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PCHunt

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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...................:confused:

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.
 
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Sticking

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.
 
Check

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

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

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

PCHunt said:
I will check the float, but wondering if a sunk float can "fix itself" like mine has appeared to do?
Click to expand...

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.
 
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Really Hot

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.
 
All of the above suggestions as well as, intermittently leaking primmer or intermittent induction leak.
Good Luck,
Mahlon
 
Worn throttle shaft bushings? disconnect the cable and look for rattle at both ends of the shaft at different throttle positions. Cheap check.
 
Report of Troubleshooting

Thanks for all the suggestions:
Throttle and Mixture linkage not sticking or loose. Good cushion on both.
Throttle plate closes normally when pushed on. Does not touch sides of venturi.
Dropped the float chamber, nothing abnormal found. Metal floats, no sign of rubbing anywhere, appeared to be set properly.
1 piece venturi ("V") stamp.
Primer is a solenoid type, and couldn't tell if leaking. Not sure how to check.
Throttle shaft bushings not sloppy.
No trash in float bowl. Inlet screen clean as a whistle.

The carburetor P/N is 10-3878.

Have reassembled everything and ground run, runs normally at all power settings. Have pulled gently on the throttle to idle, and have "slammed" it to idle from 1500 RPM, idle is normal in all cases. Engine accelerates quickly out of idle, even with aggressive throttle opening.

I did notice that the idle mixture screw appears to be fully screwed in, as the spring is fully compressed. The idle rpm rise when slowly closing mixture is about 100, indicating a bit rich for the idle circuit. Engine doesn't shut down "cleanly" or sharply when mixture slowly closed at closed-throttle idle. Stumbles along for several seconds. But if throttle is at about 1000 RPM for shutdown, the engine quits cleanly.

Intend to fly it, and see how it goes. If I can't repeat the issue, will write it up to "gremlins".

Any more thoughts?
 
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PCHunt said:
...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.
Click to expand...

Pete,

If the mixture control travel is correct, another possibility is that the mixture control needle valve or seat may have some wear.
 
Metal float

As I stated in my earlier reply, the recent rebuild contained a new float (metal type unknown brand) you could not tell from looking at the float that it had become "heavy" it is this partial flooding that causes the intermittent problem. I had to remove the float from the carb to check it. When I removed it I couldn't hear any fuel sloshing around, but it still felt a little heavy, I squeezed and rotated the float and found tiny non visable pin hole weeping fuel. Another way to tell if you have a pin hole is to drop the float in hot water and look for bubbles or fuel coming out. Side note: I cut the metal open to see why I couldn't hear any fuel sloshing around and found the float is filled with open cell foam??? Any way don't let the intermittent condition steer you away from the float level, cold engines will tolerate a much richer condition than hot ones. Gene
 
Did you ever figure this out? I never heard any mention of carburetor heat when this was going on. Is there any chance you were getting carburetor ice?
 
Carb settings

It sounds to me like your carb settings need to be reset.
Idle should be around 750 RPM after engine is warmed up. With the mixture pulled back to cut off, the RPM should rise by about 30 to 40 RPM as the engine shuts down.
You can begin to readjust by setting the throttle stop screw to the desired idle RPM and then adjust the mixture screw as necessary to get the RPM and mixture setting correct. It may take several times and you may need to readjust the idle stop more than once as you reset the mixture screw.
Each time you check it, make sure you run the RPM up to at least 1500 before you bring it back to idle to test the idle RPM and cut off RPM rise. Remember, engine should be warm. If your carb is in good shape you should be able to get it all set correctly.
Having the idle RPM too high can cause high landing speeds, since you may not be able to slow down quickly. Besides floating, this will also cause you to over heat brakes and wear them out prematurely, so there is a domino effect going on here.
Any way, thats what i would do. Good luck.
 
I am going to suggest another option based on what happened to me today. I have a RV6 with fuel injection. You mentioned you had been flying for 3 hours and were slowing rapidly in your post. I was inbound to a fuel stop today with 3 gallons in the left tank and 6 gallons in the right tank. Less then I normally like to have but still learning the aircraft. I was also slowing rapidly on downwind with the left tank still selected. I intended to switch to the right before base. I wanted to get the left as low as possible to get a accurate gauge check when refueling. As I added power going through 100 knots the engine stumbled and a quick glance showed fuel pressure fluctuating. I switched to the right tank and added a shot with the boost pump. Engine ran just fine from then on. Refueling confirmed that the left tank had 3.1 gallons. I think its possible that rapid deceleration might uncover the fuel pickup even with 3 gallons in the tank. In my case everything was normal after fueling. I know you had some issues on the ground after but perhaps you had some air in the lines. Do you recall how much fuel you had in the tank selected?

George
 
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