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Carb issue shows up during mag check?

in 912 / 914 Technical Questions 9 years ago
Hi,
The following is a quote from the ROAN Rotax 912/914 series FAQ’s pdf:

""-I have a bad mag drop, what do I do?
First determine if you have a carburettor problem or an ignition problem;
(WHAT?? Yes, the Rotax 9 series dual carb system can confuse the owner as a
carb problem may show up as a high mag drop on one ignition system)""

How is that? :huh: Why can a carb issue appear during mag check?
It is quite a technical question but any ideas would be very appreciated.

Blue skyes

Denis

  • Re: Carb issue shows up during mag check?

    by » 9 years ago

    Hi Denis,

    Two ways to look at the mag drop.

    1. Large differential between ignition modules. One normal and one way off. More likely electrical.

    2. Large drop that affects both. Let's say you have a 200-300 rpm drop on both modules, but equal. This could be fuel related.

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

      
    Report

  • Re: Carb issue shows up during mag check?

    by » 9 years ago

    Hi Roger,
    The mag drop is of one circuit only. Large rpm drop accompanied by vibration.
    I think it is probably caused by heat because the failure is intermittent and usually happens when the cowling gets hot.

    The quote says: "high mag drop on one ignition system". I understand it relates a high RPM drop to one carb failing. Is that so?
      
    Report

  • Re: Carb issue shows up during mag check?

    by » 9 years ago

    Hi Denis,

    Sorry, I thought it was just a general question not an actual problem.

    Just a few things to think about:

    Could be electrical and fuel. Have the carbs been checked for sync? You need to rule out fuel delivery so you can focus on one system.
    What are the mag drop numbers and at what rpm do you do the mag drop?
    Do you have a return fuel line? (If the engine is hot and there is no return line then vapors may form in the fuel lines since you point to a possible temp issue with the cowl on)
    From your comment I take it this never happens with the cowl off and the engine is cooler?

    You really need to try and figure out if the cowling is really making a difference as this may lead you down a different path.

    If you have a fairly large mag drop on just one ignition then I would think electrical. It is going to have vibration with a large drop.

    If a carb had poor fuel delivery then the vibration would be there all the time. You would most likely still have equal mag drops, but both would be larger than normal.

    Once fuel is out of the equation I would determine which module has the big drop and then focus on those 4 spark plugs, especially wires and wiring at the module itself.

    If I missed something here let me know.

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

      
    Report

  • Re: Carb issue shows up during mag check?

    by » 9 years ago

    Thanks for the information Roger,

    I will further investigate and perform tests this weekend and let you know how it goes.

    Cheers

    Denis
      
    Report

  • Re: Carb issue shows up during mag check?

    by » 9 years ago

    Hi Roger,

    I just did a few test to help troubleshoot the issue.

    I now suspect detonation due to poor fuel quality(due to availability I currently use a mix of AvGas and regular automotive gasoline).

    I followed static tests. The engine fails when the inner cowl temperature is allowed to rise, either from shutting down then back on shortly after or during prolonged ground run on a hot day. When failing, the engine sounds like it is misfiring.
    Full throttle, the failing engine only gives me 5000 of the 5500RPM I usually get, also I observe uneven EGT: L 670°C, R 840°C.
    At 4000 RPM I conducted a Mag test, resulting in a stunning drop to 2500RPM on either mag acompanied by the folowing EGT: L 773°C, R 790°C.
    Also at 4000 RPM the EGT where not even, the left EGT was at 675°C and the right one at 830°C.

    I could have a lean mixture since, while pulling the choke a rise of only 200RPM is observed before the engine starts to stall.

    I forgot to test the issue without the cowl.

    Do you need any other info ?

    Cheers

    Denis
      
    Report
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