New Engine Noise?

[kitfox812]

I am in denial but I think I am in trouble. I just turned over 345 trouble free hours on my 912ULS when I had a new noise last week. Kind of a slight rattling noise that comes and goes. Could not find anything loose and the engine ran fine at all speeds. Today I ran it without the cowl off and hear a definite but slight rattling,knocking sound at 2150 rpm on warm up. It goes away at 2000 and 2200 and on up. I am still flying but since I don't like off airport landings, I have to ask if anyone has any ideas.
....Please say its OK...
Oil usage is nill and has been changed on schedule since new. Average fuel consumption is 3.7 and always non-ethanol 91 octane.
The cost and loss of flying time is going to be grim. I don't even know where to get it fixed if I have to.

thanks in advance
Rich L

[Slyfox]

what kind of prop

[kitfox812]

Powerfin 3-blade 71"

[DesertFox4]

Rich , The 912 series has a stellar rep. so lets check all the easy things first. I'll assume you checked all exhaust fittings for the rattle or new rubbing marks anywhere on the inside of your cowlings.
Lets take a look at the baffle inside the muffler next. When everything is quiet tap the muffler with a rubber hammer carefully and see if it rattles. I had mine come loose on my 912 years back and it made a weird noise but not at all rpms. If it has come loose you may need a new muffler or like me you may be able to remove the baffle one small piece at a time by breaking it up with a long needle nose pliers through the exhaust pipe inlets but of course you'd have to remove the muffler to do it. Not a great fix and very tedious but it got me flying again and seemed not to affect performance at all. Just a slightly different exhaust note.

allbee already mentioned checking the prop.

I would think if it was a gearbox problem you'd hear something different sounding when you turn the prop by hand or feel extra gear "slop" while hand propping. Do you have a slipper clutch in your gearbox?

Good luck and keep us posted.

[kitfox812]

First I want to thank you all for the ideas to make me think in another direction and Im not gonna say that box thing.
I found the tab on the lower cowling which slides over the radiator in a KF-7 had somehow become weak and was not fitting as firm as it did in the beginning. I had put rubber tape under it but the fit had loosened to the point where it ratteled againsed the radiator. I am going to add a peice of fiberglass to the tab and put a thicker peice of rubber in between. It was scary but the Rotax is still running great.

thanks again for all of your input

Rich

[DesertFox4]

That is good news Rich. Good catch. Glad it was not serious.