Rough Right Mag, any thought?

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Ivan H.

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First off, little background here since I'm not sure if thats related to the rough mag I am experiencing. My airplane is a 71' Piper Cherokee 140 purchased 4 months ago and it had an engine overhaul in 2012 and its now 800hrs SMOH, and starting about a month ago, I noticed the oil quickly turns deep black only an hour of flight after oil change along with increased oil consumption(qt/2.5~3hr tach) so I asked my mechanic to do a compression check and result is #1 70psi #2 62psi #3 68psi #4 62psi, a compression test done 4 months ago during prebuy indicates #4 65psi while 70s psi on all other cylinders. While I was still debating should I just go ahead do a top overhaul or just find a shop to pull #4 and #2 out and overhaul, right mag starts being rough during run-up.

So whats happening is when I turn my key to right position at 2000rpm, rpm would start jumping around like 1950-1900-1980-1800-1850-2000, etc. I tried trouble shooting by leaning out the mixture a lot as if the engine is almost shutting off for about 3-5mins, still doesn't work. I also tried runnning on right mag in flight to check it out, roughness still there and I also noticed some mild banging sound in the engine compartment during climb and I grounded the airplane since then. So I went ahead and replaced all the spark plugs and when my mechanic pulls the old ones out, they all appear very clean and healthy in colors, except gap is widening in some of them, however roughness still exists after the replacement! Next, I went ahead to buy a NEW right mag and have my mechanic to replace and timed it to 25deg btdc, but still, the roughness didn't go away!!! I have no clue now of what item should I continue hunt for. Anyone has similar experience and could provide some opinions? Thank you all and really really appreciate for any idea possible.

Btw, anyone knows any shop/mechanic who is reliable on cylinder overhauling and fixing oil leaks around inland empire area in Socal(Chino, Corona, Riverside, etc) or anywhere close? My mechanic saids he is not too good on engine works, so any recommendation would be greatly appreciated.
 
Looks like you are into it pretty well. Years ago I had a rough engine, Cherokee 140. After checking the plugs & such ended up being a bad ignition lead. Not saying that’s contributing to your problem, but ensure the leads look ok.

I think mine had a slight ‘arc’ to it.
 
If you replaced the mag and the problem didn't go away it sounds like either a wiring issue or the P-leads are swapped so it's really the left mag that's bad.

Before pulling a cylinder you should determine where the leakage is coming from (exhause, intake, crankcase). If it's an intake or exhaust valve they can often be lapped in place. But 60's, while not great, aren't terrible, I would borescope the cylinders and if nothing looks bad, fly it some more (after the mag issue is fixed) and see how it trends.
 
Could be a plug wire, problem with p-lead, noise suppressor cap, mag switch...etc.

Ditto Dana.

Remove plug wires at plugs..check each plug wire for a high resistance reading to ground...set ohm meter to megohms scale...you should not see any reading other than infinity on a good wire. If you get a reading, you found your problem.

You can test for a fouled spark plug using this same method without removing the plugs.
 
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Did you or your mechanic look at the plugs as part of the diagnosis process? Hanging a new mag as a solution sounds a bit like throwing parts at an unknown problem. You have rough mag check and high oil consumption as symptoms. Those may or may not be related. As others have said, plug wires, p-leads, plug resistance and gaps should all be checked.
On the oil side, how "full" do you fill the engine when replacing oil? A lot of Lycomings puke out oil when filled completely full. My 180 is very happy at 6 quarts, capacity is 8. If I fill it to 8, the extra oil is on the belly in about an hour.
It could well need new cylinders, but there's a lot of pretty easy things to check before going that route and finding that wasn't the issue.
 
Ivan H. said:
should I just go ahead do a top overhaul or just find a shop to pull #4 and #2 out and overhaul, right mag starts being rough during run-up.
Click to expand...
What could be wrong with the cylinders that would result in a rough run up on one mag but not the other?
 
Hang 4 said:
On the oil side, how "full" do you fill the engine when replacing oil? A lot of Lycomings puke out oil when filled completely full. My 180 is very happy at 6 quarts, capacity is 8. If I fill it to 8, the extra oil is on the belly in about an hour.
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@Hang 4 - At what [cold] oil level on the dipstick do you add oil and when you add oil, do you always do a full qt or do you do partial qts?
 
I suggest you fix your ignition issue first, then fly a few hours at cruise rpm.
 
