power loss in flight

i want to share my yesterday experience , very close to same problem on Rotax 914.

starting problem:

1. Problem: Engine does not start  after all effort , over flooding observed. 

Solution: On opening of carburetor  found that a FoD stuck in starting jet in bowel, after cleaning a very good start observed.

weight of dry floats : 6.45 gram.

2. Problem: during Ground run  100% Throttle to 115%, rpm drop  i.e 150 to 200 rpm drop , slight roughness or extra vibration

solution: again open the carburetor (Left side only) mixing tube with main jet change.

carburetor synchronized and again start the engine , idle to 115 % : RPM 5750 to  5820 rpm achieved. RPM was stable.

all other profile checked: stable rpm observed. 

Thanks for responses. 
Please explain how the weight of floats make full power loss hiccup ? And then run seemingly fine for several minutes, then another heart stopping hiccup, then run fine through landing and taxi ?  

Do the floats decompose ? Is that what causes particulate blockage in carb passages ?

Carburetor vents ?  Air box ?  Wouldn’t a problem caused by that show up before 220 hrs ? Why would it take so long to have an issue ?  Would like to hear what is meant by “similar problem “

Nearest Rotax tech is 140 mi away. I have no desire to be a mechanic, dang.

  Will call Lockwood tomorrow for insight, since they checked them recently.

First understand that we are tring to troubleshoo by remote control and can only base our suggestions on what you report.

4 different people are going to give you 4 different opinions.

So all you can do is take our insights and apply them to you observations.

Even the Blind Squirrel eventually finds the nut!  

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New Floats weigh about 6 grams per pair,  7 grams is the overweight limit.

Typically the plastic foam of the floats slowly absorbs fuel like a poor sponge and the tiny air bubbles in the foam become flooded and they get heavy.

1 gram is Not a lot of fuel!

- - -

Here is the heavy float theory.

At idle the floats work well enough to stop any excess fuel flow.

At midde RPMs, the extra vibrations cause the float valve to bounce enough to admit slightly more fuel than is being used and the float bowl slowly fills to the brim.

The excess fuel overflows up the carb jets and the mixture becomes too rich.

Initially the engine runs rough and eventually it becomes critically too rich and faulters.

You open the throttle in response to the faultering,  admitting more air, leaning the mixture, and using more fuel due to the higher power setting, lowering the float bowl's level back to something more normal.

Then you trottle back and the cycle repeats.

Cruising at 4900-5000 is not to be recommended.  5200 - 5500 would be prefered. 

- - -

You stated that the carbs were "Checked" 30 hours ago in 2019.  So that is more than 6  months ago at the very least. 

That is over 5000 hours of soaking.

Rotax has been having a bad record with floats for the past few years.

If your old but working Floats were replaced, just for good measure, at the last "Check", The problem may have been created by the improvement.

- - -

You may be a reluctant mechanic, but someone has to do the job.

A perfectly usable Portable 200g x 0.01g Digital Scale can Be found On Ebay for well under $10.

Alternately, your local "Head-Shop" will have them in stock if you don't mind rubbing elbows with the Counter Culture.

Thanks for responses. The more I learn the more I am embarrassed I didn’t research the 912 more fully.

My original Rotax investigation had a Rotax sales video state  range of 4500 to 5400 (?) Rpm.                                  Whereas about 4800 produces a wonderful quiet smooth cruise. 5200 is much noisier and less smooth. 
( Of course Rotax Sales videos also state “can be powered by 100LL or MoGas”, then the “fine print” is read, and turns out not really the case, unless you want to wear your valves out) . 

Now am getting consensus that we should be cruising at the higher rpm, without an explanation ???

Also we are suppose to weigh floats every 25 hrs ? And the original floats were not as problematic ? WTF  My brain instantly asks why did Rotax not switch back to the pre 2012 floats?

And we are suppose to check coolant for every flight ? That involves removing the top cowling, I’ll wear out the cowling bolt holes !!

I am “in for a penny, in for a pound”  And trying not to get to frustrated. After flying 1200+ hours on 35 -50 year old Continentals (with original fuel lines, carb floats etc.) Totally did not see all this maintainance coming.

Ahh, but I digress...

Guess I will order some insanely expensive poor quality floats. Figuring even if it’s not he problem in this situation, it will be in the future.  

as a man of few words friend of mine said to me, “if you own an airplane, you don’t get to complain” ....good point 

Please keep input coming. And thanks again.

Dan Dawson 563 349 4059

patndandawson@aol.com

Hello, thanks for response. Was wondering what you meant by “similar experience “ and did it happen soon after start of flying or did It take a couple hundred hours to develope ?

Also, which wing on your RANS ?

thanks Dan

patndandawson@aol.com

563 349 4049