0-470 Sudden loss of idle below 12-1500 RPM

[Farmboy]

Quick story for those techs with 0-470 experience that perhaps have ideas.

182 with O-470, ran great albeit with a few older oil leaks and plugs. Did some avionics work and after completion flew the aircraft for an 30-40 minutes and all good.

Next day was doing an extended low rpm (idle - 1500) ground run verifying further avionic improvements when suddenly it nearly stalled. I caught it with the throttle and pumped it back to life as if it was running out of fuel. Checked tanks and selector and all good. Would only run now over 1500 RPM. Mag checks were normal.

Pulled carb heat and this allows careful rpm operation close to 1200 rpm but not below. Acts like full of ice or seriously lean. Pull the mixture short distance and kills the engine.

Checked fuel for water, all pure.
Put it away for the night thinking it may be ice as the carb was cold and no heat at idle rpm.

Ran it again a day later with same problem.
Acts like an induction leak. Runs great over 1500 RPM right to full power. Will start sliding wheels locked across the grass.
All induction system rubbers checked for obvious leaks, blowouts, and clamp tightness.
Cross over tube pulled and inspected inside.
Stuck a boroscope into carb intake to see if something was sucked into the carb.
Engine has a 6 cylinder primer system. Disconnected primer feed line to check for leak/dribble. Have not capped all six lines at the cylinders.
Engine has a backup vacuum line off the intake for instrument air. Removed from check valve and plugged.

Test ran again, no change. Mags still check fine as well. Carb heat improvement there but not as much without cowl installed.

Pulled top plugs and verified plenty of compression and plugs working, just to eliminate as variable.
Seems like are at the point of dropping the carb.

Anyone got a smoking gun idea?

Thanks,
pb

 

[Farmboy]

SOLVED.

For those inquisitive, read on, for those experienced, feel free to laugh at me.

You know how when you look at something you don’t see really often, if an item is not there your mind doesn’t pick up on the missing item?
Well two of us looked the carb over for something wrong. Both of us in our mind said “huh, usually there’s an idle adjustment screw”, but neither of us said out loud “isn’t there supposed to be…..

I pulled up the carb online to look at the diagram and first thing I notice is the idle adjustment screw.
Walk next door and look at the carb. Nothing but an empty hole. Hahahahahahaha

But the real amazing part and lucky it didn’t snow enough to plow….
Did a FOD walk down the taxiway to where it happened and there it was laying just off the centerline.

Hahahahha. Bad on me for not instantly seeing it. But happy to have found it and know I wasn’t wrong on the induction leak.
I am glad it fell out on the taxiway and not on final when he pulled the throttle to idle, on the flight home from a repair shop.





Transmitted from my FlightPhone on fingers… [emoji849]

 

[hotrod180]

My first thought was an induction leak.
I had the same sort of issue a couple times with my 470K--
both times, the clamps at the first rubber coupling above the induction Y (above the carb) on the RH side were loose.
For some reason, the other clamps are always fine but these two loosen up.
Probably should replace them, but I just drilled the thumbscrew tighteners (dunno the correct word) on both clamps & safety-wired them together.

 

[skywagon8a]

Big question....why was it loose enough to fall out in the first place? That spring is supposed to resist it from moving on it's own. Was there an issue which prompted someone to have backed it out too far? Were you able to replace it in it's proper position and operate the engine correctly?

 

[Farmboy]

Big question....why was it loose enough to fall out in the first place? That spring is supposed to resist it from moving on it's own. Was there an issue which prompted someone to have backed it out too far? Were you able to replace it in it's proper position and operate the engine correctly?

Pete,
It appears that the customer's mechanic replaced the carb within the last year. To us it seems a bit lean at idle, to the point that I added a washer to help maintain spring tension and ensure a safe flight home, with the customer and his mechanic aware of the situation and will be looking at it.

My theory is that when setup on install it was backed out as much as possible to enrichen the idle, perhaps too much, or past the point of any real spring tension.

pb

 

[skywagon8a]

It appears that the customer's mechanic replaced the carb within the last year.

That tells me a lot. Carbs seldom wear out to the point of needing to be replaced.

 

[aktango58]

SOLVED.

feel free to laugh at me.

We will, and we do!

Truth is we all seem to take turns.

 

[Steve Pierce]

Go buy a lottery ticket with that kind of luck.