Mongoose Aviator said:
@Hang 4 - At what [cold] oil level on the dipstick do you add oil and when you add oil, do you always do a full qt or do you do partial qts?
Click to expand...
I fill to 6 and add at 5. I have a sheet in my hangar where I track adds and calculate consumption. I used to fill to seven, but consumption was up and belly was damp with oil. 6 seems to be the optimum for my engine. The other thing to check is the breather hose. It should have a peak in the engine compartment. Mine had a dip and oil that would have flowed back into the crankcase was pooling in that dip and then getting puked out.
 
I agree with spark plug leads to check.

For compressions, your mechanic SHOULD have determined where the leakage is. It is not hard to do, listen at the exhaust, if you hear hissing, it is an exhaust valve. Listen at the oil filler/dipstick, if you hear hissing, it is rings. The intake means intake valve.

With your oil consumption and oil turning black, I would suspect rings. Look of the Mike Busch Ring Flush. Worth a try. Worse case, it doesn't help and you are out some money. Best case, it works and save you LOT of money.

If you are talking about pulling cylinders, check on the availability of replacements or lead time for overhaul. You might be down many months.
 
Proper troubleshooting will save many dollars. Replacing the right mag without making sure it was the right mag, as per Dana's P-lead swap comment, is a good way to burn money. And to add to this: Have the mags been off for their 500-hour inspections since the engine was overhauled 800 hours ago? Or were the existing mags maybe even bolted back onto the overhauled engine "because they're still working ok"? Mags need maintenance. Or else.

P-leads get frayed and can short against stuff under vibration, causing rough operation. The banging in the engine compartment is afterfiring in the exhaust. It makes quite a bang and can crack the muffler or other exhaust stuff.

Ignition leads need more than a resistance check. They need an arc test to see if they're shorting though the insulation to the shielding. Mechanics should have that tool.

Spark plugs need more than a resistance check, too. They need the pressure bomb spark test to see if they continue sparking at 135 PSI. If they don't they can fail to spark under compression. Internal resistor failures or cracked ceramic insulators are to blame there. The older Champion plugs had terrible resistor failure rates. I found brand-new plugs that failed the bomb test. There are a lot of those out there, still causing grief.

The mag switch can cause trouble. It's sometimes related to the Bendix Mag switch AD of 1977, where so many mechanics just assume that the switch is a Bendix and do the required annual test and sign it off, but it so often turns out to be an ACS switch instead with an entirely different AD on it, from 1993 or so, and has never been addressed. The starter contactor doesn't have the suppression diode on it, the internal switch contacts burn every time the starter is released, and that damage causes trouble with the mag contacts in that same switch.

Ground-running the engine instead of flying it causes internal engine corrosion that pits cylinders and wears rings and drops the compressions. It can contribute to valve and seat corrosion.
 
sourdough44 said:
Looks like you are into it pretty well. Years ago I had a rough engine, Cherokee 140. After checking the plugs & such ended up being a bad ignition lead. Not saying that’s contributing to your problem, but ensure the leads look ok.

I think mine had a slight ‘arc’ to it.
Click to expand...
Sounds like a direction, really appreciate it!
 
Dana said:
If you replaced the mag and the problem didn't go away it sounds like either a wiring issue or the P-leads are swapped so it's really the left mag that's bad.

Before pulling a cylinder you should determine where the leakage is coming from (exhause, intake, crankcase). If it's an intake or exhaust valve they can often be lapped in place. But 60's, while not great, aren't terrible, I would borescope the cylinders and if nothing looks bad, fly it some more (after the mag issue is fixed) and see how it trends.
Click to expand...
Thanks for the reply, my ap actually borescoped the valves and found the seal gaps in good shape, although the top of the valves look pretty white and he said I probably flew the plane too lean on mixture.
 
Doug Reid said:
Could be a plug wire, problem with p-lead, noise suppressor cap, mag switch...etc.

Ditto Dana.

Remove plug wires at plugs..check each plug wire for a high resistance reading to ground...set ohm meter to megohms scale...you should not see any reading other than infinity on a good wire. If you get a reading, you found your problem.

You can test for a fouled spark plug using this same method without removing the plugs.
Click to expand...
Thanks! I would recommend this method to the mechanic/shop next time when troubleshooting the problem, appreciate for your opinions!
 
