brayski98

brayski98

Well Known Member
I flew this past weekend and had an odd issue with a misfire on an O-320/carb in the -6A. Not sure if it was a backfire or afterfire.
Normal startup. Everything looked as expected during run-up prior to taking off. Normal takeoff and climbed out starting at around
1200 fpm with full fuel. 10 minutes later at 7500 feet, it sounded like a small bang, like the engine had misfired. It happened twice more and
I thought about landing and checking things. But right after that it smoothed back out. I kept a close eye on it and stayed close to
airports and had no more problems. As I was leaning the engine, I noticed #2 being about 50-100 cooler than the others. Continued the flight
for another hour and no problems. On the return flight at 8500, no issues either. The engine has 2 slick mags and I replaced the spark plugs
about 5 hours ago after seeing a problem on run-up with running rough on the right mag check. I did gap the plugs before I put them in.
I?m thinking it still sounds like an ignition problem, but wasn?t sure if it could be a fuel contaminate, or ???

I am tempted to at least replace the plug wires, but thought I?d gather ideas from the group before I just start replacing items.
 
Look at the mags

First thing I would do is look inside the mags for carbon tracking or just plain worn out parts. It is surprising how ratty the insides of mags can get before problems present themselves. Steve
 
harness routing

I read this yesterday in the current General Aviation Newspaper, the one with the pink pages. The reader told of a slight rough running engine on decel and at around 1500 rpm. The columinst (a longtime Lyc engineer) told him to connect his mags to the plugs using the optional method (ref Lyc overhaul manual and Lyc Service Instruction 1942). In a nutshell, this option basically swaps the top and bottom plug wires. The Lyc guy stated the miss was due to it being a carbureted engine with varying air/fuel ratios and the combustion flame propagation front being different in every cylinder. Sounded good to me.
I have the Lyc OH manual and the 2 wiring diagrams are shown but I could not find anything on the Lyc site for that Service Instruction.

Steve
 
Lyc Manual

I was looking through the operator's manual on the engine last night. When mine misfired, I was almost full throttle (2300 squared) at 7500. I do notice that it runs a little rough right after I start it, but smooths out as it warms up. I just attributed it to the engine not being warmed up yet.

I think what I will do is pull the mags and have them inspected and overhauled if needed. They do have about 600 hours on them and I don't see anything in the log books about being overhauled. And if they check out okay, I'll figure out plan B. Is there any tricks I should know about when pulling the mags off?
 
Mag Rebuild

I have two Bendix mags on my IO360 and everything seemed normal. They had slightly over 600 hours at my last condition inspection so I took them off and had them both rebuilt at a mag shop.
The difference is quite noticeable. Starts easier, idled faster, ran smoother. Quiet dramatic.
The point is, they were working OK before the rebuild, and noticeably better after. It's cheap preventative mtc that I think is required every 500 hours on standard mags.
 
Spark plugs

I just changed the spark plugs 5 hours ago. It was running rough during the preflight runup on the right side. I pulled the plugs and had the mechanic in the next hangar measure the gap and overall condition. He said they were within spec, but barely. So I bought new massive plugs and gapped them prior to installation.

Typically what does an overhaul of a mag cost? Not that it is an issue for decision whether to do it or not, but curious as opposed to buying new.
 
Another easy/cheap thing to check is the insulators in the plug connector.

If you have the old brown bakelite ones, throw them away, and get the white ceramic units.

Also, check for carbon tracking or physical damage---cracking/chipping.

Good Luck.
 
You may like to try an in-flight mag check, look at the EGTs to diagnose the mis-firing cylinder. I had a similar problem to yours (although it doesn't sound exactly the same) which was caused by a bad ignition harness. I couldn't see anything looking at it, but changing it out got rid of the mis-fire. The guy parked next to me at Osh suggested it - took a long flight at reasonable altitude on my own to recognize there was a misfire. In flight mag check quickly showed which mag and which cylinder. If you haven't had your mags overhauled in the last 500 hours now would be a very good time - it is really worthwhile to have them looked at at that interval.

Good Luck, Pete

PS get ready to switch back to both when you check the mags airborne - the engine may get real rough - don't get caught by surprise and switch both off.
 
Mags

I think it is in cylinder number 2 as it is running colder than the rest by 50+ degrees. I checked with a local place in Van Nuys who wants $295/each to overhaul. Does that seem resonable?

I do have the brown tips and the harness is the original. I thought about replacing that at the same time.
 
brayski98 said:
I do have the brown tips and the harness is the original.
Click to expand...

I think you have just identified your problem.

Also, as was stated prior, check for carbon tracks or other issues in the rotor cap.

I fought a similar problem caused by those stupid bakelite insulators for a couple of years before a buddy told me about them.
 
brayski98 said:
I do have the brown tips and the harness is the original. I thought about replacing that at the same time.
Click to expand...

Like Mike said, replace those first, then rebuild the mags if the problem persists. If you do both you won't know what the problem really was / is.
 
Mag costs

brayski98 said:
I just changed the spark plugs 5 hours ago. It was running rough during the preflight runup on the right side. I pulled the plugs and had the mechanic in the next hangar measure the gap and overall condition. He said they were within spec, but barely. So I bought new massive plugs and gapped them prior to installation.

Typically what does an overhaul of a mag cost? Not that it is an issue for decision whether to do it or not, but curious as opposed to buying new.
Click to expand...

I had mine rebuilt at Maxwell Magnetos in Florida. He did have to replace a couple parts and including the kits it was a tad over $550.00 for both mags. Turnaround was 1 day.
Great folks to work with.
 
Mags

Well I went to the hangar and pulled the mags and looked through everything closely and I think I found the culprit. I notice that there was some discoloration of the harness and marks on the oil filler. It looks like it has been arcing there.

I will buy and put in new harnesses, but I will take the magnetos to the shop and have them inspected at least. Based on what they find, I will decide on whether to have them overhauled.

thanks for everyone's input. When I get it back together and fly it, I post how it went.
 
Follow up

The problem was two fold or one may have caused the other. I sent the mags out for a 500 hour inspection and overhaul and they found a bad coil in one mag. When inspecting the wires in the harness, one appeared discolored and looks like it may have been arcing against the oil filler tube. So new harnesses as well. Starts like a champ now. Haven't flown yet as weather turned bad here. Thanks for all the suggestions.
 
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