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06-18-2018, 10:58 AM
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Join Date: Nov 2017
Location: Johannesburg
Posts: 8
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O360 running rough
Any suggestions, please. Spend a small fortune already.
Lycoming O-360 (200 hrs SMOH)
Two partial engine failures. Max rpm at full throttle 1300. Fortunately I was over an airfield both times.
On advice I overhauled the:
Alternator
Carburetor
Starter
Regulater
Blow by?s all above 77.
Symptoms:
Starts easy
Idles smoothly
Advance throttle to above 1300 rpm, runs rough.
Stays running rough at idle.
Some say:
It?s the mags
Others say, overahualed carb is the problem
Have you checked the ignition
Even the mechanics are confused, hence my reaching out.
Any suggestions would be highly appreciated. (Getting expensive). Want to avoid pulling the engine.
Thank you
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06-18-2018, 11:08 AM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Need more info
Is oil pressure adequate at all RPMs?
Tell us about your ignition. Two mags? What kind of plugs?
Finally, constant speed propeller? What propeller do you have, what governor and how old are they?
__________________
Kurt W.
RV9A
FLYING!!!
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06-18-2018, 11:21 AM
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Join Date: Sep 2006
Location: Windsor, California
Posts: 924
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Adequate fuel flow from tanks to carb? Fuel tank vent system OK? Dry tappet clearances OK? Same rough issue on each mag and/or on both?
__________________
David Heal - Windsor, CA (near Santa Rosa)
EAA #23982 (circa 1965) - EAA Technical Counselor and Flight Advisor; CFI - A&I
RV-12 E-LSA #120496 (SV w/ AP and ADS-B 2020) - N124DH flying since March 2014 - 940+ hours (as of September 2020)! 
V AF donation through June 2021.
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06-18-2018, 11:22 AM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Allison
Any suggestions, please. Spend a small fortune already.
Lycoming O-360 (200 hrs SMOH)
Two partial engine failures. Max rpm at full throttle 1300. Fortunately I was over an airfield both times.
On advice I overhauled the:
Alternator
Carburetor
Starter
Regulater
Blow by’s all above 77.
Symptoms:
Starts easy
Idles smoothly
Advance throttle to above 1300 rpm, runs rough.
Stays running rough at idle.
Some say:
It’s the mags
Others say, overahualed carb is the problem
Have you checked the ignition
Even the mechanics are confused, hence my reaching out.
Any suggestions would be highly appreciated. (Getting expensive). Want to avoid pulling the engine.
Thank you
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Primary suspect is fuel delivery. A bad coil in the mag is also possible, but far less likely. I assume this problem existed before the carb was orverhauled and remained post overhaul, which makes the carb itself unlikely (if symptoms only appeared post overhaul, the carb is highly suspect). With that assumption, I would be looking seriously at a fuel restriction or leak somewhere somewhere upstream of the carb, as well as a bad fuel pump (have you tested to see if the boost pump eliminates the problem?). You should be able to test the coils with an ohm meter. However, both would have to be bad to get these symptoms and that seem improbable. Engine will happily accelerate past 1300 RPM with one dead or bad mag.
A theory: With a partial restriction, you get enough fuel for idle. Once you go WOT, the fuel delivery is inadequate to support the engine and the fuel level in the bowl drops, causing the lean mixture, rough running and inability to support the demand at higher RPMs. When you drop back to idle, the bowl is still low and therefore still runs rought at idle (bowl height determines fuel deliver through carb, in conjunction with air flow through the venturi). If you let it is sit at idle for some time and the idle quality eventually improves, that confirms the theory.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 06-18-2018 at 11:44 AM.
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06-19-2018, 12:47 PM
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Join Date: Nov 2017
Location: Johannesburg
Posts: 8
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Thanks to all, for the questions asked and possible fault finding avenues.
To answer some questions:
Oil pressure is good through all RPM?s
Two standard Bendix Mags with Champion Plugs
Fixed Pitch Prop
Fuel flow to the Carb tested as normal
Fuel vent checked
Have not checked the dry tappet clearances, yet.
