Engine hesitation?

DavidWhite

DavidWhite

Final Approach
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Apr 19, 2011
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DW
I was turning base to final today after flying out to lunch with an airline pilot friend, and my engine hesitated when I added power. It was like the way a car hesitates with a dirty fuel filter. Anyone know what could have caused this?

David
 
hmm Was your engine leaned out too far? Forget to go rich when you descended to pattern altitude?
 
How much fuel did you have in the tank you were on?
Depending on the amount in there and how much bank you had, you might have ported the tank.
 
SO many variables that could have caused it.
Just a "hiccup" in the engine that you may never know why?

Do a REAL good run up next flight, work the throttle back and forth some. If all is good, then chalk it up to "who knows". Water in the fuel, carb sticking a little, so many things...
 
Ok thanks. I just wanted to make sure that Its nothing serious. I'll do a real good run-up to be sure.
 
I had that happen to me in a 172 when doing solo pattern work. My CFI said it was probably plug fouling.
 
Hm. Base to final. Low power setting, an ideal situation for carb ice. This is a carbureted engine, right? What were the temperature and dewpoint at that time?

Ignition. 90% or engine troubles are electrical. We can ayk on about fuel issues, but ignition is far more troublesome. When you open the throttle, the RPM is initially low, which means the magnetos' little internal alternators are turning slowly and not generating much power. Opening the throttle rapidly raises cylinder pressures, and sparkplugs have a tougher time sparking across a denser medium. Combine weak, worn mags (or contact points or old capacitors) and sudden power increase, and it'll miss.

Fuel issues get worse quickly and stay bad. More power means more fuel demand, and a clogged filter won't permit it. It'll quit. The only fuel issue that might be present here is an accelerator pump (in the carb) that has quit working. That'll cause hesitation on acceleration.

Dan
 
Gremlins...:yikes:

Carb Ice gremlins
Water slug Gremlins (carb ice gremlins in training)
Fuel bubble Gremlins
Sparkplug crap Gremlins

The list goes on and on...it's worse depending on where and what you fly.
As far as I know a Bell 47 helicopter pilot flying over Lake Superior has the most potential Gremlins aboard...


Chris
 
DavidWhite said:
I was turning base to final today after flying out to lunch with an airline pilot friend, and my engine hesitated when I added power. It was like the way a car hesitates with a dirty fuel filter. Anyone know what could have caused this?

David
Click to expand...

What aircraft?
 
During runup today I messed with the throttle with carb heat in, and carb heat out. It never stuttered with carb heat off, but when I pulled out the carbheat it would hesitate.
 
DavidWhite said:
During runup today I messed with the throttle with carb heat in, and carb heat out. It never stuttered with carb heat off, but when I pulled out the carbheat it would hesitate.
Click to expand...

Normal operation in some aircraft, what aircraft are we talking about?
 
I use carb heat in my runup, but I never adjust RPM or mess with the throttle. I just look for a RPM drop and no gain
 
I'm not so sure thats normal for a carb 172.... Are you at a high elevation airport?
 
Tom-D said:
Are you saying the engine actually stops running for a second or two? or does it drop a few RPM?
Click to expand...


drops RPM, then when I increase throttle it drops more RPM before coming back up


AJ, I'm not at a high elevation airport but density altitude was 5100' at 109 degrees
 
That may be high enough....

If you are doing your runup with the mixture full rich, your fuel/air ratio will be even richer at a higher altitude than at sea level. If you have less dense air with the higher altitude, with same amount of fuel as at lower altitude, and add carb heat, it will enrich the mixture even more.

Perhaps the engine was getting too much fuel at that altitude, with full mixture, and carb heat, causing the hesitation with throttle. Maybe?

Have you flown/operated this aircraft with temperatures so high at your airport before? Has this happend before?
 
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Just want to add, if it really is a problem with too much fuel, mixture too rich, you are at risk of fouling those plugs, as previousy commented on.
 
DavidWhite said:
I've never been told to lean on the ground before, I'm gonna start doing that
Click to expand...

Up here, if you don't you'll know it. The engine won't run well during taxi.

You can't take off full-rich up here either.

Bites a lot of low-land pilots square in the butt when they can't figure out what's wrong with their climb performance on a hot day when the DA soars above 8000'.

We lean aggressively -- and by aggressively I mean until the engine almost wants to quit -- for ground ops up here. If you don't, you're virtually guaranteed to foul a plug taxiing out.

I have to advance the mixture to add power above idle during taxi, or the engine *will* quit. Never had a fouled plug in my aircraft. Saw them a lot in the rental fleet though.

Had a broken plug wire, though. That was entertaining since out of habit from rentals, I attempted to "burn it off" in the run-up area.

The O-470 vibrates a lot when a plug is not firing at full power leaned as far as it will lean. :D

We also lean a bit for start, but that's not typically trained. I started doing it when I saw my mechanic pull it out an inch and the engine turned over and caught in one blade. I memorized the spot our engine "likes" for start, and it's probably one of the reasons our battery has lasted six years.
 
N801BH said:
lean.... lean.... lean some more..... Or switch to unleaded auto fuel...... Ah, never mind... it is full of liqour...
Click to expand...
Exactly. Lean until the engine almost quits on the ground. Don't go full rich prior to landing. Don't waste your plane and the owner's money with "runups". In short, become an informed pilot, and if this happens then, you'll _know_ that something is wrong.

