intakes 912S

by Paul Hamilton in General Tech Discussion 10 years ago

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Re: Fuel venting/dripping out air filter/intakes 912S

by Roger Lee » 9 years ago

Roger Lee

127 5.22k 4.83k

Hi Kenneth,

With 1100 hrs. it would be a good time o do a carb rebuild.

Possibly two things.
This one first because it is the most common. Did you reset the float arm height when you changed the needle? It should be 10.5mm to the top of the float arm if you measure from the carb edge straight up.

Second. Pull the floats and weigh them together. They shouldn't be more than 7 grams. It is possible one is getting heavy from fuel absorption and starting to sink and not controlling fuel level any more. When you start the engine it doesn't drip because the fuel is then being used by the engine.

You need to rule these two things out because they are the most common.

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

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Re: Fuel venting/dripping out air filter/intakes 912S

by Brent Hoskins » 9 years ago

Brent Hoskins

3 7 0

Hi Kenneth,

I addition to doing what Roger is suggesting I would also check the seats. I have a 912ULS with 1100 hours and had fuel dripping out of the carbs after a complete rebuild. I verified several times the float arm measurement and still fuel dripped after every flight. What I ended up doing that fixed my problem was to polish the seat. I used one of the old needles and epoxied it to the end of a 1/4" diameter wooden dowel and used some fine valve lashing compound (toothpaste will work) and polished the seats. I used a mityvac before, during and after to make sure it was working. Once I got a good vacuum I finished the polishing process. Worked great for me and no more leaks. It does not take much of blemish or nick in the seat to keep it from sealing.

Only do this process when the carbs are completely apart so you can get the ports of the carbs really clean to remove all lapping compound grit. If you need a picture of my homeade lapping tool let me know.

Brent Hoskins
Just Highlander
Caldwell, ID

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Re: Fuel venting/dripping out air filter/intakes 912S

by Paul Hamilton » 9 years ago

Paul Hamilton

30 145 298

Back to my problem where the 912S spitting fuel mist out of the air filters WITH NO AIRBOX at high altitudes.

Turns out my friend in Colorado operating at high altitudes is having the same problem. He also has no air box. He operates from 7000 to 12,000 MSL density altitude. As every one said, he send the carbs back for rebuild, checkout, etc and super tuned/tested - same thing once he got the carbs back. (Same thing i did)

So he raised the jet needle one notch and is installing smaller main jets to see what happens. One notch up with the needle jet raised his mid range EGT's about 100 F from 1250 to 1350, closer to where they should be.

No conclusive findings yet but some progress. Even though the 912 carbs are altitude compensation to some extent, it does not appear they do the job for constant operation at high altitudes. We shall see. Another reason for an iS for operation at higher altitudes.

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Re: Fuel venting/dripping out air filter/intakes 912S

by Roger Lee » 9 years ago

Roger Lee

127 5.22k 4.83k

Hi Paul,

I admit I haven't read all the way back in these post.

Have you adjusted the carb float armature?
Moving the needle down a notch at those altitudes is probably a good idea, but I can't see that fixing the fuel drip problem.

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

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Re: Fuel venting/dripping out air filter/intakes 912S

by Paul Hamilton » 9 years ago

Paul Hamilton

30 145 298

Yes I did everything including installing a separate carb and it did the same thing. Fuel misting out of the air filters. High altitude with NO AIR BOX.

HOWEVER I THINK WE HAVE A BREAKTHROUGH. My counter part in Colorado flying from 7500 feet had the same problem. Fuel misting out of the fuel filters as I described in detail earlier.

So he has tried a different fix. Re jetting. He raised his clip and dropped the needle down a notch. This raised his EGT mid range from 1250 to 1350 or there a bouts.

He went from the 155 main stock jet in increments to a 148 and raised his top end EGT from 1250 to 1350 and an increase in top end RPM of 75.

Now after re jetting he is not getting any gas misting out of the air filters.

So bottom line re jetting has solved the problem for him and I am going to do the same thing for me here.

For each 1000 foot increase in altitude, the main jet was reduced 1 point per 1000 to get where he is now. He may go down to a main jet of 146 we will see.

I am sure this is not common knowledge because there are few 912S operators flying at high altitudes with no air boxes.

It should be stressed that all the normal carb suggestions above MUST be incorporated before any body does any re jetting to fix fuel/carb problems. Re jetting will only help at significantly higher density altitudes.

The ROTAX 912 carbs are altitude adjusting but not as much as I thought they were.

That is it for now......

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Fuel venting/dripping out air filter/intakes 912S

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