0-320 starting to run rough over 2000rpm

Long story long

i wrecked, prop strike etc. fixed plane and built back up exp. installed 10:1 pistons, new ignition harness and champ plugs. That is all I changed on engine. So I fly it the first time running it hard to seat the rings, and I hear/see a 50 rpm 'miss' on the tach. But it does it intermittently. This was in September. When it would do it, it didn't seem to have a specific time, in cruise 2350, climb...off and on. Then go away for say 30 min and do it a few more times. Didn't really think anything of it. So I'm on skis now and have approx 40 hrs since I got it back to together, with it still doing it off and on. One day, I land for 15 min, take off and the thing runs like crap for like 30 solid mins. Wouldn't let me get above like 2200. Backfiring, popping, tach jumping all over. It did seem to get a lil better when I got close to town, but still wasn't right. Pulled plugs, the front 2 bottom were black and wet, the others looked normal. Comp check one in high 60's, rest mid 70's. Tested the new harness, its good. Mag drop is perfect at 1700, and checked it at 2200ish with 75-100 drop.

Ive tried leaning it way out, running full rich, carb heat doesn't help. I strictly run prem cargas with marvel added religiously. I even put about 10 oz in a tank and almost a qt in the oil at one time to see if it would help, thinking it may be a sticking valve...nothing. And comp check says the valves are seated. Starts fine idles fine, taxis fine etc. but as soon as I break ground and gain rpm-2300 is starts Doin it's 'miss' again'. If I back off to 2000-2100 ish it goes away. engine is a b3b with approx 300hrs since overhaul, same on mags. I currently have mags off and are having them looked at and will install new plugs when I put mags back on(Bendix). It only has ran REALLY crappy one time, other than that it's the miss thing.im hoping mags/plugs will fix it. Im no mech and have been talking to several about the issues. Is it safe to say it's not getting good spark, to light up, then it does and boom backfires? There is black residue in my tailpipe. I've checked ALL the simple stuff. It ran fine before I wrecked and only thing I changed was pistons and the harness....have researched till my eyes have bled. Any ideas appreciated thanks

Start by running a full load of 100LL avgas through it.

A few thoughts.

Did you do a full prop strike inspection on the engine?

What exhaust are you running? Maybe a loose or failing bale or spark arrestor causing intermittent blockage as it heats up.

Why auto gas with 10.5:1 compression?

Old (+30 days) auto fuel can get weird.

As I was reading your post I was wondering if you were using Marvel, then I read you were. I tried it once, fouled the plugs frequently so I quit. Flew fine after that. None in oil or fuel. I realise you may have folks say " I've been using this for years". Well maybe so, my engine didn't like it. Also used hi octane car gas for a while, found it backfired a lot when throttling down and lack of response when giving throttle. I would go back to basics, 100 LL and no marvel. I have a standard O360. Maybe you have run this combo for years, maybe something changed in rebuild, maybe it will work again someday, but for now I'd go back.


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I topped it off with 100ll when I flew it first after it was back together. I have 10:1, and have run 10:1 on several engines with prem car gas, for the exception of prolly a tank every 6 months or so. Yes, prop strike inspection

It sounds like either a sticking exhaust valve or a mag problem, with my personal inclination leaning towards a valve.

Had an identical scenario on a PA-18 with O-320 30 years ago.....turned out to be partially blocked fuel strainers in the fuel tanks restricting fuel flow.

Intermittent P lead shorting out? Could be anything but you did have two plugs on one mag fouled while the same cylinder plugs on the other mag were fine. Gotta love intermittents!

Might be a little water in the carb bowl. It'll only screw with you when you're at high throttle settings. That little bubble of water won't suck up through the main jet so it bounces around and occasionally restricts the jet. Try draining the carb and check for water. If you use MMO the red dye will bond to water and it's easy to spot. Occasionally adding some isopropyl is a good idea, especially in winter when you can't sump your tanks.