Hang 4 said:
Did you or your mechanic look at the plugs as part of the diagnosis process? Hanging a new mag as a solution sounds a bit like throwing parts at an unknown problem. You have rough mag check and high oil consumption as symptoms. Those may or may not be related. As others have said, plug wires, p-leads, plug resistance and gaps should all be checked.
On the oil side, how "full" do you fill the engine when replacing oil? A lot of Lycomings puke out oil when filled completely full. My 180 is very happy at 6 quarts, capacity is 8. If I fill it to 8, the extra oil is on the belly in about an hour.
It could well need new cylinders, but there's a lot of pretty easy things to check before going that route and finding that wasn't the issue.
Click to expand...
I basically fill the oil pretty much the same way you did, whenever its 5 I fill it to 6qt, I could see oils on the back side of the engine but not sure where its coming from. And sadly another repair shop I went to last time, said they couldn't find any significant leaks after cleaning with solvents and running up for 10 minutes.
 
Pinecone said:
I agree with spark plug leads to check.

For compressions, your mechanic SHOULD have determined where the leakage is. It is not hard to do, listen at the exhaust, if you hear hissing, it is an exhaust valve. Listen at the oil filler/dipstick, if you hear hissing, it is rings. The intake means intake valve.

With your oil consumption and oil turning black, I would suspect rings. Look of the Mike Busch Ring Flush. Worth a try. Worse case, it doesn't help and you are out some money. Best case, it works and save you LOT of money.

If you are talking about pulling cylinders, check on the availability of replacements or lead time for overhaul. You might be down many months.
Click to expand...
Thanks for the opionion, I would definitely try the ring flush method you mention earlier since my mechanic actually borescoped the valves a week or two ago and they look pretty good, and he also tighten up the oil filler too, so most likely should be the cylinder ring, only thing is we are not very sure which cylinder is it that having a bad ring.
 
Dan Thomas said:
Proper troubleshooting will save many dollars. Replacing the right mag without making sure it was the right mag, as per Dana's P-lead swap comment, is a good way to burn money. And to add to this: Have the mags been off for their 500-hour inspections since the engine was overhauled 800 hours ago? Or were the existing mags maybe even bolted back onto the overhauled engine "because they're still working ok"? Mags need maintenance. Or else.

P-leads get frayed and can short against stuff under vibration, causing rough operation. The banging in the engine compartment is afterfiring in the exhaust. It makes quite a bang and can crack the muffler or other exhaust stuff.

Ignition leads need more than a resistance check. They need an arc test to see if they're shorting though the insulation to the shielding. Mechanics should have that tool.

Spark plugs need more than a resistance check, too. They need the pressure bomb spark test to see if they continue sparking at 135 PSI. If they don't they can fail to spark under compression. Internal resistor failures or cracked ceramic insulators are to blame there. The older Champion plugs had terrible resistor failure rates. I found brand-new plugs that failed the bomb test. There are a lot of those out there, still causing grief.

The mag switch can cause trouble. It's sometimes related to the Bendix Mag switch AD of 1977, where so many mechanics just assume that the switch is a Bendix and do the required annual test and sign it off, but it so often turns out to be an ACS switch instead with an entirely different AD on it, from 1993 or so, and has never been addressed. The starter contactor doesn't have the suppression diode on it, the internal switch contacts burn every time the starter is released, and that damage causes trouble with the mag contacts in that same switch.

Ground-running the engine instead of flying it causes internal engine corrosion that pits cylinders and wears rings and drops the compressions. It can contribute to valve and seat corrosion.
Click to expand...
Thank you Dan! this clears up my direction even better, I would definitely tell the mechanic this and see if it helps when troubleshooting, really appreciate it.
 
IMHO The way to troubleshoot a mag drop is with a cold cylinder check.

Start cold engine and IMMEDIATELY go to bad mag. Run about 30 secs.

Check exhaust stacks temp. Cold is bad one.

A laser temp would be good here as it seems it is firing intermittently.

A really bad one may allow grabbing the pipe.

Your Tech may have a “ Harness Tester” that allows checking Leads with high

voltage. A Lead that is close to the exhaust may have the insulation destroyed

internally and allow the spark to run to the Shielding.

You can’t do this check with an Ohmmeter.
 
Ivan H. said:
Btw, anyone knows any shop/mechanic who is reliable on cylinder overhauling and fixing oil leaks around inland empire area in Socal(Chino, Corona, Riverside, etc) or anywhere close? My mechanic saids he is not too good on engine works, so any recommendation would be greatly appreciated.
Click to expand...

