O-320-E2D head scratcher
 

The issue:
Engine occasionally bogs down a bit, loosing 150 rpm, slight vibration and louder, irregular bark from exhaust. Same on all magneto positions.
Troubleshooting so far:
-plugs checked, none worn out, cracked, shorted, resistances all below 1500 ohms. Plugs serviced, gaps all okay, reinstalled: no change. All new plugs installed. No change.
-ignition harnesses replaced about fifty hours ago due age.
-magnetos Bendix with impulse on left, 190 hours since 500-hour shop visit. Impulse working and timing at spec.
Dismantled Bendix ignition switch. Worn but no signs of pitting, arcing, no carbon tracks. Cleaned metal surfaces and reassembled with dielectric grease, reinstalled: no change.
Fuel samples always clean, always 100LL and aircraft hangared. Carb bowl drain pulled and no debris or water drained out. Suspected a fuel flow issue but will turn up full static rpm for thirty seconds, albeit while intermittently trembling.
Always starts happily and easily, albeit with some mechanical clanking from left side of engine, which was built up in 1967 and has 2100TT. Last time cylinder was pulled, about 200 engine hours back, cam lobes and Followers visually inspected, not shiny any more, fish-scale appearance but no pitting or surface defect that catches a fingernail. Paper oil,filter always clean of ferrous metal, running Aeroshell W100Plus with Camguard.
When run at 1700, after ten seconds of smooth running, slowly gets rougher and rpm drops 125, no change on individual mag nor with leaning the mixture.
Exhaust valves checked for sticking, all free and slide/rotate without issue.four hundred hours since top overhaul.
Removed air filter, carb heat box; no change during brief ground run. Carburetor throat looks intact, venturi, jets all appear secure. During second last ground test, experienced triple the usual drop when selecting carb heat, left heat on,no subsequent rise, so heat off, rpm did not recover. So removed all that stuff to try again, no change.
Ground run with P-leads disconnected, eliminating ignition switch and P-leads, filters and wiring, feels and sounds better but still not as "happy" as four flights ago, when this started happening intermittently and for but a few seconds at higher power settings but not at lower power settings.
During today's ground tests, it would start running within 2-3 blades and run happily until running at 1700 for about ten seconds. Then it would start to bark out the exhaust, vibrate and loose about 125 rpm. Always turned up full static rpm but not willingly.
I'm still thinking the issue is heat related. The chances of two separate mags developing an internal issue simultaneously seems very remote.
Unlikely to be fuel flow related as it can turn up and sustain the required full-power rpm, albeit with random issues like this.
Tapped the exhaust piping, nothing loose internally anywhere.
Now, after three days of scratching my head, I don't have that much hair left to scratch...

maybe a broken intake valve spring or a lifter that is defective and not pumping down???? if you have an engine monitor, monitor cht and egt closely when this is happening to try and discern a cylinder that indications change on, while it is going on. if no monitor let it run badly as long as you can and see if you can find a cooler than the others exhaust pipe or cylinder head.
Good Luck,
Mahlon

one bit.
 

I agree with "M", if you have a four lead CHT & EGT it would make it nicer to try and track this one down. One trip we have found down through the years is and I am just throwing this out as an idea to look for. How old are the distributor caps and do they have any carbon tracks built up on the inside surfaces. If you have a cap that has any even smudges of carbon on the inside, when you bring the engine up under load you sometimes get a jump fire inside the cap. You may wish to check for any cracks or carbon jump fire traces inside those and cross that off your list as well, if they are clean and don't have any leaking electric or misdirected electric current.
Hope this helps, Yours, R.E.A. III #80888

Good suggestions above. The portion quoted above sticks out for me. With a healthy ignition, removing the P leads should have NO impact on performance other than an inability to turn them off. The P lead is tied directly into the mag's primary winding circuit. Grounding this lead prevents the mag from charging the primary side of the coil. Poor charging of the primary winding results in a poor output on the secondary side and therefore a weak or non-existent spark. The fact that performance improves with the P leads removed, indicates that possibly your P lead is leaking to ground and reducing the current in the primary circuit. It is possible that this could also be an indication of a problem in the mag, most likely the condensor shorting or a bad coil. Given the positive improvement when removing the P leads (assuming this wasn't a misread), Ignition is the most likely culprit and where you should spend some diagnostic time. It is very common for a coil to perform well under light load or low RPM, and start to stumble at higher RPMs or higher loads.

Given the symptoms with the P lead, you will need to verify that your mags are truly dropping out when you command them to. You are relying upon your mag drop test to diagnose, but I don't believe that you can fully trust they are dropping given the symptoms. You can put an automotive timing light on the plugs to positively confirm whether or not a plus is firing.

Do you typically get 125 RPM drop when pulling one mag during runup at 1700? If so, that would be another tell tale sign pointing to a problem in one of the mags

Mixture?
 

Having read the OP a couple times, I'm surprised mixture hasn't come up. I read the the bowl was checked, but the slow to come up to static and rough after 10 sec suggests POTENTIALLY leaning out following consumption of the accelerator pump fuel. Bowl is clean, but what if it's because the junk is the the spray bar? Anyway, it should be easy to test, pull back on the mixture a a couple of engine speeds. If it's ROP like it should be, speed should go up. If, as speeds increase, the speed increase (egt rise?) gets less and less, maybe there's less reserve fuel flow....but betting against Mahlon in these types of discussions is usually ungood. ;)

Interesting situation. Good luck.

Thank you for taking the time to reply.
Today, I did what I should have done at the beginning,
Sit on my milk crate Thinking Seat and look
at the airplane firewall forward but
I'm Afraid that I still haven't solved this one.
While it used to run differently when on a single
magneto( 125 drop and 50 drop), its
now 125 on both. So it is likely that one magneto
was previously keeping the engine running more
smoothly and happily and now that one is
also suffering some deterioration, perhaps the
coil as was suggested.
I do have a spare carburetor and can try that and also another
ignition switch now.
All that my sitting and thinking revealed today
was a finger-tight intake band clamp.

Also, it's not slow to come up to static nor to to
achieve full static rpm but it is rolling back from any rpm
Around runup and higher, after a few seconds.
The exhaust is a light brown which makes me wonder about
Under-fuelling.
I'm not sure that the accelerator-pump well is ever depleted

Last week I posted a long description of what might be a similar problem including photos and a video of the black smoke coming out of the exhaust stacks. But since my Lycoming engine is mounted in a Lancair rather than RV, apparently that information isn't welcome here. The mod(s) deleted my post. Too bad. I thought we were all here to learn...

I just hope that your engine is not a pilot of dog ship.

(Sorry, can't help myself, 'tis how I read your user name / signature. Every. Time.) :rolleyes:

A leaky primer plunger if you have one can cause very similar situation