Boost pump makes no difference to the symptoms
Left, right, both, mags are rough.
Fuel is not contaminated
Primer deactivated
Thanks Larry for your theory, we having a look. But the condition only improves after a day?s shutdown?
Both Mags have have been send in for inspection and recon. (Which was done two years ago). I highly doubt it?s the mags, have a better chance a being struck by lightning twice.
I too suspect it?s a fuel problem. Once it?s rough and I advance the throttle, black smoke is visible. Interestingly, once it?s running rough, keeping the mixture rich, cuts the engine. It?s only able to tick over at a very rough 1300rpm max with WOT, when I lean the mixture out considerably.
I hope we can find the problem, it?s just all so bizarre.
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06-19-2018, 02:58 PM
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Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,958
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Quote:
Originally Posted by Allison
I too suspect it?s a fuel problem. Once it?s rough and I advance the throttle, black smoke is visible. Interestingly, once it?s running rough, keeping the mixture rich, cuts the engine. It?s only able to tick over at a very rough 1300rpm max with WOT, when I lean the mixture out considerably.
I hope we can find the problem, it?s just all so bizarre.
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Almost sounds like the carb float is sinking or the needle is somehow sticking open.
Are you using a quality 100LL fuel or auto gas? Higher vapor pressure of auto gas combined with high temperatures can cause the float to sink (especially if winter blend in the US, not sure about blends where you're at). 100LL usually has a lower vapor pressure requirement, and more tightly controlled, than auto gas.
If using 100LL already, the carb still sounds a bit suspect. Perhaps a bit of debris is floating around, keeping the needle from seating?
__________________
Kurt W.
RV9A
FLYING!!!
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06-19-2018, 03:26 PM
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Join Date: Oct 2013
Location: Schaumburg, IL
Posts: 5,297
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Quote:
Originally Posted by Allison
Once it’s rough and I advance the throttle, black smoke is visible. Interestingly, once it’s running rough, keeping the mixture rich, cuts the engine. It’s only able to tick over at a very rough 1300rpm max with WOT, when I lean the mixture out considerably.
I hope we can find the problem, it’s just all so bizarre.
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Ok, that's classic overly rich behavior. I would normally suspect a bad needle/seat, allowing the bowl to overfill. However, very unlikely that happens with two different carbs. I would be looking at an excessive fuel pressure that is over-driving the float / needle seat setup. Mechanical fuel pumps don't typically fail this way, but they can. They have a relief valve (ball and spring) that sets the pressure. If this froze or seized, it would cause high pressure, especially at lower fuel flow levels.
Larry
__________________
N64LR - RV-6A / IO-320, Flying as of 8/2015
N11LR - RV-10, Flying as of 12/2019
Last edited by lr172 : 06-19-2018 at 03:30 PM.
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06-20-2018, 08:12 AM
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Join Date: May 2014
Location: Bountiful, UT
Posts: 145
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To piggy back on LR172's overrich scenario....
Is the wrong mechanical fuel pump installed? Injected Fuel pump (High Pressure) when it should be Low pressure. Do you have a fuel pressure guage? Is it installed between the carb and the fuel pump?
What is the history of the engine before this started happening or is this the maiden voyage?
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Donation for 2019 complete! Thanks VAF and DR! (Worth every penny!)
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06-20-2018, 12:29 PM
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Join Date: Aug 2016
Location: Asheville, NC
Posts: 2,104
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Quote:
Originally Posted by Allison
Once it?s rough and I advance the throttle, black smoke is visible.
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That's the smoking gun (no pun intended) for too rich a mixture. I used to have a similar problem when the engine was very hot, like when idling on the ground waiting for takeoff on hot days.
__________________
(2020 dues paid)
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06-20-2018, 08:10 PM
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Join Date: Feb 2014
Location: Granbury, Tx
Posts: 30
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Running rough
Hey Allison!
Sorry to hear about a rough runner. Has it been that way since the overhaul 200 hours ago? Kinda sounds like the cam/crank timing might be off a tooth or more. Have you checked that? Lycoming has a way to check that without too much disassembly.
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