Flooded carbs

by Stephen Foreman in General Tech Discussion 7 years ago

Hi all

Just a question does anybody know what could cause the cabs on my 912ul to flood on startup to the point of fuel pouring out of the vent tubes. We started the engine from cold and it fired once and instead of running up as normal it just stopped dead. Then the engine wouldn't even fire and we started to look round to see if we could find the cause and eventually discovered that the cabs were overflowing with fuel. After i emptied the bolws and pumped fuel back in to get some fuel pressure prior to starting all was well the engine ran as normal. Incidentally up until today the engine has been starting perfectly everytime. And was last flown a week ago. Thanks for any advice.

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Re: Flooded carbs

by Roger Lee » 7 years ago

Roger Lee

127 5.21k 4.79k

Just some food for thought. Mentally go over your own scenario that day and see if any of these fit.

Several things can cause the carbs to vent fuel.
Carbs not synced, rough running engine to the point the floats can't control the level, too much throttle on startup (common) which may be in conjunction with too high an idle setting, float control arms out of spec, carbs with lots of time that need some maint. or an aux fuel pump with too high a pressure rating and on during start. People who use primers over a choke setup are more prone to flood their carbs.

Since this seems to be a one time issue it may be just a throttle position error?

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

Thank you said by: Stephen Foreman

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Re: Flooded carbs

by Stephen Foreman » 7 years ago

Stephen Foreman

3 14 0

Thank you for your thoughts, just for information our idle is about 2000 rpm and the engine has done around 50 hours. Its the first time this has happened and i have been using 912 engines for 12 years and i can't remember ever flooding the carbs on start up before. Which is why i am concerned. But i must have just done something wrong i guess. Thanks again for your reply

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Re: Flooded carbs

by Roger Lee » 7 years ago

Roger Lee

127 5.21k 4.79k

It would help if you reduced your idle rpm to around 1700+/- rpm. You won't idle there, but it makes shutdown easier. Having the idle rpm too high can make starting harder and more prone to flooding.

When you start now do you use full choke and throttle shut or crack the throttle? You can do it either way, but idle rpm will play a role here. High rpm with a cracked throttle may flood. A low idle rpm with a cracked throttle isn't an issue and many times will help and keep the rough low rpm knocking out at first start.

If you had a rough start with more shaking than normal then that may have accounted for the carbs venting. It's possible it was a one time deal? (or not) Like you said it hasn't happened before.

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

Thank you said by: Stephen Foreman

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Re: Flooded carbs

by simon reeve » 7 years ago

simon reeve

0 1 0

Might be a bit late here but....
I purchased a flexwing with a 2006 (@5-600 hrs) Rotax 912s a few months ago. Initially I had no issues starting but then one day the situation described above occurred. After perhaps 3 or 4 attempts the engine wouldn't start although the prop turned ok I thought. On investigating straight away I noted fuel running out of the overflow tube. Both carb bowls were full of fuel. After emptying the bowls the engine stated first time.
Start up with the engine warm seems to be no problem and I thought that the problem may have related to the low outside temp. At the time. This recurred so I used jump leads from a leisure battery which seemed to prevent the problem. This despite the fact that the aircraft battery was adequately charged when tested. I subsequently purchased a battery booster and starting was fine once more, even when it was cold.
Recently I went to start the engine with the booster attached and the carb bowls flooded once more. Very frustrating until I quickly found that a booster lead had disconnected.
This is the current situation and I am considerng buying a replacement battery with a higher cold cranking value.
I wonder if the engine shaking on a series of failed startups had caused the carb bowl flooding.

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Re: Flooded carbs

by Pete Christensen » 7 years ago

Pete Christensen

5 10 0

Just a thought. A few times over the years my engine would run rough and spit fuel out the overflow tubes after I checked and cleaned the float bowel during my annual condition inspection. Seems like sometimes a float would stick at the bottom of the bowel. Removing the bowel and putting the bowel back on with gas in the bowel the float would then not stick. Happened a couple weeks ago again. Once I had to replace the little brass thing that pushes the needle valve closed because it would catch and not close the valve.

Pete

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Flooded carbs

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