Author Topic: Mysterious "thump" while flying my EZ-Help!  (Read 8157 times)

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Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« on: October 12, 2005, 09:25:32 PM »
I own a Long EZ that has a CDI system (2 coils and a magneto) and has been flown close to 400 hrs with it. Since I didn't build the aircraft, I've  invested time and effort studying ever aspect of the aircraft. I would like to share my experiences with your expertise and hopefully we can identify the inconvenience I'm having right now.
 
While flying my airplane I've noticed every now and then a "thump" which ususally makes my heart skip a beat. At the beginning I thought it was maybe moisture on the fuel or bad fuel, however, the problem has not gone away. After experiencing some engine roughness on a flight around 10,000 ft, I was suspecting tht my carburator of my Lycoming 320 engine need an overhaul given that I didn't have supporting documentation that any service had been done to the carburator. I ordered a new carburator, spark plugs &cables were in good condition, no visible moisture on the coils, air intake clean and with good integrity, etc. We did all the calibration to the carburator and I took it to a test flight. To my surprise the every now and then "thump" didn't go away, but the engine run pretty smooth with the new carburator. Trying to troubleshoot this I am looking into the CDI systems to be able to understand it better and try to determine if the CDI might be a "suspect" in this case. Is it possible that the coil might not be giving the proper charge to the system while in running mode? I've discarded the magento because every amg check is smootha and on tolerance. Do these coils loose efficiency over time? Should they be replaced?
 
Thanking you in advance...

Best Regards,
 
Afif E. Saybe, LONG EZ, HR-ATQ
President
Saybe Engineering S. de R.L./Honduras

Offline Drew

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Mysterious "thump" while flying my EZ-Help!
« Reply #1 on: October 13, 2005, 09:57:40 AM »
Is the thump still there if you only run with the mag?  When the thump occurs is there any change in rpm, mp, egt?  What kind of configuration are you in when the thump occurs (climbout, cruise, descent------gear up/down, speedbrake up/down)?  Gascolator condition?----check for sludge in the bowl, change filter.  Fuel lines---are they old?--do they need to be replaced?  There is a fuel flow test that is listed in the plans that you could check.
Drew Swenson
Cozy N171ML

Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« Reply #2 on: October 13, 2005, 12:00:48 PM »
Thanks for you respose. I haven't tried just using the mag. However, the engine "thump" is noticeable in climb and just yesterday, I noticed it when powering down turning to final in landing configuration. Gascolator is ok. Since I just changed to a new carburator, we verified all the lines, gascolator, filter, etc. In other wods is like the engine is skipping a beat every now and then. I will try to run only with mar and see waht happens. Suggestions a kindly welcome. Thanks you.

Offline Drew

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Mysterious "thump" while flying my EZ-Help!
« Reply #3 on: October 13, 2005, 02:50:36 PM »
It sort of sounds like the same thing I had with my 0235 Longez (2 mags and carb).  I would notice it throttled back and in the landing pattern (gear/board down).  I would get no change in mp, rpm, egt, or anything----but my ear would hear it.  Never really did figure it out.  But thought that it was related to the dirty airflow off the nose gear and board or due to the fuel atomization at the lower power settings---or both.  When I put my 0320 in (2 mags and carb also), I did not have this problem (downdraft cooling-----but ram air to the carb came from below).

You felt as though the problem may be in the electronic ignition----and it may be possible-----but the good way to tell is to shut off the electronic ignition, get in the configuration that seems to cause the event, and see if you can isolate the electronic ignition as the problem.  You can also check the mag in the same manner.

I wonder if others have had similar problems with solutions.  Let see---electrical (mags/elect ignition, intermittant grounding), fuel (restrictions, water, other contamination), tank vents (bent, clogged, disturbed).  That's all I can think of

If you have an engine monitor with a record function, you may be able catch the problem after the flight---usually on EGT.

Hopefully there are some A&Ps that can jump in here.
Drew Swenson
Cozy N171ML

Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« Reply #4 on: October 13, 2005, 05:15:34 PM »
Thanks for the insight. I difinetly will continue to look into all the suggestions and hopefully isolate the problem as you suggested. I'm forutnately have a good A&P familiar with the engine, but not with the electronic ignition system. One peculiar thing is that after we calibrated the carburator for idle and max throttle, after the flight, the idle RPM was a lot lower when I landed (at the same setting) and in the first instance the engine quit while exiting the runway. That also happen yesterday after the second test flight, while bringing it to idle, the engine sputtered substantially and then it quit. ???? Thank you very much for your time.

Afif

Offline Bill James

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mysterious thump
« Reply #5 on: October 14, 2005, 11:35:15 AM »
Along the way try looking at wiring while idling on the ground in the dark with the cowls off? I caught a bad plug wire arcing that way.
Bill James, Fort Worth VariEze N95BJ
Downdraft Plenums, QuickCowls
There was supposed to be anhedral?
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Offline spy

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THUMP
« Reply #6 on: October 14, 2005, 01:35:53 PM »
HELLO ALL,
    for AWHILE i thought it was just me.... i too hear  " thumpinG"...only on decent..power down or idle....
      i am using a 0-235...w/ a p-51 scoop...and a cessna type carb heat box...   WHAT IS HAPPENING ...is when i am  decending....the airspeed is still high...and i pull back the throttle...the  RAM AIR is pushing into the  airfilter box which is directly facing the scoop...
    THE  fuel/air mixture becomes extremly lean...causing a backfire..."THUMPING".... ALL I DID IS reverse the carb heat box...facing away..from the RAm AIR... and ..no more thumping>>> 8)

Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« Reply #7 on: October 14, 2005, 02:38:07 PM »
Thank you very much for all of your insights. I will try each and every one of them to hopefully eliminate this uncomfortable feeling on the air. Klaus Savier from Light Speed Engineering is also helping troubleshoot this issue on the CDI part. Thank you all!!!