I recommend Ben's Motorworks at KFUL.

714-396-6090
 
Ivan H. said:
Thanks for the opionion, I would definitely try the ring flush method you mention earlier since my mechanic actually borescoped the valves a week or two ago and they look pretty good, and he also tighten up the oil filler too, so most likely should be the cylinder ring, only thing is we are not very sure which cylinder is it that having a bad ring.
Click to expand...
Replacing parts as a troubleshooting technique is some A&P's wet dream.

Given your compressions, I assume you're referring to an oil scraper ring. Pretty sure a coked up oil ring would not cause rough running. It might cause excessive oil consumption.

Personally, I would fully troubleshoot one system rather than change things in multiple systems. I'd start with the ignition system. Has the old magneto been IRAN'd? Follow Magman's suggestion. If you find a cool cylinder it may be that single plug lead.
 
Domenick said:
If you find a cool cylinder it may be that single plug lead.
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Or the plug.

Troubleshooting usually seems to be the last resort, when it should be the first. Troubleshooting takes time, which costs money, but replacing stuff until the problem goes away also takes a lot of time and adds the price of those parts on top of it.

When I was in the air/vacuum/hydraulic brake remanufacturing shop, I sometimes took the cores that were returned and tested them. A good two-thirds had nothing wrong with them at all. The mechanics simply did not know the systems nor how they worked, and didn't care to study and learn and get smart about it, and would just start replacing stuff. That gets expensive for the truck or heavy equipment owners. That phenomenon applies in aviation, too, unfortunately.

Imagine your doctor just doing one surgery after another, taking out various parts of your guts, until the symptoms go away.
 
I’m not a mechanic.

Piper Cherokee oil level I would keep it around 6-6.5. When you have more than 6.5/7 it starts to leak out anyway.

2.5/3hrs per qt is a lot of consumption. Oil getting dark quickly happens on older engines, in my experience too, doesn’t necessarily mean a problem. Just checked my records on oil consumption, I’m getting about 12-20 hours per qt (and that’s not science as sometimes I’ll fill it up extra aka 7/7.5 for longer trips whereas for local I’m totally fine with 6 so I’m not consistent at the fill level). After one oil change it was around 10 hours for 1.5 qt but I think this is because oil was filled to the top so it blew out, not sure as I didn’t write any notes down on this. Do a dye test to see if any cylinder leaks?

Wires come lose or get damaged quickly, check those.

Other than that I’m not sure, do share the solution with us once you figured it out.
 
Domenick said:
Replacing parts as a troubleshooting technique is some A&P's wet dream.

Given your compressions, I assume you're referring to an oil scraper ring. Pretty sure a coked up oil ring would not cause rough running. It might cause excessive oil consumption.

Personally, I would fully troubleshoot one system rather than change things in multiple systems. I'd start with the ignition system. Has the old magneto been IRAN'd? Follow Magman's suggestion. If you find a cool cylinder it may be that single plug lead.
Click to expand...
Got it and appreciate it, I would troubleshoot the ignition system first. And per the mag, right mag has actually been replaced with an overhauled one at august 2022 and left mag was inspected around 300hrs ago at 2021.
 
Dan Thomas said:
Or the plug.

Troubleshooting usually seems to be the last resort, when it should be the first. Troubleshooting takes time, which costs money, but replacing stuff until the problem goes away also takes a lot of time and adds the price of those parts on top of it.

When I was in the air/vacuum/hydraulic brake remanufacturing shop, I sometimes took the cores that were returned and tested them. A good two-thirds had nothing wrong with them at all. The mechanics simply did not know the systems nor how they worked, and didn't care to study and learn and get smart about it, and would just start replacing stuff. That gets expensive for the truck or heavy equipment owners. That phenomenon applies in aviation, too, unfortunately.

Imagine your doctor just doing one surgery after another, taking out various parts of your guts, until the symptoms go away.
Click to expand...
Very true, but finding a good experienced mechanic at reasonable price nowadays is very hard unfortunately.
 
I think there are folks on the forum that can assist with troubleshooting.

In many cases certification is not needed for these checks.

My take is if it is a smooth rpm drop it is usually timing that will affect all jugs.

A rough drop is generally you are partially losing one or more cylinders.

In this case it could be a plug or a lead or even the Distributor Section of the mag.

