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Why did my 912UL performance go to pieces overnight?

  • Adam Messer
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4 weeks 22 hours ago #17284 by Adam Messer
Adam Messer created the topic: Why did my 912UL performance go to pieces overnight?
After weeks of patient tweaking, Tuesday I finally had my Rotax and Sensensich 3-blade ground adjustable prop exactly where I wanted them. On my first cross-country traversing mountainous terrain, as opposed to following the coastline, everything was perfect. The airplane easily climbed to 4500 feet, with RPM stable in climb just above 5100, the max oil temp was 208 and cooled to 200, CHT 234, prop pitched to give about 5500 rpm on WOT, EGTs 981/1020. Finally!

But what could have happened overnight? Wed AM, setting off another cross country over low mountains, everything went to pieces. That flight, aborted, and two subsequent flights, have shown reproducibly severely degraded performance.

RPM: 5100 on takeoff, but above 1500 feet decays to 4900 and of course airspeed drops. Engine feels sluggish, but rpm recovers when you go straight and level. Never tried to push it above 2000 feet, always immediate return to airport. Engine clearly not happy.

Oil temp: Set new record highs 230-240 °F. OK, within limits but under tougher conditions earlier this week 40 degrees lower. Ain't right.

CHT climbs above 240, OK within limits but never previously observed this high.

EGT identical at t/o, but now after temps stabilize in flight there is a 150-200 °F spread between the max (1150) and the min.

No observable issues with fuel flow, pressure, etc. Made one flight with the Facet pump on, the other off, and no difference in performance. No apparent electrical system issues. OAT pretty much the same in all cases—28-30C on the ground; 22-24C at altitude.

I replaced the carb vent hoses, which are both very short, and there was no improvement.

Fuel is from the same supplier. Fuel passes through 25 micron in line mesh filters and an Andair gascolator. Carb floats, installed 24 hobbs hours ago, replaced as per SB-912-067UL R1. Carbs synched 30 engine hours ago, spark plugs have 24 engine hours on them.

I just can’t figure out why the engine performance would degrade overnight. I didn’t touch a thing! Ideas? I’m wondering if the carbs fell out of sync or something. How should I approach troubleshooting this?

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  • Roger Lee
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4 weeks 16 hours ago - 4 weeks 15 hours ago #17290 by Roger Lee
Roger Lee replied the topic: Why did my 912UL performance go to pieces overnight?
You may have an air leak that would lean out one side or you may have a partial blockage in one carb? Sounds like something may have changed the fuel delivery.
After looking at your numbers before I wonder if you were running a little rich. Those are fairly cool numbers especially for an 80F-86F OAT. Maybe something was stuck in the carb? Something like a float rubbing or hanging up on the bowl side wall or the vent tubes in a location where the air pressure was too high, low or there was a differential between the two carbs? Maybe the throttle cables or system changed?
I believe it's fuel and you need to start a diagnosis from the beginning. Start with the throttle cable and see if they open and close all the way and equally, sync the carbs again, check the filters, ect... Be methodical and leave no stone unturned. I would bet it's a simple fix.

p.s.
Use gauges to sync the carbs. You can diagnose with them and you can't with an electronic device.

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last Edit: 4 weeks 15 hours ago by Roger Lee.

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3 weeks 2 days ago #17362 by James Ott
James Ott replied the topic: Why did my 912UL performance go to pieces overnight?
Could your ground adjustable blades have slipped in the hub, giving a much coarser pitch, which might explain the lower rpm's and higher temps.

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3 weeks 2 days ago #17370 by Adam Messer
Adam Messer replied the topic: Why did my 912UL performance go to pieces overnight?
Thanks for all your suggestions. I methodically traced everything, tightening cables, doing mechanical and pneumatic carb synch, replacing vent lines, replacing air filters and the problem recurred. OK, now have to remove the lower cowl half. Next step, pull the carb bowls, clean them but they are not that messy and the floats move freely. Reassembled the carbs, checked prop bolt torques, dragged the aircraft out to the apron for a runup, so that maybe I can fly tomorrow. Big mismatch in EGT at higher rpms. Hmmm.

In the sunlight I noticed that the rubber flange on the right carb was cracked around the bolt, and that if I yanked the carb it the crack would open up to provide a serious gap. Like at high rpm. So this is possibly the simple fix. Too bad I didn't think of this one earlier. But hey, if this is really the problem at least now I know that the carb bowls are clean, the carbs are precisely synched, the air filters pristine, and prop pitch OK. We'll know in three weeks!
The following user(s) said Thank You: Wayne Fowler

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3 weeks 1 day ago - 3 weeks 1 day ago #17373 by Roger Lee
Roger Lee replied the topic: Why did my 912UL performance go to pieces overnight?
Carb rubber flanges are a replace every 5 years. They can crack as years pile on or if someone had an old style flange without the spacer in the screw clamp. They could over tighten the flange and cut into the rubber. If you have visible cracking on a flange this would be a good place to start to find your problem. You should replace both rubber flanges. There is an "O" ring under it to replace to.

How old is your engine?

Roger Lee
LSRM-A & Rotax Instructor & Rotax IRC
Tucson, AZ Ryan Airfield (KRYN)
520-574-1080 Home (TRY HOME FIRST)
520-349-7056 Cell
Last Edit: 3 weeks 1 day ago by Roger Lee.

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