Misfire / Engine 'Blip'

·         Ryan Mollaun

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Hi Guys,

Love this forum and loving my new (well used) J400.

It has just had it's 100 hourly and about 530 hours in total.

I must first note, I am a newly qualified pilot and am not comfortable with the slightest engine murmur, however I just through I should ask...

I was flying yesterday around the Sunny Coast and at about 3000ft I reduced power for a decent and the engine went 'putt, putt' then resumed it's normal purr. It gave me a fright and after a while I started feeling very slight turbulence and thought maybe it was just turbulence and not the engine. I continued to fly around climbing to 4000ft then started another descent and when pulling power back (gradually) the exact same 'putt putt' happened again then normal operation.

Not sure what this could be due to but I pulled carb heat immediately and continued on back to the airport uneventfully.

Any thoughts would be fantastic.

Ryan

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·         Doug Smith

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Hi Ryan,
Firstly apologies for the slow reply: it's been pretty hectic lately!
As to the symptoms you've given there are a few candidates. The gurus tell me that the most likely is carby icing. The 3300 is fairly susceptible to it and in the right conditions it can happen at cruise power or less. The "putt-putt-purr" can come from a little build up of ice being disturbed by the power setting change.
There are other possibilities of course (carby float level/tuning, the carburettor sense tube, various ignition parameters etc) but start with the easy one first. We'd recommend you get in the habit of paying close attention to your carby heat and don't be afraid to use it (in accordance with the pilot operating handbook of course - the main thing is to have it fully ON or OFF - no half measures).
The classic symptoms on a long flight are a gradual drop in RPM over time (or finding you need to keep advancing the throttle to keep the same RPM) and/or rough running. On a shorter joy flight with changes in speed and power setting it can be a little harder to pick but if you've got any doubt, set the aircraft up flying straight and level, make a note of your RPM and then pull the carb heat on for a minimum of 30 seconds. Then turn if off and watch the tacho: if the RPM increases to a level higher than it was before you applied carb heat then most likely there was some ice which you've cleared.
I remember a day near Biggenden where I had to apply carb heat regularly (it felt like every few minutes) to keep the ice at bay and it wasn't especially cold or humid.
Hope that helps!
Doug.

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·         frank marriott

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Interesting indication on carb ice indications. I had ice indications regularly after about 2 hrs of cruise - application of carby heat would fix indication for 10 to 15 mins then return - in conditions where an identical aircraft had no problems.

Eventually problem located (by Mark at Jabiru) The carby needle had a growth of green "gunk" on it, After cleaning problem fixed - By the time the problem was located it had damaged the exh valve on number 6 (probably about 15hrs flight time).

Why only after a couple of hours flying time, I wouldn't guess (heating from too lean over time???) but bottom line the problem disappeared.

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·         Doug Smith

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That is an interesting one. Carby heat tends to richen the mixture a little so it stands to reason that it would temporarily help with a misfire from lean running... as to why it only happened after a couple of hours in the air is a tricky one though.
Some aircraft tend to cruise on the cusp of the "shoulder" of the needle - where it goes from the leaner cruise setting to the rich full power area - so possibly the gunk caused some some stickiness between needle and jet and over time that would tend to make it sit a little low in the jet. Were you using green AVGAS or was the green gunk verdigris/tarnish?
Glad to hear it was sorted out anyhow.
D.

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·         frank marriott

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Not sure what the gunk was, but use straight avgas.

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·         Ryan Mollaun

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Thanks so much for your responses - haven't had any re-occurrences as yet but will keep a finger on the carb heat and ensure the carb needle is checked if it happens again (and the carb heat doesn't relieve it).

Cheers,
Ryan

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·         Ryan Mollaun

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Yes ! I am making sure I don't save a buck on fuel at the expense of the power plant. Avgas all the way!