Afif Saybe
Long EZ HR-ATQ, Honduras

Offline Jack

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Mysterious "thump" while flying my EZ-Help!
« Reply #8 on: October 16, 2005, 09:34:45 AM »
Quote from: "asaybe"
Thanks for you respose. I haven't tried just using the mag. However, the engine "thump" is noticeable in climb and just yesterday, I noticed it when powering down turning to final in landing configuration. .


Can we assume it's engine related?? Engine performance gauges should reveal something. Another area to check is your fuel venting. When I installed new lines, it made a helluva of an improvement at low power.

I don't have the time right now to give you the quick troubleshooting checklist. Am sure others know it. Currently I am in the middle of freaking Iraq and at times it gets a lil exciting.

Good luck;

Jack

Offline Waiter

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Mysterious "thump" while flying my EZ-Help!
« Reply #9 on: October 16, 2005, 11:19:53 AM »
I not sure what your calling a "thump"?  Is it something you can hear. or is it something you can feel. Do you feel it in the flight controls, or does it seem to be a "miss" in the engine?.

ENGINE RELATED:

1) Water in fuel. Check fuel system paths and make sure gasculator can trap water, i.e. is it plumbed correctly, in / out. Check the gasculator AFTER the flight and see if there is any water in it. If its plumbed correctly, It should capture most of the water, but some can still get by. I would suspect if this is the culprit, you'll see a lot of water in the gasculator.

2) Valve sticking, usually not one thump, but a series of thumps.

3) Plug fouling, usually a perceivable roughness, unless the other plug isn't firing at all, then its "very" perceivable. ALSO. If you see large balls of lead on the spark plugs, these can become dislodged, and if the timing is absolutely just right, it could lodge between the valve and the seat, keeping the valve from closing for a cycle or two.

4) Long Shot, Carb Ice / intake ice ingestion, Probably not this, as you would probably suffer from engine performance as the ice builds up.

5) Another long shot. FOD, stuff getting through the intake system and fouling the valves.


AIRFRAME RELATED: Something shifting or not secured properly. I would expect these to cause unusual vibrations, but I guess a "thump" could also happen.

1) Look very carefully at the engine mount, and the mount extrusions. Also look at the rubber motor mounts. Is the engine "moving"?

2) Look at the landing gear attach points. Visually inspect these, but ALSO, reach into the Hell Hole and place your finger tips on the attach points and fuselage, Have someone rock the plane front to back and sideways, you'll be able to "feel" the movement.

Also check the wheels/brakes, and of course, the wheel pants.  Fly with  the wheel pants OFF and see if the problem is still there (Caution, you will notice a significant difference in pitch between "Wheel Pants" and "No Wheel Pants".

3) Same goes for the wing, Put your finger tips across the gap in the leading edge between the wing and strake. Have someone at the wingtip left up, down, forward and back. If you to feel a "difference" between the wing and the strake, then take a closer look at the attach bolts.

Good Luck

Waiter
LongEZ-RG   >>    N961EZ
O-320 160hp  >>    MT Constant Speed Prop
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Offline Waiter

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Mysterious "thump" while flying my EZ-Help!
« Reply #10 on: October 16, 2005, 11:42:08 AM »
Could this be a backfire?

A lean mixture could cause hotspots. lean mixture could be caused by leaks in the intake system, i.e. intake gaskets cracked, Flanges on intake tubes cracked, rubber connection hoses cracked, manifold vacuum leak anywhere in the system.

Waiter
LongEZ-RG   >>    N961EZ
O-320 160hp  >>    MT Constant Speed Prop
F-16 Performance, On a Piper Cub Budget
www.iflyez.com

Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« Reply #11 on: October 16, 2005, 06:28:15 PM »
Thank you Waiter.   The thump is an engine thump, like it skips a heart beat. We did a detailed inspection on the whole engine assembly yesterday. We tighten every significant nut, check leads, crimps, fuel flow, checked the spark plugs cables with a tester, adjusted the mixture travel cable, and cleaned the whole assembly for moisture. Took it for a test flight and tried to re-create the thump scenario and I couldn't not make it happen. I had a pretty smile on my face. I just need to take it up to 10,000 ft and evalute the performance which is where i first noticed the malfunction and do the final tune up to the new carburator. According to a Klaus Savier from Light Speed Engineering, I might also have to change the CDI cable from a twisted shielded pair, to a single shield conductor in an effort to reduce inductive coupling on the wires. I'll post further results..... Nevertheless, I greatly appreciate all your "educational" check list and probable causes which will definetly help me understand the future similar scenarios.

Afif Saybe
LongEZ HR-ATQ, Honduras

Offline Snappy

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Mysterious "thump" while flying my EZ-Help!
« Reply #12 on: October 17, 2005, 07:48:30 AM »
Has anyone mentioned airbox turbulance?  You may be experiencing an agravated airflow duing certain carburator openings vs. attitude/pressure in the airbox.

Just a thought,
Steve
Snappy
Long EZ 988AB

Offline asaybe

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Mysterious "thump" while flying my EZ-Help!
« Reply #13 on: October 17, 2005, 07:53:40 AM »
Thank you Steve....That is a likely situation and should not be discarded. After Fine tuning the new carburator, I will post the findings.

Afif
HR-ATQ, Honduras