Be careful adding oil to fuel in the cold. Oil displaces fuel and serves to reduce octane and lean the mixture. Not good in the cold unless your carb is set unusually rich.

Things that come to mind.
Do only one change at a time!! . Make sure you talk to the rebuild shop and see if you found a broken tooth or other problem. Mags should be inspected after prop strike, it is often not done but it is not that hard to brake a tooth off or crack something.

Do not replace plugs until you have tried them with the rebuilt mags, I have had a plug that ran fine below 2100 RPM and cut out above that.

Did you do a mag check in flight at 2400 rpm? This will pick up a poorly preforming plug/mag (can also scare the piss out of ya when it really runs rough).

What are your CHT's? High compression pistons run hot and it is pretty easy to accelerate valve wear, I have had a exhaust valve that would intermittently fail to seat at only 500 hours ran like crap when it happened, than cleared and ran fine. At rebuild (not inspection) did it get new jugs or valve guides?

Drain you float bowl to make sure you don't have any water in it.

I would say the high rpm miss is from a plug or mag issue. A bad single plug should not cause the prolonged poping, backfire. I would look at a broken mag tooth for that. Could be a valve (do a wobble test). But I will lean to the Mag for this one.

Keep us posted!!

DENNY

jkarlovich said:

Had an identical scenario on a PA-18 with O-320 30 years ago.....turned out to be partially blocked fuel strainers in the fuel tanks restricting fuel flow.

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Good idea, but nothing was done with tanks before the wreck, and it ran flawlessly before...

DENNY said:

Things that come to mind.
Do only one change at a time!! . Make sure you talk to the rebuild shop and see if you found a broken tooth or other problem. Mags should be inspected after prop strike, it is often not done but it is not that hard to brake a tooth off or crack something.

Do not replace plugs until you have tried them with the rebuilt mags, I have had a plug that ran fine below 2100 RPM and cut out above that.

Did you do a mag check in flight at 2400 rpm? This will pick up a poorly preforming plug/mag (can also scare the piss out of ya when it really runs rough).

What are your CHT's? High compression pistons run hot and it is pretty easy to accelerate valve wear, I have had a exhaust valve that would intermittently fail to seat at only 500 hours ran like crap when it happened, than cleared and ran fine. At rebuild (not inspection) did it get new jugs or valve guides?

Drain you float bowl to make sure you don't have any water in it.

I would say the high rpm miss is from a plug or mag issue. A bad single plug should not cause the prolonged poping, backfire. I would look at a broken mag tooth for that. Could be a valve (do a wobble test). But I will lean to the Mag for this one.

Keep us posted!!

DENNY

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thought about Doin one thing at a time, but my hangar time is limited, and winching it into the hangar is a pain. So is trying to work on it outside. I figured I'd have spare plugs, if that's not the case, 200-300 bucks won't kill me.

Used same cylinders. Just had them honed & re-ringed. Not saying it can't be a sticky valve or spring, but don't see it doing it from day one once I started flying again..it seemed to slowly get a lil worse. I don't know crap about mags but had heard that with strikes it can do damaged internally, which I'm assuming would throw timing outta whack...??

hunt/trapak88 said:

thought about Doin one thing at a time, but my hangar time is limited, and winching it into the hangar is a pain. So is trying to work on it outside. I figured I'd have spare plugs, if that's not the case, 200-300 bucks won't kill me.

Click to expand...

no mag check at 2400, as the day it really ran bad, at times I could not get to 2400+....

DENNY said:

Things that come to mind.
Do only one change at a time!! . Make sure you talk to the rebuild shop and see if you found a broken tooth or other problem. Mags should be inspected after prop strike, it is often not done but it is not that hard to brake a tooth off or crack something.

Do not replace plugs until you have tried them with the rebuilt mags, I have had a plug that ran fine below 2100 RPM and cut out above that.

Did you do a mag check in flight at 2400 rpm? This will pick up a poorly preforming plug/mag (can also scare the piss out of ya when it really runs rough).

