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Old 06-21-2018, 01:10 AM
Yen Yen is offline
Join Date: May 2010
Location: Benaraby Queensland. Australia
Posts: 184

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.
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Old 06-21-2018, 01:35 AM
M1chaelw M1chaelw is offline
Join Date: Mar 2014
Location: Buford, Ga
Posts: 34
Default Do some more troubleshooting

Iím no engine expert, but Iíve 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 itís 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.
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Old 06-21-2018, 03:38 AM
GStrassburg GStrassburg is offline
Join Date: Oct 2015
Location: Germany
Posts: 2

If installed, it might be a leaking engine primer.
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Old 06-21-2018, 07:09 AM
mahlon_r mahlon_r is offline
Join Date: Jan 2005
Posts: 978

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?
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."
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Old 06-21-2018, 09:57 AM
Bill Boyd's Avatar
Bill Boyd Bill Boyd is offline
Join Date: Sep 2006
Location: Landing field "12VA"
Posts: 1,300

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).
Bill Boyd

Hop-Along Aerodrome (12VA)
RV-6A - N30YD - flying since '98
RV-10 - N130YD reserved - under construction

donating monthly to the VAF - thanks, Doug
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Old 06-21-2018, 12:59 PM
Allison Allison is offline
Join Date: Nov 2017
Location: Johannesburg
Posts: 8


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 thatís 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, Iíll post the finding.

Also Iím 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 canít 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. Itís so darn frustrating!
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Old 06-21-2018, 01:31 PM
mahlon_r mahlon_r is offline
Join Date: Jan 2005
Posts: 978

"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,
"The opinions and information provided in this and all of my posts are hopefully helpful to you. Please use the information provided responsibly and at your own risk."

Last edited by mahlon_r : 06-21-2018 at 01:36 PM.
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Old 06-21-2018, 01:41 PM
snopercod's Avatar
snopercod snopercod is offline
Join Date: Aug 2016
Location: Asheville, NC
Posts: 1,693

Marvel Schebler carburetors are available in a variety of part numbers, each with different jets. If you have the manual for your engine, look up the P/N of the carb and make sure you have the right one. Maybe this chart will help:
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Old 06-21-2018, 02:09 PM
Canadian_JOY Canadian_JOY is offline
Join Date: Aug 2007
Location: Ontario, Canada
Posts: 1,961

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.
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Old 06-21-2018, 04:09 PM
spitedge spitedge is offline
Join Date: Dec 2010
Location: Sydney Australia
Posts: 7
Default 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!
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