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  #1  
Old 09-18-2012, 10:09 PM
BobAir BobAir is offline
 
Join Date: Mar 2011
Location: Georgetown, TX
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Default Need Engine Troubleshooting Help: Mags or Carb? (Long Post)

Folks,

I have an old Lyc O-360-A1A of uncertain provenance that I've been flying for about 500 hours in my E-AB GlaStar with great performance and reliability until now. I hope you don't mine me posting here to get the benefit of this huge community -- I am helping a friend build his RV-8A! :-)

Recently on engine start, Cyl #4 wouldn't fire -- very rough engine, no CHT or EGT rise on Cyl #4. The first couple of flights, it cleared up after a minute or two and run-up was normal. The situation progressed so now neither Cyl #3 or #4 will fire -- engine very rough & shaking like a dog. CHT's & EGT's rise on Cyl #1 & #2 but not on #3 & #4. I have to shut it down after a couple of minutes.

Here is what I have done to isolate the problem and the results:

1. Compression check: All Cyls at 75 PSI or higher.
2. Timing: Correct on both mags at 25 degrees BTDC.
3. Spark Plugs: Cyl #1 & #2 look good -- nice tan color. Cyl #3 & #4 have carbon build-up, but not too bad. (Swabbed exhaust stack with finger -- some carbon soot but nothing I think is abnormal. No noticeable black smoke from exhaust.) I cleaned/blasted and checked the gaps on the plugs and tried another ground run. Same results. I don't have a spark plug tester so I wasn't able to test the plugs themselves. Next ground run, I'll switch plugs from rear cylinders to front cylinders to see if problem follows them.
4. Ignition leads: Will test those tomorrow with borrowed lead tester. They are about 4 1/2 years old and 500 hours since new (Champion).
5. Mags: Older Bendix, again of unknown provenance. Impulse coupled mag on left and shower-of-sparks on right (shower-of-sparks not used).
6. Carb: Marvel Shebler (sp?): New/overhauled 500 hours (and about ten years) ago. Mixture and throttle linkages and throws are fine. On one ground run, I got a slight backfire. On another, as I pulled the mixture to idle cut-off, as Cyls #1 & #2 started to die, Cyls #3 & #4 seemed to fire up for a few engine revs before the whole engine died.
7. Intake manifold tubes. Inspected rubber hose connectors between intake tubes and oil sump. They look OK. Inspected attach points of intake tubes to cylinders for any evidence of air leaks (which would lean mixture). No evidence of air leaks found.

I lean aggressively while on the ground but don't lean in flight until at 5,000' MSL or above.

My first hypothesis (which was wrong): I have a Van's primer system with the solenoid valve on Cyls #3 & #4 only. Since it seemed like the engine (at least Cyls #3 & #4) was running rich, I thought the solenoid valve had failed and additional fuel was being sucked into Cyls #3 & #4. (I also confirmed that the solenoid was installed correctly re "in" and "out" ports.) I completely disconnected the primer system and blocked the primer inlets in the intake manifold. No change in behavior during subsequent ground runs.

So, other than spark plugs and ignition leads (which I'll test tomorrow), it looks like I'm running out of "cheap parts" as the root cause of this behavior. It seems to me that it is coming down to either the mags or the carb.

My question to you experienced hands out there: Are there any diagnostic procedures that I can run that will definitively point me to whether it is the mags or the carb? I don't want to just start replacing or overhauling them willy nilly to see if that fixes the problem ($$$). A neighbor A&P/IA suggested pulling all plugs and looking for sparks by grounding each plug in turn to the cylinder while motoring the engine with the starter. Could this be a definitive test?

Or it is something else entirely that might be causing this? The consistent, but mystifying, behavior I see is that neither Cyl #3 nor Cyl #4 appears to be firing at all based on lack of CHT and EGT rise. But that implies the failure of two mags, two sets of ignition harnesses and four spark plugs (or some combination of them), or a carb/intake manifold that is feeding a too-rich mixture to two cylinders and the correct mixture to the other two.

Thanks for any help or suggestions you have for me.

Best regards,

Bob Falstad
GlaStar N248BF
~500 Hours
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  #2  
Old 09-18-2012, 10:22 PM
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rocketbob rocketbob is offline
 
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Simple things first to eliminate. Change the plugs. Then run it with the p-leads disconnected. I've seen shower of sparks boxes miswired to the keyswitch that caused misfiring.
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  #3  
Old 09-18-2012, 10:39 PM
aerhed aerhed is offline
 
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Do what Bob says, even if you just borrow a set of plugs. Then if your mags are bad, yank 'em. They're overdue for service anyway.
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  #4  
Old 09-18-2012, 10:56 PM
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RV6_flyer RV6_flyer is offline
 
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A friend had a similar problem in a 200 HP RV-7. Ended up being a sticking valve.

Hard to believe a stuck valve could be the problem with that many hours.

Once the plugs are changed and it still has the problem, do the valve wobble test.
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Last edited by RV6_flyer : 09-19-2012 at 04:19 AM. Reason: spelling
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  #5  
Old 09-19-2012, 06:12 AM
jabarr jabarr is offline
 
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Location: Fayetteville, Georgia
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I agree with the above about plugs and mag servicing- the mags are due for inspection anyway. I would also consider a valve issue-- ref Lycoming SB 388b.
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  #6  
Old 09-19-2012, 06:46 AM
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Doug Doug is offline
 
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When you pull the plugs measure their resistance, greater than 5 k ohms is the limit (I'm told).

My own experience saw a cylinder not firing properly at startup, missing at mag check plus some random rough running - major improvement after replacing a single plug that measured 6k ohms. This was at 90 hours only.

Good luck.
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  #7  
Old 09-19-2012, 07:00 AM
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Walt Walt is offline
 
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Quote:
Originally Posted by RV6_flyer View Post
A friend had a similar problem in a 200 HP RV-7. Ended up being a sticking valve.

Hard to believe a stuck valve could be the problem with that many hours.

Once the plugs are changed and it still has the problem, do the valve wobble test.
I agree with Gary, the chances of both plugs and mags being bad are highly unlikely. Sounds like typical Lycoming "morning sickness".
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  #8  
Old 09-19-2012, 08:51 AM
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rocketbob rocketbob is offline
 
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I tend to think stuck valve but due diligence has to be done first.
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N9187P PA-24-260B Comanche, flying
N678X F1 Rocket, under const.
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  #9  
Old 09-19-2012, 09:02 AM
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G3i Ignition G3i Ignition is offline
 
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A common element that is share between cylinder #4 & #3 are cam lobes. Doing an old school rotating compression test might show common results if a lobe is bad. Rocker arm lift measurement would be next in order. A low valve lift would still have a good leak percentage. Hope that this is not the problem.

Thomas S.
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  #10  
Old 09-19-2012, 09:04 AM
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DanBaier DanBaier is offline
 
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Default

What the others have said.

It's just time for:

plugs need at least clean and rotate if not replacement.
mags need some TLC.
Lyc wobble test is due.

You should consider doing all of these regardless of which item solves the rough running.

$.02 (probably overpriced).

Dan
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