Engine issue, looking for troubleshooting ideas

[PCHunt]

Had an engine "issue" today at the end of a 3-hour flight. O-360-A1A, Carburetor, Bendix SL4N mags, approx 1600 SMOH.

On downwind, was a bit high and fast, and closed the throttle, doing about 140. As I slowed, I noticed that the prop blades were going by slow enough to count them!!!! Opened the throttle, and the engine burst BACK into life! It did it again on final. A bit disconcerting. After landing, I tested it. Couldn’t close the throttle without the engine quitting. It would idle at 900+ just fine, and run at high power normally, but would shut down at closed throttle.

After several times of abnormal shut-downs caused by closing the throttle, I shut it down twice normally using the mixture, from about 1000 RPM. After that, it now starts, runs, idles perfectly even at closed throttle, and seems "back to normal." I can “slam” the throttle closed from 1200 rpm now, and it idles happily at about 700 RPM. It accelerates out of idle very normally, and quickly.

Not sure what the issue is, but will be removing the cowl, and the FAB and air filter to have a detailed look at the carburetor before flying again. I will check the plugs, as well. Champion REM37BY plugs. Some resistors tested bad recently, I didn't trust my results, and reinstalled all plugs. Mag check has been normal.

All thoughts appreciated...................

PS: Since I have owned the plane, (18 months) the shutdown using idle cut-off mixture has been a bit slow and "ragged", but it always quits after a short "argument". In other words: not a "clean, sharp" cut off.

[WAM120RV]

This sounds like a sticking problem in your system..... in that when you pull the throttle closed less firmly it comes to what you think is the idle position and idles happily at 700.

When you are more firm with the pull to idle as you probably are under flying conditions, you pull it back past the point where its sticking and to its true stop...... which is set too low and allows the engine to stall.

Get rid of and sticking in the system and eliminate this first.

[Andrew M]

to see if the throttle plate can close more using pressure on the plate, not the arm. I'm thinking ram air could be closing something more.

[Walt]

You may want to check the carb floats as well, symtoms sound pretty typical of an over rich condition from sinking floats.

[GeneL]

Had this happen on a fairly new carb rebuild (1 year) new float was bad. Replaced with solid blue one, no more shutting off in the pattern.

[PCHunt]

Originally Posted by Walt

You may want to check the carb floats as well, symtoms sound pretty typical of an over rich condition from sinking floats.

I will check the float, but wondering if a sunk float can "fix itself" like mine has appeared to do?

[rv9av8tr]

Originally Posted by PCHunt

I will check the float, but wondering if a sunk float can "fix itself" like mine has appeared to do?

It certainly sounds like a mixture issue and I too would be first suspicious of the float/needle position. The pitch of the plane in flight will be different than on the ground, so the relative position of the float in carb bowl can be a little different. My experience has been proper float position range tends to be finicky. Even when you split the carb it might be hard to tell of the float is OK or not, other than it's weight is obviously wrong. There is a procedure for measuring float swing for setting that stop tab. I would really recommend getting a mechanic involved who is familiar with carbs when it comes to messing around with the float.

[petehowell]

Hi Pete,

My O-320 will act like is has a sunk float on the ground after flying when the WX is really hot. When it cools down, it is magically "fixed". Seems as if there is fuel boiling in one of the carb circuits when it is really warm.

Does yours seem to be temp related? Just a thought.

[mahlon_r]

All of the above suggestions as well as, intermittently leaking primmer or intermittent induction leak.
Good Luck,
Mahlon

[aerhed]

Worn throttle shaft bushings? disconnect the cable and look for rattle at both ends of the shaft at different throttle positions. Cheap check.