Rotax 912 Carb float fowling chamber

by Neil Pearse in 912 / 914 Technical Questions 4 years ago

Approx. 3 years ago I sent my Rotax 912 carbs for the 600 Hour overhaul, picking up floats that suffered the sinking syndrome. I replaced the floats last year on the warranty scheme and have just opened the float chambers as part of my annual inspection (18 hours after changing to non sinking floats).

Please see attached picture.

I think the yellow tinge is from percolation of the MOGAS I was using, fixed by using UL91 (not present in the other carb.)

I'm concerned about the line that appears to have been causes by the float fowling the side of the float chamber. I'm sure it wasn't there last year as I'd have spotted it. Is this something anyone has seen before?

I'm at a bit of a loss as to know what to do.

Ideas so far:

1. Take off the bowl and fill with fuel to see how the float bobs around/compare to the old float

2. Put the float in fuel but without being mounted on the guide to see if it has a heavy end

3. I've already weighed all four floats their each 3.2g give or take a nats whisker

4. Could the float have been incorrectly seated/slightly stuck when I refitted last year? The plane doesn't start as well as it used to which I thought was due to percolating fuel. Maybe I should try some cold start cycles to see if its still needs to crank for a few seconds before firing. Hot start issue was fixed by using UL91 fuel rather than MOGAS with ethanol in it

5. Continue to fly and monitor every 5 hours, photograph each time

I'm really not to sure what else I can do about it. There is no obvious damage to the float (not used an eyeglass to examine as yet.

Any advice greatly appreciated :-)

8722_1_20210711_125816.jpg (You do not have access to download this file.)

Re: Rotax 912 Carb float fowling chamber

by Roger Lee » 4 years ago

Roger Lee

131 5.38k 5.44k

The floats should not rub on the bowl sides. IF it does rub then float pin may have been tweaked at some point. Put the float back in and look to see that is has a bit of clearance. If it rubs the side tweak the float pin outward just a tad so the float does not rub. The pin should be straight up and down. If it doesn't rub leave it alone. The brown stain isn't that uncommon. Just use some carb cleaning fluid and either wipe it out or use a scotchbrite pad to lightly scrub it out.

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

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Re: Rotax 912 Carb float fowling chamber

by Tyler Hathaway » 4 years ago

Tyler Hathaway

3 478 1.39k

What exactly do you mean by "percolation"?

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Re: Rotax 912 Carb float fowling chamber

by Neil Pearse » 4 years ago

Neil Pearse

2 4 19

Percolation is when the fuel boils off in the float Chambers. I proved this by attaching a thermocouple to the float chamber and monitoring the temperature following shutdown. It quickly reached 50degC. I took a test tube of mogas an put in a waker bath at 50degC and proved that the fuel was boiling.

On the carb floats rubbing front. I checked the pins they look OK. I also looked at the floats and brass valve actuator that were chamged during the 600h check. These show signs of rubbing, hence I've convinced myself that it's all very close in the float chamber and rubbing occurs!

I also floated current and old floats in the chamber in fuel and monitored them whilst I nudged them. They were free to move, clearances looked reasonable.

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