A weird issue.... Help!

[GoingHawgWyld]

Here is a weird issue my flying partner has with his 912 UL. When he is flying straight and level, every once in a while he gets a little engine blurp. Does it a 2-3 times a flight. Never in a climb. So we decide to pull his cowling off the top of the motor and take a look. Had him start the motor and run it up. It will not go over 3000 RPM with out running rough with the cowl off... I zip tied down all the wires to make sure it is not the thrust moving anything around and causing an electrical issue. Checked and double checked wires. Still does it. He flew it a few times and still blurps in flight. Pulled the top cowl off and no go over 3000 RPM. Runs rough and seems to be carburetion. Has anyone seen this before? Could it be due to the thrust changing the pressure at the carb either at the carb inlet or maybe the float bowl vent? Maybe this is also the issue causing the blurp in flight? I am thinking of checking the jets and trying a size change to see if it is maybe running lean. But the plug colors says it is running perfect! Any ideas for troubleshooting?

[BigJohn]

I have seen this issue before with the 912 and it most of the time was related to the float bowl vent tubes being in a location other that ambiant pressure, I.e. either in a location that has preasure increase due to air flow against the tubes by the prop or direct air due to air speed or the tubes are placed in a location where there is low pressure occuring and causing a pull on the vacuum. Some people i know have corrected this by moving the tubes to a neutral ambiant air pressure location under in the back area of the cowl or some have put a small hole in the airfilter and ran each vent tube into the air filter if there is no air box, if there is an airbox T the lines and run it into the airbox.

Just a suggestion.

[Av8r3400]

Bigjohn is onto something. A friend just solved his intermittent hiccup issue by addressing the vent tube location. LEAF recommended no longer on those tubes than 4-3/4" too.

[GoingHawgWyld]

Wednesday I will go out and see where his are. I know he does not have the tuned intake so his tubes are probably the cause as it takes very little pressure differential on the vent lines to lean the carb out... This may be something that people may want to check on their motors. May not be causing noticeable issues but can cause an imbalance and make the motor run rougher than it should especially if it is only affecting one carb... Thanks for the help! I was kinda leaning towards something like this. It is noce to have many people with years of experience on here that can chime in with real life experiences...

[cap01]

Got to ask . If leaf recommends no more than 4 3/4 in max , how can anything be done different than the way they come under the float boel bail?

[Av8r3400]

Got to ask . If leaf recommends no more than 4 3/4 in max , how can anything be done different than the way they come under the float boel bail?

That is the question. My friend wound up routing his into the back side of the air cleaner on each carb.

Mine are longer extending back to the firewall. I haven't had any problems (yet).

[dholly]

I have the carb heat box and plumbing setup on my 912ul, not individual air filters. Initial entry in my engine log notes the engine quit shortly after takeoff on the first and second flights, imagine the excitement! A subsequent call to LEAF resulted in a factory reminder that the ONLY recommended vent tube length and routing was the short tube secured by the carb bail. Indeed, the original builder had not followed spec. with the vent tubes. Change made and no further issues noted in the log. I just installed new vent tubing per Rotax instructions and the engine runs great.

[kitfox2009]

I have had mine routed to the back of the small K&N air filters for the last 100 hours or so. Seems to work fine. I like the setup because if for some reason fuel did exit the vent tubes it would dump back into the carb instead of possibly igniting.
Have has no issues yet with this arrangement.
Good luck with however you do it!!
Cheers
Don
Vixen 912 UL 370 hours total

[egp8111]

I've had a similar issue with my 912ul on a KFIII. I would get an occasional hiccup and it just didn't seem to run a smooth as it should. The carbs were synched etc. I was thinking it was electrical. I had the carb vent tubes running into drip pans below the carbs but on a whim this morning I drilled holes and routed the tubes into the air cleaners as suggested in this thread. Flew it for an hour and guess what ? Much smoother! You might want to give it a try. EG

[GoingHawgWyld]

I Googled this issue and found it very common. There are several articles and technical bulletins on this subject. Here is one of them that describes it best...

The clear plastic vent line, from the carb, must be vented to the same pressure air as that entering the carburetor. If your installation uses a K&N air filter on each carburetor, without an air box, you must leave the vent line end at the carburetor. This is how the carbs are set up from the factory. The K&N air filters cause a minimal drop in pressure, so venting the carb float bowl next to the carb works fine. Some owners make the mistake of extending these vent lines (one per carb) down to the belly of the aircraft; that may seem like a good idea, but it is not. Extending the carb vent lines in this fashion will cause the engine to run poorly because of the pressure differential between the carburetors and the belly with its higher velocity air stream (increased velocity causes reduced pressure). When using an air box, the carburetor vent lines must then be connected to the air box.