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EGT Readings

in 912 / 914 Technical Questions 11 years ago
I recently (20 hours ago) re-synchronized the carbs on my 912 ULS (247 hours TT) and all seems to be running fine. But my EGT's don't always match. The engine idles smoothly at 1800 RPMs. It spools up smoothly. It is set for 5600 WOT. When warming up the engine prior to takeoff I note that one EGT is about 100 degrees off the other. I note this in the air as well, but only at times. Often as I change heading relative to the wind, that difference often disappears, or drops to maybe only 50 degrees off, or even less. Previously, however, both EGTs sat at around 1487 consistently. Now one sits at around 1400, and the other a little below 1400 with the engine running at around 5100 RPMs.

So my questions are 1) is this a matter of re-syncing the carbs?, or 2) could there be another cause? Could it be that there might be too much heat transfer paste on the spark plugs? Could there be residual paste on the spark plug holes from prior installations (I have been flying the airplane since 2004)? Could spark plug gapping be an issue? (It is approaching winter in Alaska: Lockwood has always recommended .028 for gapping on this engine, but I note rotaxowners.com recommends .023 in cold weather, .027 in warm. Any recommendations? Or is this a problem I need not worry about if the engine is running smoothly?

I truly hate synchronizing the carbs if I do not have to. Every now and then it spits gas, and with a poly-carbonate windscreen it can cause real problems. I am near having to replace it now. Thanks.

Tom Skore
Eagle River, Alaska

  • Re: EGT Readings

    by » 11 years ago

    Hi Tom,

    With the cold weather they have in Alaska .023 for plugs is better. Keep in mind narrow gaps for cold and wider gaps for hot weather.
    First EGTs can be any where from equal to 100F apart and be normal and it changes with throttle position or rpm settings. Nothing you can do about most EGT differences. Mine and every one else EGT's vary.
    So I take it the carbs are recently synced and they were both equal? Do you have any header warp cloth on the exhaust pipes and touching the EGT probes? It is possible to have a tiny air leak.
    The difference since it changes and gets only up to 80F isn't worth worrying about unless it keeps getting worse. You could if you wanted try a new sync, plug gaps do make a difference, air leaks make a difference, improper carb function can make a difference, header wrap cloth touching the probe makes a difference.If the thermal plug paste gets on the plug electrode it will cause mis-firing that you may miss, but cause a cooler EGT. How is the mag drop?

    It might be worth the time to just watch it a few flights and see what happens.

    The spitting fuel I would take care of and this may be another cause of a cooler EGT. You may have a bad needle valve or the float height in the bowl is off causing the spitting, then fuel may come out the vent tube. The carb vent tubes should be routed to prevent fuel spraying on the windshield and engine.

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

    Thank you said by: Tom Skore
      
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  • Re: EGT Readings

    by » 11 years ago

    something as trivial as a small difference in the location of the probe compared to others will give different indications, if all is smooth, starts well, and everything else seams normal, than i wouldn't worry.

    i found the best way to test ignition and mixture is do a mag check at cruise at 5100Rpm. and watch each cylinder EGT rise, this will give a better indication if a plug isnt firing properly compared to others. a dead plug will result in no egt rise with one ignition off, and a lagging, or plug causing later ignition will show up as a higher egt rise compared to other cylinders. eg, one cylinder might rise 200 deg, when the other 3 rise 100. when on one ignition system at cruise power.
      
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