Sorry if I'm overly harsh - I'm just not in the mood for old wives tales tonight :)
 
ajstoner21 said:
That may be high enough....

If you are doing your runup with the mixture full rich, your fuel/air ratio will be even richer at a higher altitude than at sea level. If you have less dense air with the higher altitude, with same amount of fuel as at lower altitude, and add carb heat, it will enrich the mixture even more.

That is not true, the ventrui acts the same as a wing, high DA effects it in the same manor. The higher the DA, the less suction will be created, thus the less fuel transferred to the airflow..

Perhaps the engine was getting too much fuel at that altitude, with full mixture, and carb heat, causing the hesitation with throttle. Maybe?

Have you flown/operated this aircraft with temperatures so high at your airport before? Has this happen before?
Click to expand...


The accelerator pump in the carb is set too high, the engine is running full rich, and when the throttle is advanced the accelerator pump squirts more fuel into the intake and the engine goes way too rich for a moment.
 
DavidWhite said:
I've never been told to lean on the ground before, I'm gonna start doing that
Click to expand...

Lycoming and Continental both now recommend leaning to best RPM at all power setting and altitudes.


Remember any power change, is a mixture change. IOWs change the throttle, you change the mixture.
 
Tom-D said:
The accelerator pump in the carb is set too high, the engine is running full rich, and when the throttle is advanced the accelerator pump squirts more fuel into the intake and the engine goes way too rich for a moment.
Click to expand...

I didn't mean EXACTLY the same amount of fuel. I just using that as simplified explanation of why the mixture would be too rich at higher altitudes, without adjusting the mixture.

But, thanks for explaining about the venturi suction. I hadn't thought that much about the carb before. Although I should, I'm starting A&P school in a couple months.
 
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Quote: "That is not true, the ventrui acts the same as a wing, high DA effects it in the same manor. The higher the DA, the less suction will be created, thus the less fuel transferred to the airflow.."

It might suck a little less fuel but not enough to compensate for the DA. The carburetor senses air velocity through the venturi, not air density, which is why we have to lean as we climb. Velocity will be constant for a given throttle setting and RPM, and the pressure drop created will also be relatively constant and will draw about the same amount of fuel, but the decreasing weight of air makes the mix richer.

Dan
 
ajstoner21 said:
I didn't mean EXACTLY the same amount of fuel. I just using that as simplified explanation of why the mixture would be too rich at higher altitudes, without adjusting the mixture.

But, thanks for explaining about the venturi suction. I hadn't thought that much about the carb before. Although I should, I'm starting A&P school in a couple months.
Click to expand...

There are three things that influence the fuel transfer from the float bowl to the venturi area of the carb.

Atmospheric vent in the float bowl. The mixture valve, and the minus pressure developed in the venturi.

Close the vent, the flow stops, close the valve and the flow stops, close the throttle, the flow stops.

WE normally can not do any adjustment to the bowl vent. (pressure carbs yes)

WE can adjust the mixture valve, by the control provided.

WE can change the pressure developed in the vanturi by the throttle valve changing the amount of air flowing.

The actual change of atmospheric pressure exerted on the fuel in the float bowl changes with actual atmospheric pressure, which will influence the fuel flow. as you go up, less pressure is exerted on the fuel in the float bowl, and a lesser amount of fuel will transfer.

The amount of minus pressure created in the venturi, will be determined by the throttle setting and atmospheric pressure, you will vary this pressure when you change the throttle setting. but the atmospheric pressure will remain the same at both places. thus the rise in altitude will not change mixture.

At full rich setting the mixture valve is wide open, and the engine is at the mercy of the two pressures to determine fuel flow, and will be very rich.

This is why we are allowed to lean by partial closing of the mixture valve, this how we get best power and are able to run lean of peak.

When we get a High DA, the air its self changes, now the molecules of the air are larger, and fewer get thru the carb. but the pressures remain the same, thus the fuel flow will also,

But the engine will receive fewer molecules of O2 to burn. thus the rich mixture. this is why we now must lean to get the proper mixture to develop best power.
 
Dan Thomas said:
Quote: "That is not true, the ventrui acts the same as a wing, high DA effects it in the same manor. The higher the DA, the less suction will be created, thus the less fuel transferred to the airflow.."

It might suck a little less fuel but not enough to compensate for the DA. The carburetor senses air velocity through the venturi,

(Q) how does it sense that? (A) a drop in pressure. Velocity = pressure, think of the wing, more velocity, the more lift.

not air density, which is why we have to lean as we climb. Velocity will be constant for a given throttle setting and RPM, and the pressure drop created will also be relatively constant and will draw about the same amount of fuel, but the decreasing weight of air makes the mix richer.

Dan
Click to expand...

you are thinking MAP, not density

Density decreases as the temp goes up. the greater the temp the lesser amount of O2 gets into the cylinder under the same pressure.

MAP can remain the same and the mixture will increase. due to the size of the air charge in the cylinder.
 
Tom-D said:
When we get a High DA, the air its self changes, now the molecules of the air are larger, and fewer get thru the carb. but the pressures remain the same, thus the fuel flow will also,
Click to expand...

Tom-D said:
When we get a High DA, the air its self changes, now the molecules of the air are farther apart, and fewer get thru the carb. but the pressures remain the same, thus the fuel flow will also,
Click to expand...



fixed that for you. The molecules themselves do not change size.

-Skip
 
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