Ignition Check Irregularity

Hi Jim,

What rpm are you doing your pre/post flight ignition checks at?

How long after flight, are you doing the post check?

What fuel are you using?

All the best for the New Year

😈

"Exactly the same, rolling"

Do you mean the rpm goes up and down on a specific mag while testing?

How many rpm drop is there?

A few thoughts and we could use some more info.

 Is it just on one ignition module? It may not be an ignition module because each one operates four plugs and the mag drop would be more significant.

Do most common causes and least expensive things first.

How many hours on the plugs? If they are NGK's and have 100 hours then change them. What's the gap? Out of the box they are .030. Gap them between .023 - .027. Pick .025 it's in the middle. Don't start at the widest gap.

It could be a plug wire at the plug cap. Unscrew each plug cap and cut off 3/8" to get clean new wire. Then apply a dab of dielectric grease to the wire end and screw the cap back on all the way down until it stops. Then do a run and see your rpm drop again.

I just did an inspection on a 912ULS. The plug gaps were ,055 and the tips destroyed and almost gone.

Gentlemen, thank you for your suggestions.  Let me try to answer your questions.  I do ignition module checks at 4,000 RPM.   Post-flight check is immediately after clearing the active runway.  I run high-test car gas, Valero brand.  Yes, the problem is on only one module.  The RPM drop during the trough of the roll is 250-300.  Plugs are NGKs and have less than 20 hours on them.  I always check plug gaps before installing.  Boots were replaced less than 100 hours ago.

 I will re-gap as per your specs and apply the dielectric, then test-run.  If problem persists, I will perform surgery on the wires as you specify.

I would very much like someone to tell me if the "rolling" of RPM is indicative of any known Rotax engine malady.  I've been doing run-ups for over 50 years, and I've never heard anything quite like this before.  Almost made me think it was a fuel issue or something.

Again, thanks for your prompt response.    Jim

Hi Jim,

Your information above does not support my gut feeling - fuel vapourisation.

My reasons for thinking vaporisation;

The problem arises after flight - engine hot.

Taxi time may be a contributing factor, especially if down wind (less cooling air entering cowling).

You fly in the USA where winter fuel (higher volatility) problems seem to be common.

"Rolling"/fluctuating engine rpm can be caused by short term loss of liquid fuel, due to vapour " lock"

Follow up questions.

Have you made any changes to;

The fuel reticulation system, fuel line insulation or routing, especially within in the cowling.?

The cowling air in/out size, location, shape. Added/subtracted ducts or deflectors?

Does your aircraft have a cowl flap that may not be functioning correctly?

Are you using the same propeller & spinner, as before the problem arose and if in-flight-adjustable, any changes to your ground management, after landing?

😈

On only one module and only 250 - 300 rpm sounds like one plug. Did this all start after you replaced plugs and caps?