912UL Issues

[Slyfox]

fantastic, way to go. If you want someone to fly along side you give me a call. my fox is always wanting to go for a flight.

[Av8r3400]

I finally got out to the airport to get some work done. Tis project is kind of like watching dominoes fall...

I got the gearbox together only to find a leaking fuel pump.

Swapped out the pump with the one off my project plane only to find the main valve leaking. Took that apart to replace o-rings and was finally able to run the motor!

It ran well being very cold. Once it warmed up the vibrations are virtually gone now. Yeahhhh!!

During the test run I found the supply line between the carbs leaking now...

Oy vey!

[Dorsal]

Still great news, glad to hear!!

[DesertFox4]

Like that news Larry. You can handle a few small repairs now that the large one is solved.

[SkyPirate]

Awesome Larry aka Gear Box Guru

[Av8r3400]

Awesome Larry aka Gear Box Guru

I have earned that title in my EAA Chapter already…

I don't know that I am a guru of anything but not being able to fly my plane for the last 5 months! I hope I can remember how...

[Av8r3400]

I'm still struggling with a rough running engine from 2000-3000 rpm.

It is dramatically better since the gearbox rebuild, but it's still not where it should be. At 1800 and lower, the engine is butter-smooth. Above 3000 it smooths out again, totally.

Some "experts" are suggesting that because of the reversal of the intakes on a I-IV 912 instal, the shaking of the carbs will eventually ruin them. I'm not completely buying this.

I'm now into a chicken and egg paradox. Did the carb trouble cause the gearbox trouble or vise versa?? Or is there even carb trouble to begin with…


Back to the hangar tomorrow to tear things apart again. Maybe I should've bought a Rans…

[Danzer1]

That's a tough one Larry. I do have a few suggestions though.

Is this the same place it was rough and same "kind" of roughness as before the gearbox rebuild?

Is it rough when cold and all the way through full warmup to operating temperature? Does it get better or worse or no change as it gets to full temp?

What do your plugs look like? (I'd start with reading the plugs) - that can give you a pretty good clue if it's carb related and then go from there. If carb related in mid range only, most likely sources are - clip height, out of round or bent needle, out of round or partially clogged needle jet, slide not moving freely and/or proper fuel height in the bowl.

Have you rebalanced the gearbox/prop combo after the rebuild? Changing to the new gears and removing and reinstalling everything can cause out of phase, only noticeable at certain rpm's. I'd definitely want to rebalance after the work you've done - especially so as to not burn up the new gears.

You have an advantage now if you balance with a Dynavibe II (or equal) as it is (1) much more accurate (2) is much quicker and more precise (3) uses 2 sensors - one on the front of the engine - essentially to balance the gearbox/prop combo and another on the rear of the engine that compares with the sensor on the front. This combination can then point to other sources of misalignment or out of balance condition and location - such as a misaligned alternator or flywheel, etc. This could then solve your issue or point you in a new direction as to the source of the problem. In any case, you would then know that your newly reinstalled prop/gearbox is balanced as close as humanly possible and not the problem.

Good Luck

[Av8r3400]

We only have the single sensor Dynavibe and I did refresh the balance with it. (Not much change) I have also re-synced the carbs to the point of near perfection.

The vibration is different than what it was before. It is a "rough running" rather than "vibration" action. It is also no where near as sever as it was before the rebuild. It actually gets worse as the motor warms up.

My plan at the moment is to pull the slides out of the carbs and inspect them for wear and damage per the Rotax "experts" recommendation. I have the carbs off my project plane in hand, too if I decide to rob some parts from them...

I like your spark plug inspection idea too, that will happen first (easiest).

[Danzer1]

Sounds like a carb issue so far. Getting worse as it warms up generally means a rich issue. If it got better as it warmed up that would lead to a lean issue. So, still could be needle clip height, needle trueness/wear, needle jet out of round (letting to much fuel through) or float bowl fuel height (lower = leaner).

I'm guessing this is being done on the ground (static)? If so, you might see what happens (if anything) if you prop the tail up to level flight attitude. That might give you a clue if it's a float height issue. Just one of the reasons I've never been a fan of carbs on aircraft - fuel delivery changes with every attitude change.

You mentioned Rans - can't wait to see the new S20 Raven they are building with the 180hp UL engine - I hear it's getting close! I'm also hearing all good things about the 912 version now flying. Hmmm and only a 5 hour drive from here! Okay back on topic.

Post some pics of the plugs (ends, side and angled down into the insulator) if you'd like another viewpoint!