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  #1  
Old 09-06-2017, 01:33 PM
Jesse's Avatar
Jesse Jesse is offline
 
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Default Lose 2 Cylinders at Idle

I have a plane here with a Lycoming O-360 that has been down for a panel upgrade. When we start the engine, Cylinders 1 & 4 drop out at Idle. At power (pretty much anything over 1,000RPM) the EGT's and CHT's come up, but at idle, they drop back out. It flew fine and made good power. The engine has 2-300 hours.

We checked for stuck valves and they are fine, not stuck or even a little sticky.

We can test new plugs, but the chance of having both plugs on two different cylinders dropping out the same is very low.

We have looked for intake leaks, and haven't completely ruled that out, but everything looks great on those cylinders.

Leaning out at idle doesn't change anything, but the RPM does increase significantly.

Any other thoughts, things to try?
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Last edited by Jesse : 09-06-2017 at 01:37 PM.
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  #2  
Old 09-06-2017, 01:37 PM
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Raymo Raymo is online now
 
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My engine did the same on one cylinder. The flow divider had piece of junk in it, causing the idle fuel flow to be restricted. At higher fuel flow/RPM, the plunger inside raises the internal valve, allowing normal fuel flow.
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  #3  
Old 09-06-2017, 01:46 PM
mahlon_r mahlon_r is offline
 
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Quote:
Originally Posted by Jesse View Post
I have a plane here with a Lycoming O-360 that has been down for a panel upgrade. When we start the engine, Cylinders 1 & 4 drop out at Idle. At power (pretty much anything over 1,000RPM) the EGT's and CHT's come up, but at idle, they drop back out. It flew fine and made good power. The engine has 2-300 hours.

We checked for stuck valves and they are fine, not stuck or even a little sticky.

We can test new plugs, but the chance of having both plugs on two different cylinders dropping out the same is very low.

We have looked for intake leaks, and haven't completely ruled that out, but everything looks great on those cylinders.

Leaning out at idle doesn't change anything, but the RPM does increase significantly.

Any other thoughts, things to try?
Missing intake port pipe plugs, leaking primer, leaking stand by vac system, really has intake leak written all over it :-)
Good Luck,
Mahlon
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  #4  
Old 09-06-2017, 01:50 PM
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Jesse Jesse is offline
 
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Quote:
Originally Posted by Raymo View Post
My engine did the same on one cylinder. The flow divider had piece of junk in it, causing the idle fuel flow to be restricted. At higher fuel flow/RPM, the plunger inside raises the internal valve, allowing normal fuel flow.
Not fuel injected, so I think we can rule that out.
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RV Hotel (hangar, room, car) at X35 in north FL

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Dynamic Prop Balancing, Prebuy Inspections, Condition Inspections, Repairs and Mods, Injector Tuning, Airframe, Engine, Panel Upgrades, Ferrying, etc.
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  #5  
Old 09-06-2017, 01:51 PM
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Jesse Jesse is offline
 
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Quote:
Originally Posted by mahlon_r View Post
Missing intake port pipe plugs, leaking primer, leaking stand by vac system, really has intake leak written all over it :-)
Good Luck,
Mahlon
That's exactly what I have been thinking. We'll check all of those things. Thanks!
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Jesse Saint (Saint Aviation, Inc. - VAF Advertiser)
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RV Hotel (hangar, room, car) at X35 in north FL

Commercial ASEL/S A&P/IA EAA Technical Counselor

Dynamic Prop Balancing, Prebuy Inspections, Condition Inspections, Repairs and Mods, Injector Tuning, Airframe, Engine, Panel Upgrades, Ferrying, etc.
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  #6  
Old 09-06-2017, 02:07 PM
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Bob Martin Bob Martin is offline
 
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Default Basics

Jesse,
I am by no means a engine guy....
But basics
Air fuel spark.
Air would just be intake spider....and gaskets. Leaks cause poor running engine, especially when idling because the ratio of leak to combustion air is high. if all looks good, most likely not your smoking gun
Fuel...carb is carb...and if spider is ok, then fuel should be ok as well.
Ignition: assuming mags and aviation plugs, you did not say age/condition etc. Two mags wired normally would have a mix of wires from both mags so both mags failing is improbable. Plugs could be bad on those two cylinders and an easy thing to check/clean/test/ohm.
Primer---is the primer only on 1 and 4?
Good Luck
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  #7  
Old 09-06-2017, 02:22 PM
ty1295 ty1295 is offline
 
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Leak Down test?

I am a firm believer in them, much better than compression. I spent a couple years on a top fuel motorcycle race team, and one of the first things we did after every run was a leak down test. We would know instantly how much work we would be needing to do to be ready for the next round and where the issue was if it was mechanical.
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  #8  
Old 09-06-2017, 02:31 PM
rv7charlie rv7charlie is offline
 
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I had an O-360 lose one of the pipe plugs in a primer port on the way to OSH about 15 years ago. Was running fine until I pulled power to descend for a fuel stop, then ran horribly. Bringing power back up, ran fine.

Now, losing two, at the same time.....
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  #9  
Old 09-06-2017, 03:39 PM
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Jesse Jesse is offline
 
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We found the problem we are pretty sure. Just about to run a test.

This is what happened. The primer (electric) was sticking and staying on. The primer goes to cylinders 1, 2 and 4. Cylinder 3 has the manifold pressure pickup in the primer port. It turns out that cylinder 2 had a leaky primer fitting, so just cylinders 1 and 4 were getting full time primer, which must have just made them run rich at high power, but so rich at idle that they stopped firing at all (or were firing but the EGT's were just hugely low because they were hugely rich. Kind of the opposite problem as we were thinking, but similar symptoms. We fixed the sticky primer and fixed the leaky primer line into cylinder 2 and are about to test.

Thanks for the ideas, guys.
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Jesse Saint (Saint Aviation, Inc. - VAF Advertiser)
352-427-0285
jesse@saintaviation.com
RV Hotel (hangar, room, car) at X35 in north FL

Commercial ASEL/S A&P/IA EAA Technical Counselor

Dynamic Prop Balancing, Prebuy Inspections, Condition Inspections, Repairs and Mods, Injector Tuning, Airframe, Engine, Panel Upgrades, Ferrying, etc.
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  #10  
Old 09-06-2017, 06:10 PM
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JonJay JonJay is offline
 
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Jesse - you have a lot of experience, yet you still put things out there when you find issues. That really helps all of us learn. Thanks for posting.
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