new carbs venting fuel at 2500rpm and below

by jay white in 912 / 914 Technical Questions 12 years ago

I have a 912. Its cold here in North Idaho. The engine is taking 10 minutes or more to warm up and during the warm up the engine is running rough and venting fuel out the vent tubes. At 2500 rpm the engine smooths out and the venting fuel problem goes away but I should be able to idle at lower RPM and not vent fuel. After the engine is at operating temprature the problem goes away completely. After take off I smelled fuel in the cockpit so I terminated the flight and pulled the cowl. no evidence of fuel venting. I took off and flew for an hour but smelled fuel at the beginning of the flight.
I already taped off the oil cooler and got the temp up to 180 during flight at cruise but I am sure the plane is not supposed to vent fuel like this. Does everyone have this problem? The carburators are only 3 hours old. I'm three hours out of a new annual inspection. i'm stumped and too new at Rotax to know whether this is normal or BAD.
Jay

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Re: new carbs venting fuel at 2500rpm and below

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

Hi Jay,

It could be just a matter of too low and rpm for warm up. 2300-2400 in the cold.If it's real cold there then slowly close the choke a little at a time and not right after it starts and not all at once.

Start at the beginning and double check everything in the carb setup. A good step by step diagnosis is worth many hours of frustration trying to hop all around in a hit and miss approach.
A couple of things you might take a look at. Make sure both vent tubes are getting the same air pressure. They should be located in the same spot on the engine and not in a pressured air flow area or out in the air stream. If you have the stock Rotax airbox that shouldn't be a concern. Double check the carb are sync. The sync should be checked every 100 hr and Annual inspection. If the carbs are only 3 hours old maybe something moved in the throttle linkage as far as the chokes and sync are concerned. Make sure the chokes are shutting completely off and or they are equal when open. Don't idle and warm up at too low an rpm. If it's cold out like it is there then 2300-2400 rpm warm up. I think the Fuel smell is from the venting of fuel. If the engine is running rough because the carbs are poorly synced or your idling too low the floats don't control the fuel level as well with the engine running real rough and fuel can vent.

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

Thank you said by: jay white

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Re: new carbs venting fuel at 2500rpm and below

by Joe Horta » 12 years ago

Joe Horta

2 4 0

Hi Rodger,

I have a 914 with 335 hours, I have never had any carb venting until a few days ago it has started dripping right after shutdown. More out of the left side than the right side and not after every shutdown. Where would you start to find the cause?

Another issue, Do you consider cruising at 5200 to 5500 RPM to fast?

Thanks to anywons sujustion,

Joe

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Re: new carbs venting fuel at 2500rpm and below

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

Hi Joe,

Cruising between 5000-5500 is fine. 5100-5300 is common and your 5200-5500 is just fine. I typically cruise at 5150-5200 (912ULS), I use 91 Oct. auto fuel(it has ethanol) and have my prop setup to get 5600 rpm WOT. If you run 100LL then 5300+ is better along with the lead scavenger Decalin to help keep the lead blown out.
A couple of questions;
How old is your 355 hr. engine? Have the carbs been worked on lately? Any changes made to the carbs lately?
As far the leak goes the common causes are; quite common to have a carb bowl gasket leak into the drip tray. If it is truly coming out the vent tube then I would ask if the vent tube location has recently changed, when did you balance the carbs last and you may need to adjust the float level in the carb. A poorly synced engine that vibrates will puke fuel out the vent tubes at times because if the engine is vibrating bad enough the floats can't control the level. You may even need at some point to check the carbs fro debris inside around the float needle.
Always start with the easiest and simple check first on a problem and work in a logical procession until you find the problem. I tend to look for the most common solution up front and work toward a more exotic solution if one isn't found along the way.

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

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Re: new carbs venting fuel at 2500rpm and below

by Mike Goodrich » 12 years ago

Mike Goodrich

8 123 0

Roger
Correct me if I'm wrong but I thought the 914 (Joe has a 914) vent connections were connected to the inlet manifold.
I haven't got my manual with me so I'm working from memory only.
Mike G

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Re: new carbs venting fuel at 2500rpm and below

by Roger Lee » 12 years ago

Roger Lee

125 5.12k 4.52k

Hi Mike,

It depends if he has a factory setup or if he has another experimental type setup. If it is factory then your right that wouldn't be the issue, but so many are set up differently. I was trying to cover that base. I hope we hear back from him soon then we'll know. I see some with the vents to the Rotax supplied airbox and then some buy engines second hand or put vents in other various places because they don't use the Rotax airbox and that sometimes can be an issue.
It's in the Installation manual section 16.2.3 page 101

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

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new carbs venting fuel at 2500rpm and below

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