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O360 running rough

Allison

I'm New Here
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
 
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?
 
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?
 
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

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
 
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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.
 
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.

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?
 
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.

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
 
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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?
 
Once it?s rough and I advance the throttle, black smoke is visible.
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.
 
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.
 
It looks like rich mixture.
What is the altitude of your home strip? Isn't Joburg about 5000'
If it is and you are rich normally that could contribute to your problem.
 
Do some more troubleshooting

I?m no engine expert, but I?ve owned a few old airplanes. The first thing I would do is warm it up and get it to run rough and then take a look at each cylinder and exhaust with an infrared heat gun and see if it?s all cylinders or just one. Then if there is no difference in cylinder or exhaust temps, I would pull the carb off and take a shop vac with the hose on the output or exhaust side and tape it tightly to the base where the carb was and turn it on. Check all the points of the intake lines for air leaking. I am saying all this assuming you have checked the resistance of every spark plug and ignition wire and done a full compression check. If you happen to narrow it down to a particular cylinder then it becomes easier to isolate.
 
Plugged or restricted engine breather? High Crankcase pressure causes the fuel pressure to increase.Then after it sits the case pressure returns to normal and the fuel pressure is normal until the crankcase pressure builds up again. You make no mention of the fuel pressure..No guage installed?
 
You overhauled the starter, alternator and regulator for these symptoms?

I'm NO engine guru, but that's some pretty counterintuitive advice you were given.

BTDT with a smoking-rich O-320 that straightened up with carb replacement to get the bowl properly vented for the airspeed regime of the RV vs the Cessna it came out of (or so it was explained to me).
 
Update,

The two overhauled mags were put back today, and as suspected no change. She still runs rough above 1300 rpm. A lender Carb will be fitted soon, to make 100% sure, but the guy who overhauled the carb swears that?s not the problem.

Failing this, I am pulling the engine. The local aircraft maintenance organizations have called to help, yet they cannot make head or tail of it either. When we do find the problem, tens of thousands of dollars later, I?ll post the finding.

Also I?m no engine expert, but if there is spark, fuel and compression, an engine must run. All have been checked fine.

To answer some questions posted earlier:

There is no primer.
Compression tests all above 77/80
The problem only recently occurred, last 200 hours since engine re-con was fine.
I agree, the starter, alternator and regulater was a strange suggestion to me aswell, but before the engine cut, I had a compass swing of 30 degrees, then a radio failure, then an alternator failure. So the techs pushed to check that out.
It can?t be the timing because then it would run rough all the time.
I have only ever used 100LL and am well familiar with the high altitude leaning requirement.
The fuel pressure is normal, tested by flow rate at several points.

Thanks for all the suggestion, please keep them coming. It?s so darn frustrating!
 
"The fuel pressure is normal, tested by flow rate at several points."

Flow and pressure are two totally different things.
Do you actually know what the fuel pressure is when the engine is misbehaving? I would confirm that it was proper before pulling the engine, if it were me.
Easy to check if the breather is clear. Just remove the dipstick and blow in to the oil filler tube like it was a tuba... It should blow with absolutely NO resistance to your breath. If there is significant resistance that is likely your problem!
Good Luck,
Mahlon
 
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Since there are no primers attached to the cylinders this rules out the number one culprit. This issue bit me big time on my 3rd engine run when I failed to close and lock the manual primer pump.

A hangar neighbor with an O-360-powered RV6A saw similar symptoms. Engine ran very rough as soon as the throttle was advanced. He followed a similar troubleshooting process, overhauling carb and mags and becoming incredibly frustrated (and considerably poorer) in the process.

Ultimately his issue was a fuel issue. Our local fuel station for mogas is a Shell station. This aircraft had been burning Shell's V-Power premium auto fuel quite extensively prior to the rich-running incident. After many hours of troubleshooting the aircraft owner decided to defuel the airplane and refuel it with pure 100LL fuel. He drained all the lines to the engine and ensured, to the greatest extent possible, that the fuel system was purged of the mogas. Once 100LL was installed the aircraft ran flawlessly.

Interestingly, that same Shell V-Power fuel that wouldn't run in the O-360 ran just fine in the owner's Toyota Camry!

Investigations with Shell seemed to point toward the fuel in the gas station's tanks being a mix of summer and winter blends, and some of that winter fuel might also have been pretty old.

It might be worthwhile to drain a fuel tank and as much of the fuel system as possible, then try adding fresh 100LL and trying to run the engine on that fresh fuel.
 
Rough running

Hi there,
I had the same issue about 11 mths ago ( lycoming 0-320) and tried everything right up to installing a new electronic ignition system with no change. The problem turned out to be a mixture issue and the engine was running to rich causing a reduction in power at high throttle setting. Carburettor jet was replaced and never had a problem again!
 
"The fuel pressure is normal, tested by flow rate at several points."

Flow and pressure are two totally different things.
Do you actually know what the fuel pressure is when the engine is misbehaving? I would confirm that it was proper before pulling the engine, if it were me.
Easy to check if the breather is clear. Just remove the dipstick and blow in to the oil filler tube like it was a tuba... It should blow with absolutely NO resistance to your breath. If there is significant resistance that is likely your problem!
Good Luck,
Mahlon

I don’t know if you guys are aware of this but Mahlon is one of the most respected engine gurus/builders in the USA, it seems a little like his advice is being ignored. Before you spend anymore cash follow his guidance, the test he suggested is free and if it bears fruit the fix is free too!
 
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I don?t know if you guys are aware of this but Mahlon is one of the most respected engine gurus/builders in the USA, it seems a little like his advice is being ignored. Before you spend anymore cash following his guidance, the test he suggested is free and if it bears fruit the fix is free too!

Oh and I would never pull an engine off my plane for misbehaving until I knew the reason, most problems can be solved on the airframe.
 
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