Scary idle mixture screw

Carburetor drivers:
There are a couple of stories in the archives about people who have had engine failures due to the idle mixture screw falling out. After adjusting mine, I noticed that it sure is pretty loose. I can turn it easily with two fingers.

There is a "tensioning spring" installed behind the screw, and I don't feel like I've unscrewed it very far -- 3/4 turn from where aerosport left it. Even when I screw it back in a bit, it still feels pretty loose.

This seems like a risky thing to leave unsafetied. Has anyone come up with a way?

Paul

I'm one

Paul,

I'm one of the ones who had a problem, the screw fell out and departed the airplane! The engine would still run fine above 1500 RPM, but was very rough and coughing below. When I got my new screw and spring, I stretched the spring a bit more than it was, reinstalled and put torque seal on it. I can see it easily on the preflight by looking up thru the exhaust stack from the bottom to see if the screw has moved.

I will be definately figuring out a way to safety this screw when I get to that point!

Hmm, I've never heard of this problem before. If it's consolation to anyone, there are TONS of carbs flying unsafetied on certfied airplanes. I've never seen one safetied.

Use RTV?

Having had years of experience with buzzy, hi revving motor cycles, all sorts of screws would eventually loosen and fall out if not caught soon enough. I discovered that if you were proactive and removed the most likely candidates, coated them with high temp RTV and reassembled, there was never any further problem. Plus the RTV allowed easy dissasembly if required. It wasn't really a thread locker, it's just that the extra friction of the rubberized threads prevented complete loss if the fastener managed to loosen. It seems the idle adjust screw might be an ideal candidate for this fix if the existing tensioning spring seems a bit weak for some of the applications out there. Being careful of course to keep the stuff only on the outer most threads.

-mike

Serration Spring

I had a similar case years ago with my C-140. I installed a little spring steel retainer, looks kind of like the pocket clip on a pen, except the arm is attached to a washer and the other end fits into the needle serrated head. Don't have a picture available.

I have seen this!

When I was turning wrenches on planes for a living, I saw this happen twice.

It is usually with high time carbs and the threads have fretted away somewhat from constant vibration I think that some over haulers miss repairing the threads. The difficult part is they look normal after they re-anodize the housings. The threads would need to be checked with the correct gauge.

There is such a thing as prevailing torque helical wire inserts, by the trade names of Helicoil, Microdot, Perma-Thread and others.

I don't know if these would help or a special one would need to be made, as I remember they have a non standard thread pitch.

I also have seen a little gadget that when placed under the spring is almost U shaped and both inside surfaces provide somewhat of a detent on the outside surface knurls of the adjustment knob.

The easiest test I know of is to rock the adjustment screw after backing it out a ways (count the exact turns for reinstallation) from side to side and see if it wiggles, if it does your threads are shot. If it doesn't that is not a guarantee that you wont have problems, but if it does wiggle much you better get it fixed. If your not comfortable doing this get a good aircraft mechanic to look at it.