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Engine Take Off Cough


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So I fixed our rough engine operation with overhauled carb but now the engine coughs at first application of power on take off roll.

We do not jam the throttle forward.. it is a smooth slow advancement of the throttle.

Timing was ok when we (maint.) were trying to diagnose the rough engine prior to overhauled carb installation.

What might we be looking at to cure the cough??

 

Thanks

GHW

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Last month I had a similar problem while advancing throttle as always slow and smooth. I would get a slight hesitation (cough) enough to know it wasn't right  (mag check at run up was good). I whent back to the hangar got all the plugs cleaned and tried again with same problem. I ordered all new plugs (this time fine wire)  and problem resolved, runs like new. The old plugs only had 300 hours on them. 

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8 hours ago, George said:

Presumably.... will be checking today when I go pay for the labor...ugh!

George,

with the engine warmed up and idling, smoothly pull the mixture control to cut off.  If the tachometer drops off with no decernable rise, the idle mixture is too lean and requires adjustment.  Too lean an idle mixture can cause the stumble you describe.

Clarence

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3 hours ago, LANCECASPER said:

After startup do you lean for taxi? The plugs can foul easily at full rich low rpm.

George, I lean my C between 2/3-3/4 of the way to Idle Cutoff on the ground. It's enough to taxi with, but not uphill. Any more throttle than moving in flat ground requires me to push the mixture forward some, or it will cough and sputter and sometimes try to die. Keeps the plugs clean, and mag checks generally 50 RPM, sometimes less.

This would not have occurred to me, but it's another easy, free thing to check.

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On ‎1‎/‎16‎/‎2017 at 3:34 PM, Pirate said:

Last month I had a similar problem while advancing throttle as always slow and smooth. I would get a slight hesitation (cough) enough to know it wasn't right  (mag check at run up was good). I whent back to the hangar got all the plugs cleaned and tried again with same problem. I ordered all new plugs (this time fine wire)  and problem resolved, runs like new. The old plugs only had 300 hours on them. 

New Tempest (not fine wire) plugs about a month ago maybe 30 hours on them. Cleaned a few weeks ago prior to carb overhaul.  I do lean after engine start. I may need to look at the idle mixture as I do not get a tach rise when I pull the mixture to cut off as described by Clarence above.

Thanks

HW

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23 hours ago, M20Doc said:

George,

with the engine warmed up and idling, smoothly pull the mixture control to cut off.  If the tachometer drops off with no decernable rise, the idle mixture is too lean and requires adjustment.  Too lean an idle mixture can cause the stumble you describe.

Clarence

It does not rise...we'll give it a try tomorrow. Any hints on the procedure or is it a matter of increasing idle mixture until we get the tach rise?

Thanks

GHW

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GHW,

To observe a rise in rpm may take a relatively slow pull of the mixture back while idling.  Pulling back too quickly just kills the engine without allowing it to react to the change in mixture.

the rise of rpm is probably caused by using all of the air going to the engine.  Probably caused by a small EGT peak to go with it.  You would see the rpm rise as the EGT rises and exhaust volume increases...

slow and steady on the mixture pull.

There is so much going on at the time of engine shut-down it may be challenging to observe

PP thoughts only, not a mechanic or CFI.

Best regards,

-a-

Edited by carusoam
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You just enrichen the idle mixture until you get the proper tach rise as you gently pull out on  the mixture to cut off. A 100 RPM rise is too much and no rise id too low a mixture enrichment. Engine warmed up correctly first. Be careful of the prop !  Make sure the mags are off when you go out there to adjust it or when anyone is out there adjusting it. Its easy to forget the mags when the engine dies with the mixture and you need to go out there readjust the mixture. 

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  • 2 weeks later...

Interesting, Lionudakis...

The purpose of the accel. Pump is to boost the mixture's richness to stay ahead of the increase of airflow as the throttle plate opens...  the fuel flow is increased as a reaction to the airflow.  Without the accel pump, the FF is too slow to respond properly...

How would a person know if the accel pump is working properly?

- how well the accel pump works when priming the engine for start?

- how much fuel pressure is lost with each pump of the throttle during the priming process?

- is there an adjustment for the accel pump?

- if pushing the throttle in slowly works, this slowing of the process is covering for something else not working.

Mixture control during acceleration in a modern car requires a nice computer with a couple of sensors.  In a Mooney, we typically accelerate once... And the carb usually keeps up with the needs, flawlessly.

Getting the carb set up properly may take some experience...

PP thoughts only, not a mechanic.

Best regards,

-a-

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2 hours ago, carusoam said:

Interesting, Lionudakis...

The purpose of the accel. Pump is to boost the mixture's richness to stay ahead of the increase of airflow as the throttle plate opens...  the fuel flow is increased as a reaction to the airflow.  Without the accel pump, the FF is too slow to respond properly...

How would a person know if the accel pump is working properly?

- how well the accel pump works when priming the engine for start? I can't tell about this. I build pressure with the electric pump, then turn it off, for my first start of the day since Fuel Pressure = 0 at that point. For later starts, I just pump the throttle a couple of times, like I used to step on the gas pedal in my old truck before turning the key.

- how much fuel pressure is lost with each pump of the throttle during the priming process? Generally less than one notch on the gage. ½ psi?

- is there an adjustment for the accel pump? I've never looked . . .

- if pushing the throttle in slowly works, this slowing of the process is covering for something else not working.

Mixture control during acceleration in a modern car requires a nice computer with a couple of sensors.  In a Mooney, we typically accelerate once... And the carb usually keeps up with the needs, flawlessly. This has been my experience. Idle, Rich mixture--push the big lever forward and the engine speeds up immediately.

Getting the carb set up properly may take some experience...

PP thoughts only, not a mechanic.

Best regards,

-a-

See what works for my C above.

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