What are your CHT's? High compression pistons run hot and it is pretty easy to accelerate valve wear, I have had a exhaust valve that would intermittently fail to seat at only 500 hours ran like crap when it happened, than cleared and ran fine. At rebuild (not inspection) did it get new jugs or valve guides?

Drain you float bowl to make sure you don't have any water in it.

I would say the high rpm miss is from a plug or mag issue. A bad single plug should not cause the prolonged poping, backfire. I would look at a broken mag tooth for that. Could be a valve (do a wobble test). But I will lean to the Mag for this one.

Keep us posted!!

DENNY

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i only have one cht thermocouple on back left jug by exhaust. I can get it above 400 when running it hard(if it'll let me) and how much I've got it leaned. But will drop below 400 in cruise. I have a very basic panel with no Elec, so gauges are minimal.

Denny, Joel sture says hi. He's one of my roughnecks...

I am still not clear on how much time is on the valves/guides since they have been replaced?
DENNY

I changed mags, plugs, wires, even topped the engine looking for valve problems due to rough running at high rpm....then checked fuel flow to the carburetor which proved insufficient and led me into the tanks. Another expensive lesson for me and checking fuel flow is fairly simple. Could some trash have found it's way into the tanks during repair? The other ideas posted here are all more likely scenarios, but I'd still look at fuel flow based on personal experience.

DENNY said:

I am still not clear on how much time is on the valves/guides since they have been replaced?
DENNY

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Nor am I...not sure on guides/valves. Just know when cylinders were replaced...which I would think came with fresh guts..I've looked back at the engines old logs. No comments on the above..

Sounds like something breaking down in the ignition system, high rpms are when the highest resistance is produced. Look for carbon tracking in distributor caps on mags, coils cracked, arching. Plugs breaking down...

Installing new valve guides is not cheap so people usually put that in the logs when they overhaul jugs. I would due some checking to see if it was done. If not you may have more than one problem. Joel can fill you in on my single cylinder CHT rant:lol:
DENNY

DENNY said:

Installing new valve guides is not cheap so people usually put that in the logs when they overhaul jugs. I would due some checking to see if it was done. If not you may have more than one problem. Joel can fill you in on my single cylinder CHT rant:lol:
DENNY

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Haha Rodger. I believe they were brand new jugs, not around logs now so can't know for sure. Will cover entire ignition if I have to. If it's not ignition and I have to spend a couple grand fixing this, I may so piss on it and bolt up a 10:1 360

It is possible that you have one or more of the "bad" Champion spark plugs that have broken down internally, resulting in high resistance. I'd have my mechanic pull the plugs and run a resistance check on them, replacing any that show high resistance. Newer Champion plugs (finally) are being built the same way as the Tempest plugs that don't seem to experience this problem, so make sure you get "new" (new stock, not new old stock) plugs if you go back to Champion. Myself, I'm done with them, it's Tempest for me from now on...

hunt/trapak88 said:

Used same cylinders. Just had them honed & re-ringed. Not saying it can't be a sticky valve or spring, but don't see it doing it from day one once I started flying again..it seemed to slowly get a lil worse. I don't know crap about mags but had heard that with strikes it can do damaged internally, which I'm assuming would throw timing outta whack...??

Click to expand...

If you did not O/H or replace the mags after a prop strike, you should have.

sounds like carb ice to me. what happens when you pull carb heat when it acting up?

had an intake leak that sounded like what you are experiencing. Check intake hoses.

An intake leak is unlikely, as they manifest themselves most at low manifold pressure (high vacuum) and have much less effect at full throttle. In effect, they "go away" at high throttle openings. His problem doesn't surface until he gets to a high throttle opening.

Carb heat does nothing, as in nothing to fix the problem. I'm having a '500 hr' inspection done on mags. I will be buying some rem37by tempest plugs. Heard nothing but bad about champions.

check mags and make sure all the plastic? internal teeth are still on all the internal gears.... prop strike and cracked teeth do happen, then the jump internal timing...

Have checked all intakes, I doubt it