Ignition miss? above 5100rpm

by Michael Nault in 912 / 914 Technical Questions 13 years ago

I don't like starting a new thread if it's redundant but I've searched the forum for this topic and not seen it. I've a 912ULS with 440 hours on it. Until recently I never got over 4800 rpm. After calibrating my tach (OK) and checking linkage for full throttle I increased the pitch on my prop from 14 degrees to 16.5 and the rpm at WOT increased by approximately 300rpm. That's the good news. The bad news is that when I run at WOT for any length of time I notice a very slight miss or stumble as if one plug was mifiring. If I back off WOT to 4800rpm, the miss goes away. Since I recently changed plugs I pulled them and inspected as well as checked the gap and checked ignition wire attachment but found nothing that appeared abnormal. The engine idles smoothly, temps are all in normal range and the engine is easy to start.
I overhauled both carburetors at 425hrs and did both the mechanical and pneumatic synch and changed the fuel filter. I need some ideas on where to go from here.

Re: Ignition miss? above 5100rpm

by Roger Lee » 13 years ago

Roger Lee

131 5.38k 5.44k

Howdy Micheal,

Out of curiosity you said you now have 300 rpm more than when you ran at 4800 WOT. Does this mean you can only get 5100 rpm WOT in flat and level flight? If it does then take out another 2 degrees to get up to at least 5550-5700 WOT in flat and level flight at your average altitude.

On the missing subject. When you do your mag drop between 3500-4000 rpm does the miss show up there? It should if it's electrical and that noticeable to you while flying. If you are having a plug misfire it should show up at the mag drop check. Are you absolutely sure it's the engine that you feel. I have had several pilots in the past that thought this and it was something else and not the engine.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-349-7056 Cell

Thank you said by: Michael Nault

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Re: Ignition miss? above 5100rpm

by Michael Nault » 13 years ago

Michael Nault

9 19 0

Thanks Roger - yes I was only getting 4800rpm until I changed the pitch and I plan on increasing it another 2 degrees. I wasn't until I became a regular reader of the forum that I learned what the rpm should be, how to properly synch my carbs and a number of other things about the Rotax.

On the miss I'm not convinced it is electrical. When I do my run up I don't notice and when I was synching my carbs at 4000 rpm I didn't notice it so my ignition miss theory may be way off base. Whatever it is, it is not very pronounced. I feel in the seat of my pants more than any other indication. Next time I'll watch the tach and see if I notice any difference but the meter may not be quick enough to catch it.

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