Do the diagnostic and easiest first.
 
mandm said:
I’m not a mechanic.

Piper Cherokee oil level I would keep it around 6-6.5. When you have more than 6.5/7 it starts to leak out anyway.

2.5/3hrs per qt is a lot of consumption. Oil getting dark quickly happens on older engines, in my experience too, doesn’t necessarily mean a problem. Just checked my records on oil consumption, I’m getting about 12-20 hours per qt (and that’s not science as sometimes I’ll fill it up extra aka 7/7.5 for longer trips whereas for local I’m totally fine with 6 so I’m not consistent at the fill level). After one oil change it was around 10 hours for 1.5 qt but I think this is because oil was filled to the top so it blew out, not sure as I didn’t write any notes down on this. Do a dye test to see if any cylinder leaks?

Wires come lose or get damaged quickly, check those.

Other than that I’m not sure, do share the solution with us once you figured it out.
Click to expand...
For oil level, I fill it to 6qt everytime it reaches 5qt, and probably 6.5 for longer trips. And yup, I would definitely post what I found after getting it fixed.;)
 
Magman said:
I think there are folks on the forum that can assist with troubleshooting.

In many cases certification is not needed for these checks.

My take is if it is a smooth rpm drop it is usually timing that will affect all jugs.

A rough drop is generally you are partially losing one or more cylinders.

In this case it could be a plug or a lead or even the Distributor Section of the mag.

Do the diagnostic and easiest first.
Click to expand...
Yup plug, harnesses/leads, mag switch are what I am asking to check next since the timing has already been adjusted when mag was replaced.
 
Ivan H, Read this very carefully, it may be a Cessna, but the symptoms are the same.

Our Cessna 150 had your symptoms after an annual.

We took it back for correction, they adjusted the idle, no improvement.

I flew it to another airport, and requested they fix it, bad time to ther there, they had a hangar party preparation, an apprentice tried to fix it, and no change. I flew home.

Two unidentified shade tree mechanics each read the engine maintenance manual, met at the airport after the shop closed, and checked the mag timing. Both were 4 degrees too much advance. They were both at the max retard position, so we had to take one off, and center it, then time correctly. On that mag, the engine ran perfectly, so we pulled the other one and set it with the correct timing, center of the range.

The next day, the head shade tree mechanic test flew the plane, and it ran smoothly at all throttle settings, and was within 1% of book sea level airspeed, after all temperature and barometric pressure corrections.

A later conversation with the apprentice mech at the home field determined that the problem was he had a note in his book that Continental engines were X degrees, no additional guidance for 4 cylinder, as his number was correct for the 6 cylinder. He asked where we got the correct number, we handed him the manual with our name on it, and pointed out that we had "borrowed" that book from the shelf 3 days before, from him. "Cool, I will use that the next time I do your annual".

Shade tree mechanics get an undeserved bad rap due to the ones who do not read the appropriate manuals.
 
geezer said:
He asked where we got the correct number, we handed him the manual with our name on it, and pointed out that we had "borrowed" that book from the shelf 3 days before, from him.
Click to expand...

The engine data plate provides that information
 
Just knowing the degrees is just the start for shade tree mechanics.
Properly measuring the make break point,
proper torque when retightening the mag
Safety steps before moving the prop

The manual had a wealth of information, and the apprentice had not ever opened a Continental O 200 manual, but had removed, serviced, and replaced the mags.
 
full
 
Those mags might still be mistimed. The engine data plate may or may not have been edited. There is an AD that forces the mag timing on the O-200 to be reset to 24°BTDC instead of the data plate's 28°, depending on what cylinders are on the engine.

Here's the AD: https://drs.faa.gov/browse/excelExternalWindow/5FD3727F609E1E0686256848004B6075.0001

Here's the TCM service bulletin referenced by the AD:

http://www.tcmlink.com/pdf2/msb94-8d.pdf

The second-to-last page has the stuff on O-200 timing and how to determine whether it should be 28° or 24°. The problem is that earlier timing results in higher cylinder pressures that can cause failure of the deficient cylinders.

Yes, many mechanics don't use the manuals, and many don't even have them. It's why I used to find so many airplanes way out of rig, including all the flight and engine controls, especially flight controls. They just fool with the turnbuckles and pushrods until it looks about right, but it's not right.

