One of our club planes (flying club in the netherlands), a Icarus C42 with a Rotax 912 ULS (100 hp), has a (for us so far) hard to solve problem:

During long flights, the engine occasionally makes a "hiccup" (mis fire)
It skips a beat...
Usually during long tours with constant power, rarely during circuit flying with T&Gs, but after some months also during landing and roll-out, where the engine stops completely.

Now also during a run-up, failure, speed drops, engine stalls, but starts again easily.

After a hour of flying, the engine runs again as if there is no problem.


The engine is from 2021 and has about 350 hours

It has had maintenace acording to Rotax scedue.


Last year we have had a similar problem with an other C42 wit the same tye of engine (also Rotax 912 ULS)

After a lot of searching the problem / malfunction turned out to be completely solved by replacing the NGK DCPR8E spark plugs with the Rotax 297656 spark plugs

NGK spark plugs have 1 ground electrode
Rotas spark plugs have 2 ground electrodes

So the Rotax 297656 spark plug works very well in the other plane

The same problem developed with this plane, but this was not solved by the use of the Rotax spark plugs

Subsequently, the following things were investigated

Carburetors disassembled, float chambers of the carbs disassembled,
Floats weighted, 3x3.15 grams 1 x 3.16 grams

All nozzles disassembled and cleaned, everything ok.

Carbs reassembled and synchronized
air filter, fuel filter and tank ventilation checked, everything ok.

Ignition modules changed with the other plane, no difference observed

Inspected the wiring of the contactors, visually pulled back the shrink tubing, measured it and tightened it properly with the motor running.

Spark plugs removed, cleaned and replaced with NGK spark plugs.
The engine runs a little better on Rotas spark plugs, they are now back in.

Any sugestions about how to takle this problem?


Best regards


Coen Jansen

  • Re: 912 ULS with "hiccup" (mis fire)

    by » 2 years ago

    Where are the float bowl vent lines routed?

    Thank you said by: Coen Jansen

  • Re: 912 ULS with "hiccup" (mis fire)

    by » 2 years ago

    I similar symptoms, progressively getting worse over about two years. After trying much of what you mentioned, the cause turned out to be an air leak past a bad o-ring on the stem of the fuel tank selector. There was seldom any indication of leaking fuel but it was bad enough that the suction from the fuel pump was drawing air into the fuel causing it to foam or spurt into the carburetor. Fuel pressure changed hardly at all because the displacement diaphragm Rotax mechanical fuel pump, pumps air just as well as it does avgas. As the air leak got worse, it would quit at low rpm because the pump wasn't moving enough volume to keep the engine running. A couple of months later, besides the occasional cruise miss, it started missing at full throttle as well. Proof was had by disconnecting the fuel line after the gascolator and, with an orfice to restrict the flow and provide need back pressure for the electric auxiliary fuel pump to work, putting the end of the hose into a small container of fuel and turning on the pump. What fuel came out, spurted out mixed with a continuous flow of air. Oddly, after fixing the air leak, my cruise fuel burn *dropped* by a full gph (or 4 liters per hour). I interpret that to have been caused by the irregular flow splashing fuel out the bowl vents. With the intake air log installed, the overflow goes right into the intake and leaves no trace to be otherwise seen. Plug color was definitely on the rich side, which also cleared up after the repair.

    Now, I have a low wing plane. I don't know how this would happen with a high wing, gravity feed, but I presume you still have a fuel pump, so it could happen. The symptoms of your problem sound identical to mine!

    Thank you said by: Coen Jansen

  • Re: 912 ULS with "hiccup" (mis fire)

    by » 2 years ago

    A friend of mine had a Cherokee 6 that had a similar problem as Jeffry describes in the post above, except that the air leak was an O-ring around the screw holding the bowl on the gascolator.  Same thing, there was no obvious leak but it would suck air.  We found it by temporarily placing a section of clear tubing in the fuel line and observing air bubbles.   

    Thank you said by: Coen Jansen

  • Re: 912 ULS with "hiccup" (mis fire)

    by » 2 years ago


    Thanks for the suggestion

    The vent line are routed "from factury" into the air filters 
    The plane has no airbox, but an air fliter on each carb.

    Vent lines are clean, flexibel, not obstructed.

  • Re: 912 ULS with "hiccup" (mis fire)

    by » 2 years ago

    Hello Jeffry an Jeff

    Thanks for the suggestion.

    We have checked the fuel lines, but haven't found any obvious problems...
    We have not checked for airbubbels in the fuel yet.

    Yesterday evening we replaced one of the ignigton coils, but of  because of bad weather and darkness we did't no test it. (other then start it, without any problem) 
    Today is bad weather, so no flying, on saterday we probebly wil do at testrun and if that goes fine we wil test fly it

    We also temporary installed a  precision fuel pressure gauge, (which is not standard in the C42)  to monitor fuel pressure.

    If the problem is not fixed, we ca use the line from the fluel gauge to check voor air bubbles.

    I will keep you posted.


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