Thread: Engine vibration during descent

I recently had a Rotax 200hr service done and had engine mounts replaced.
Upon flying home, I got a severe vibration during descent, when throttling below
4000RPM. Pulling back to min. throttle stopped the vibration. The problem was
the pressure pickup tube that goes through the airbox and connects to the carbs
with the small clear tubes. The pickup tube has a hole in it, and that hole
must be positioned *inside* the airbox. Flight design knew about this and
advised me correctly. I had carb balance rechecked after the repair.
The incorrect altitude compensation would unbalance the carbs.

I just wanted to warn others in case this happens to your CTSW, a simple
pre-flight check of that tube may save you the trouble.
I posted this in CTSW, since I believe the problem is specific to the airbox
in this type. I'm not familiar with the tubing connections in LS or MC.
Maybe the post should have been under 912.

Hi CCCT,

Your absolutely correct and nice trouble shooting. It is more of an issue for the SW's.Those CT's with the Rotax airbox aren't as likely to have this happen unless someone pulls it off its nipple on the airbox. The planes with the airbox on top of the cowl like most of the SW's have that white plastic carb vent tube run into each side of the airbox. I is a continuous tube and there are 3 holes cut into this line. If one or more of these holes get exposed outside the airbox then you do have a terrific carb balance issue that like you will experience and feel. You only have to look at this and see if the holes are outside on either side of the airbox. If your engine runs smooth then yours is most likely ok. If for some reason one of the holes does protrude to the outside of the airbox just push it and pull from the other side to slide it back in place.

Maint tip:
Take 2 plastic wire ties and put one tightly on the outside edge of the plastic tube against the airbox and then one on the other side. This will keep the tube from being accidentally pulled or push out. It will more or less lock it ion place.

I forgot to say I replaced the nylon tie-wraps on the tube, to prevent its
shifting in the future. Not sure how effective that its, the tube is
slippery PTFE (teflon?)

I've had a strong vibration during decents when power is pulled back. I saw this thread and checked my carb balance tube. You guys were exactly right about the hole being exposed in the balance tube. On visual inspection of the tube there were no holes found. Running fingers on the backside of the tube at the right side of the airbox revealed an exposed hole. The tube was repositioned to insure that all holes were back inside the box. Tie wraps were applied on the tube on the left and right sides of the airbox to insure that the tube didn't slide back out to re-expose the holes. My vibration is now gone. Love this forum!

Dick

Hi, I spoke with John Hurst @ Sebring. He suggested that the pilot pull on the carb heat in this circumstance.
It would equalize the pressure and reduce the vibration until one can land and fix the tube.
I also met Roger F. in person. Nice folks.

Thanks John !

Originally Posted by BMcCand

I also met Roger F. in person.

I think maybe you met Roger Heller. Roger F. hasn't been in Florida this week.

Arrgh. I should have mentioned, prior to demonstrating, that I'm bad with names.Apologies to Roger H.
I even changed handle here from CCCT to BMcCand, and he did it for me.

Really for Roger F - I have a son in Santa Clara. I'll look you up if I land at RHV. I'm considering RHV or PAO for parking...
RHV looks lower hassle.

Originally Posted by BMcCand

Really for Roger F - I have a son in Santa Clara. I'll look you up if I land at RHV. I'm considering RHV or PAO for parking...
RHV looks lower hassle.

PM me if you come out this way. RHV is probably your best choice, for location, services, airspace, weather (depending on time of year), etc.

Be careful about putting ties on your tubes. I did that to mine, pulled them too tight and over time one of the tubes collapsed internally, shutting enough to CAUSE the problem I was trying to prevent. My mechanic has since swaped the tubes for some stronger ones, and used some epoxy on the tube and grommet on the outside of the air box to hold the tube in place. - WF