, several threads back from the electrodes so as not to contaminate them.

Make sure all plug wires are securely on each plug.

I not only checked wire security, I did the pulloff force test. The wiring harness is like new, and pressures are correct.

Carb heat in some planes does not register a drop and in some it is a round 25 rpm.

His shows about 40 - 50 RPM drop at 4000 RPM runup, but no discernible drop at 3000. I don't think that's a problem.

What is the mag drop at around 3500 or where ever you do a mag drop?

We got a normal drop and a very small difference at 4000 RPM. (Richard has the exact numbers.)

Thanks for chiming in, Roger. I always welcome your input.

[drseti]

What about carb jets?

Remember that everything was smooth at the 4000 RPM runup; it was only at low RPM that we felt the vibration. IMHO, that result is not likely to be related to jets; we'd have an issue at all power settings. But, I'd certainly welcome Roger's input.

[designrs]

When you are feeling the roughness you are feeling the difference in power pulses. At higher RPM the roughness in the power pulses is likely still there, but mask by the different frequency.

This! The problem could very well be at all RPM's but masked at high RPMs!
Or perhaps it could be a low RPM carb issue.
Or it could be ignition / plug related.

[drseti]

Most likely an ignition/plug issue. There is no logical reason for a carb issue to manifest during a mag check. After all, the two carbs feed the right and left pairs of cylinders respectively, but the "right" and "left" ignition modules each drive all four cylinders.