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Need Engine Troubleshooting Help: Mags or Carb? (Long Post)

BobAir

I'm New Here
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
 
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.
 
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.
 
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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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.
 
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.
 
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".
 
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.
 
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
 
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.

When I read your post this was the first thing that came to my mind as well. The #3 & #4 intake valves share a common lobe, and the intake lobes and lifters are a common issue in Lycomings, especially if they have seen periods of inactivity. As others have said, check all the easy stuff first, but you may have to pull a cylinder and have a look at the cam. Like Thomas, I hope this isn't the issue. Please let us know what you find.
 
Cam and valves...??

Would a common intake lobe failure affect just #4, and then some time later affect #3? Would a cam lobe failure proceed this rapidly? ( I think no and no.)

Carbon on plugs points to a rich mixture. Anything blocking airflow either upstream or downstream of the 3 & 4 cylinders? What kind of exhaust system does it have? Such a condition like wouldn't clear itself as the engine warms up though.

Pulling the prop through blade by blade gives a quick check for sticking valve.

Spark plugs on head is simple test, provides valuable insight.

Looking forward to your discovery of the true root cause. Good luck!

- Roger
 
Need Engine Troubleshooting Help... Root Cause Report

Folks,

With help from Seth Hancock, an experienced engine builder and multiple RV builder, and Jerry Stofer, currently building an RV-8A, (EAA 187, Austin, TX), we found the root cause why cylinders #3 & #4 wouldn't fire.

First, we checked the cheap(er) stuff. Ignition harness wires checked OK with test box. With new plugs, we saw sparks from both mags/all 8 leads. So we pulled the rocker arm covers...

The exhaust valves in both Cyl #3 & Cyl #4 were stuck. In fact, in Cyl #4 the rotator that (I understand) rides between the valve stem and the rocker arm had dropped out of position -- we found it lying in the bottom of the head cavity below the exhaust valve (see photo). All the parts were in their proper position on Cyl #3 but that exhaust valve was stuck, as well.

To see how badly stuck they were, Seth pried on them with a flat-blade screwdriver. They are stuck pretty badly.

Now I've got to decide what to do. My options seem to be to pull both #3 & #4 jugs and have them overhauled ($$), replaced ($$$) or, in light of the unknown history of the engine (incomplete logs -- serial number says engine was built in 1966), overhaul the whole thing ($$$$).

I was pleasantly surprised to find my post on the Van's Air Force home page this morning! That's quite an honor for a GlaStar driver. And thanks to everyone for your suggestions.

Best regards,

Bob Falstad
GlaStar N248BF
~500 Hours (And holding for now)

(After writing this reply, I find I'm having trouble attaching the photo. If I can figure it out, I'll attach it to another post.)
 
If the engine was running well and had good compression and reasonable oil consumption then I would suggest a third option, Comply with the Lycoming SB for the valve wobble inspection, during said inspection run the appropriate ream thru the guide to remove the carbon build up, this is all done with the cylinders on the engine.

Whatever you decide to do, you need to start leaning your mixture in flight, your lack of leaning is probably your root cause for the stuck valves. Regardless of your altitude you can and should lean at or below 75% power.
 
Diagnostic testing...??

Bob, Thanks for revealing the answer. Just thinking about diagnosis for future. Pulling the prop through slowly, feeling compression resistance, listening as it bleeds off likely would have indicated the stuck exhaust valves with a 'whoosh' through the exhaust pipe. A great plane (GlasStar) deserves a great engine, I vote for overhaul. - Roger
 
Bob, Thanks for revealing the answer. Just thinking about diagnosis for future. Pulling the prop through slowly, feeling compression resistance, listening as it bleeds off likely would have indicated the stuck exhaust valves with a 'whoosh' through the exhaust pipe. A great plane (GlasStar) deserves a great engine, I vote for overhaul. - Roger

Pulling the prop through or a compression check will not detect the stuck valve. Typically the valve is moving open and close but has enough resistance that when the engine is running, it does not close all the way till it just sticks open and bends a push rod. That is the case with the 200 HP engine that I helped with back in April.

Running a rich mixture near sea level was IMHO the cause of the stuck valve that my friend had.
 
Glad it's not the cam!

Would a common intake lobe failure affect just #4, and then some time later affect #3? Would a cam lobe failure proceed this rapidly? ( I think no and no.)

Carbon on plugs points to a rich mixture. Anything blocking airflow either upstream or downstream of the 3 & 4 cylinders? What kind of exhaust system does it have? Such a condition like wouldn't clear itself as the engine warms up though.

Pulling the prop through blade by blade gives a quick check for sticking valve.

Spark plugs on head is simple test, provides valuable insight.

Looking forward to your discovery of the true root cause. Good luck!

- Roger

Roger- Thanks for pointing out that the symptoms didn't occur simultaneously in the two cylinders. That would indeed rule out the cam lobe. That's the beauty of this forum. We each see the thing from a different angle, which greatly increases the collective intelligence.:)

Bob- Glad the cam seems to be eliminated from suspicion. Pleas let us know what you end up doing. Best of luck!
 
Pulling the prop through or a compression check will not detect the stuck valve. Typically the valve is moving open and close but has enough resistance that when the engine is running, it does not close all the way till it just sticks open and bends a push rod.

Yeah, how did we get from '75/80 in all cyls' to 'couldnt move the valve with a screwdriver'? Sticky valves tend to present "morning sickness" symptoms during early development. Thanks for the reminder to lean. - Roger
 
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