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Old 10-24-2014, 02:40 AM
abnranger69 abnranger69 is offline
Join Date: Oct 2010
Location: Sacramento, ca
Posts: 11
Angry Engine riddle for you..... Two dead cyl on left Side?

I have a Lycoming O-320 on my RV 4 that has decided to run on only two cylinders. I would have thought that virtually impossible to happen. Especially both being cold/dead on the (same) left side. It?s a carbureted/aspirated, mid-time engine. Shut off with mags a couple weeks ago to check ground. Exhaust is two pipes w/crossover. No recent maint done/changes before last flight. Fuel sumped.

Flew it in just fine, never skipped a beat, left it outside in average weather for a few days, went to start it up one morning and shook like after normal easy start. Knew I was in uncharted territory by the sound it was making. Tried running it up to 1000-1200 RPM thinking a fowled plug, leaned it, cycled mags with 30-40 drop each side. Shut it down after 1-2 min thinking She never gives me any trouble.

Took it over to maint shop, removed the cowl, started up again with same results and tests for the A&P. No backfiring.
Both left cylinders dead cold, hot on R side. Trouble shooting included the obvious:

Good visual insp, nothing noted

Removed, cleaned, gapped, tested plugs. One failed at 120 PSI. All were normal color, no excessive deposits or wear. Replaced. Nothing out of ordinary noted looking into the cylinders.

Again, started up, tried various mixture settings, etc? Same results

Compressions checked, mid 70s all around.

Valve covers removed, pulled prop through all cyl, valves moved appropriately. So nothing sticking open/closed.

Tested each plug wire, visible spark noted from each one while grounding it out.

Mags checked/opened up. One shaft/bushing with some sloppiness. Removed and sent out for a 500hr insp/check. No visible arc/carbon pattern noted that would maybe indicate a cross firing.

Didn?t pull the air filter (round K&M with wire mesh inside/outside element) So no rats could get in and plug two left intakes (reaching here I know}

Sooooo, I?m open to any suggestions from all the experts out there. I?m stumped.
Feel free to throw out some less obvious trouble shooting suggestions. Just not making any sense to me
Will compile and start over when mags get back in a couple weeks. Personally, I don?t think it?s the mags??.

Thanx all.


Last edited by abnranger69 : 11-05-2014 at 02:39 AM. Reason: Removed all of the expletives. Not appreciated.
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Old 10-24-2014, 05:06 AM
PaigeHoffart PaigeHoffart is offline
Join Date: Jun 2005
Posts: 375

This is a bit of a reach, but do you have primer lines to the left cylinders only? A stuck primer solenoid causing them to run way over rich at idle?

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Old 10-24-2014, 07:05 AM
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Aiki_Aviator Aiki_Aviator is offline
Join Date: Jan 2010
Location: Melbourne, Australia
Posts: 250
Default Fuel?

Are they getting fuel at all? A blockage of some sort on the fuel flow?
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Old 10-24-2014, 07:32 AM
flynwest flynwest is offline
Join Date: May 2008
Location: Sacramento
Posts: 398
Default Carb or injection

Does it have injection?
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Old 10-24-2014, 07:55 AM
Speedpants Speedpants is offline
Join Date: Aug 2012
Location: Plymouth ma
Posts: 7

You might want to start looking at intake pipes, there might be cracks a long the flange where they meet the cylinder, also check for blockage in the intake system or in the exhaust, the primer lines being broken at the cylinder would cause a rough running engine but not that excessive, but you never know.
Also as a last 2 checks before cylinder removal inspect the cylinder barrel to cylinder head joint I have seen that develop cracks in the past, while it is rare it does happen, and since the piston is usually past the crack during compression check you would not pick it up during one. If all is good from there I would boroscope the cylinder and see what's going on if you have a good one, but personally I would just remove the jugs at that point to see what's going on with my own eyeball.

Hope this helps

Mitch, A+P IA

If you have any questions feel free to email me directly,
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Old 10-24-2014, 07:56 AM
RKellogg RKellogg is offline
Join Date: Oct 2009
Location: Newark, IL
Posts: 287
Arrow good puzzle...

Dave, a very professional problem description. I love a good puzzle. But at this point I have no clue. Can you swap plug leads right to left?
When you pull the prop through slowly by hand, are the appropriate hissing noises where they should be? Do all four cyls feel the same.
Will ponder as I wander...

Last edited by RKellogg : 10-24-2014 at 08:00 AM.
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Old 10-24-2014, 08:10 AM
BillL BillL is offline
Join Date: Sep 2007
Location: Central IL
Posts: 6,313

After running and shut down, did you check and see if the lower plugs were wet? With compression, and valve movement, then it is either spark, or spark at the wrong time, and fuel.

If the plugs are wet, then narrow to spark. Since you checked for spark, then I would say spark at the right time and place. This would say check the mag caps to see if it is carbon tracked and you just happened to catch it with some absorbed moisture in the insulator that caused dissipation of the ignition energy.

When I have checked everything and there is nothing wrong, but it still does not work, then it is time to look closer.

If the plugs are not wet, then suspect some plugged orifice or tube in the carb that might send fuel to only one side. I don't know this carb that well, maybe RocketBob can help there.

Good puzzle there - actually sounds like a good reward and interesting find when you get it solved. Let us know what you find!

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Old 10-24-2014, 09:17 AM
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rv7boy rv7boy is offline
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Originally Posted by flynwest View Post
Does it have injection?
Fourth sentence: "carbureted".
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Old 10-24-2014, 09:43 AM
RV7ator RV7ator is offline
Join Date: Jan 2005
Location: Boise, ID
Posts: 1,010

Since nothing's mechanically broken it will run given fuel, air, and spark.

"Left outside a few days..."? I'd be looking for air blockage due to a critter invasion.

John Siebold
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Old 10-24-2014, 09:43 AM
tjo tjo is offline
Join Date: Dec 2009
Location: La Center,wa
Posts: 246

Interesting problem. You know that what you need is fuel, air, and spark, at the right place and right time. You have verified that there is spark and can assume that the timing is OK because two of your cylinders work. You know the fuel is flowing because it is carbed and two cylinders work. So I would start with the assumption at this point that it is either an air problem, or a primer overfueling problem. For the air problem, I would start with looking at the exhaust side then move to the intake. Also, if you start looking at the exhaust and it is dry (no raw fuel in it) then it is not an overfueling problem. If it's not that, well, it wouldn't be the first time I was wrong...

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