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).

[Jerrytex]

This got to me thinking about my vent tubes. I have the Carb box that came from Kitfox. It mounts on the gearbox and has two scat tubes that run to the carbs. Anyone run the drain tubes back to this airbox? It would be extending the tubes considerably.

Page 50 of the Rotax manual addresses this and talks about having the drain tubes connected or else there is a chance that they could drain on to the hot exhaust.

http://www.ultralightnews.ca/rotax91...tionmanual.pdf

[GoingHawgWyld]

Jerrytex,

I don't think it is the length of the tube that is the issue. The reasoning is that the longer the tube the greater chance you are placing the end of it in a location that is of a different atmospheric pressure than that of the carb opening. I would think running them to the air-box is good as your carbs are also connected there and should be the same pressure.

This morning I looked at my friends set up that was having the issues and when the cowl was removed it was obvious! He ran his vent lines into a metal tube that ran down the firewall to the bottom of the plane. An obvious pressure differential. So we poked a hole in his K&N filters and put the vent tubes in there. Problem solved! Ran excellent! Thanks for all your help! He said it was like getting a new plane! He has been worried about this for the last 250 hours! He probably is very fortunate that he did not have an engine out at some point...