FAR 43.13 says some important stuff (underlining is mine):

SECTION: Sec. 43.13
Amendment Number: 43-37, Effective Date: 04/27/2001

TITLE: Performance rules (general).

SECTION RULE: (a) Each person performing maintenance, alteration, or preventive maintenance on an aircraft, engine, propeller, or appliance shall use the methods, techniques, and practices prescribed in the current manufacturer's maintenance manual or Instructions for Continued Airworthiness prepared by its manufacturer, or other methods, techniques, and practices acceptable to the Administrator, except as noted in Sec. 43.16. He shall use the tools, equipment, and test apparatus necessary to assure completion of the work in accordance with accepted industry practices. If special equipment or test apparatus is recommended by the manufacturer involved, he must use that equipment or apparatus or its equivalent acceptable to the Administrator.

The regs weren't written for fun. They were written in blood.

Aside from all that, the O-200 has a certain method of carburetor spider mounting that is not seen on any other engine, and mechanics often miss that because they never check the O-200 manual. If the spider is just bolted solid, that engine can run rough and/or lose RPM and all sorts of ugly stuff. Go here https://www.pilotsofamerica.com/community/threads/0-200-rpm-loss-on-take-off.137511/#post-3238296 and look at post #2.
 
Corona Aircraft Engines in Corona CA.
 
geezer said:
Ivan H, Read this very carefully, it may be a Cessna, but the symptoms are the same.

Our Cessna 150 had your symptoms after an annual.

We took it back for correction, they adjusted the idle, no improvement.

I flew it to another airport, and requested they fix it, bad time to ther there, they had a hangar party preparation, an apprentice tried to fix it, and no change. I flew home.

Two unidentified shade tree mechanics each read the engine maintenance manual, met at the airport after the shop closed, and checked the mag timing. Both were 4 degrees too much advance. They were both at the max retard position, so we had to take one off, and center it, then time correctly. On that mag, the engine ran perfectly, so we pulled the other one and set it with the correct timing, center of the range.

The next day, the head shade tree mechanic test flew the plane, and it ran smoothly at all throttle settings, and was within 1% of book sea level airspeed, after all temperature and barometric pressure corrections.

A later conversation with the apprentice mech at the home field determined that the problem was he had a note in his book that Continental engines were X degrees, no additional guidance for 4 cylinder, as his number was correct for the 6 cylinder. He asked where we got the correct number, we handed him the manual with our name on it, and pointed out that we had "borrowed" that book from the shelf 3 days before, from him. "Cool, I will use that the next time I do your annual".

Shade tree mechanics get an undeserved bad rap due to the ones who do not read the appropriate manuals.
Click to expand...
Thanks Geezer, I will also have another repair shop to cross check everything while getting the problem fixed.
 
geezer said:
Properly measuring the make break point,
Click to expand...
I just noticed that. We don't use feeler gauges for the points gap. There are tools from the mag manufacturer used to set the e-gap, the number of degrees after rotor neutral. Feeler gauges can't get that accurately, and if it's not right, the spark is weaker. The magneto manufacturer's manuals have the stuff on that.
 
Even a blind squirrel may find the nut - eventually.

Gravity sucks! So the lower plugs on the “ bad” mag may be oil fouled.

Oil consumption for the entire engine may not be relevant as much as how much

is one jug pumping.

Your statement that all plugs were clean may put the engine out of the picture.

Assure the Tech brings the High Voltage Cable Tester on the next trip.


fyi- Old, thick , black Ignition Leads were not tolerant of heat is an

understatement. You likely have the newer silicone type that generally provide

good service. However; a hot exhaust stack can still destroy the insulation and

allow it to “ leak “ to the shielding. This is what the Tester is for. Takes about

5 minutes per Lead to test.

Maybe I missed it but did you ever state Mag mfg & Model and Spark Plug model?
 
Magman said:
Even a blind squirrel may find the nut - eventually.

Gravity sucks! So the lower plugs on the “ bad” mag may be oil fouled.
Click to expand...

Post #1 says plugs replaced.
 
Replacing an oil fouled plug would only make the problem go away until the

plug fouled again. He did say they were clean but I’m not sure how closely they

were examined.

There was a comment earlier (#7) asking what could be wrong with a cylinder that

would cause a problem on one mag and not the other.

Foiling of any type generally will be on the lower plugs and hence cause a drop.

If both plugs were to foul you would have a dead cylinder.

I doubt if the plugs are the issue and would test Leads.
 
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