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Intermittent stumble on O-235-C1

lanat

lanat

Registered User
I need some help!




The O-235-C1 on my PA-12 has been acting up. I've run out of things to troubleshoot. Perhaps you all can help?




I have an intermittent stumble/miss. It is at cruise power-- 2000-2300 RPM. Does not show up at idle or full power, or any other RPM, just this range. It only happens when the engine has high oil temp (180-200) and especially after I've run it hard (pattern work). It is a shudder that you feel in the floorboards, happens a couple times in a row and goes away for a bit. RPM drops about 50-75 RPM when it happens, and it occurs a couple times a minute. It doesn't happen if I run it on one or the other mag, only when it is on both mags.




Here's what I've done:

Resistance tested and bomb tested the spark plugs. Tried with a known good set of plugs. (Plugs are not lead fouled, oily, nor does the color indicate running too rich or too lean)

Ran with a known good harness

Capped off my primer system (in case it was leaking)

Ran it without the air filter.

Ran it with p-leads disconnected. No difference.

Run it on either fuel tank. No difference.

Set the valve clearances (none of them were significantly off)

Checked and set mag timing.

Overhauled carburetor

Replaced points and coils on the slick mags. They bench tested fine, but that was at room temp and we were just testing for spark. Haven't tried with known good mags... yet.

Removed the valve springs to look for a broken spring and inspect the valve stem. The stems were clean and a visual inspection of the faces showed some build-up on the intake valves. Little bit of slop on the exhaust valves, but consistent across cylinders. Doesn't appear to be a sticky valve or an issue with the guide, though this is not an area in which I claim to be an expert.

Checked for intake leaks by pressurizing the system.

Checked the fuel line from the strainer to the carb for swelling.

Checked fuel caps to ensure vents are clear.

Compression check shows all cylinders in the mid 70s.

Ran it up on the ground and found that cylinder 4 has a lower EGT. I have a slight exhaust leak around the flange there, even after a new gasket. It's a slow leak, but I can't seem to get rid of it. That may explain the temperature differential, but maybe not?

No obvious cracks in any cylinder that I can see without removing baffling or other cylinders




So... what else should I look at?
 
How much time on the motor?? "little bit of slop on the exhaust valves" is the key. Your exhaust valve could be miss seating than reseat properly. Look up wobble test. Did you borescope it and look at the valves. I usually blame slick mags for all problems but this time it is second on the list, get a set of Bendix and see how they do.
DENNY
 
A intermittent leaky intake valve will cause a stumble. A stuck open intake valve will cause the engine to quit, suddenly. Then after a seeming very long period of time the engine will restart as though nothing has happened, until the next time.

Been there done that, in a 250 Comanche in a snow flurry, down low over woods. Then suddenly popped over a hill with an airport on the other side, when it did it again. Found a broken cam follower jamming the intake valve open. One cylinder was indicating leakage in the intake valve. Removed the cylinder, lapped the valve. Upon reinstalling the cylinder I noticed some strange cleaner spots on the bottom of the piston. Looked inside and found the mushroom broken on the lifter and the remaining portion of the mushroom jammed into the case splitting open the follower bore. That engine was scrapped with a broken case.
This was a particular airplane which I hadn't flown before, which I had been sent to pick up to bring home. During the run up I "felt" something strange but couldn't identify. The run up was normal. I figured it was just a quirk of the particular plane. It wasn't. This was in the days prior to multi-cylinder EGT gauges, even single cylinder gauges.

Seldom will you find equal EGTs on all cylinders. You can't use that as a trouble shooting guide. Only when the EGTs change their relationship to the others is there something afoot. That small exhaust leak is not your stumble.

I agree with Denny as I too usually blame slick, not this time.

ps: I'm not suggesting that this is your answer. Only that there are other than normal possibilities and sometimes you need to think outside the box and the textbook.
 
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I'm suspicious of the description 'It doesn't happen if I run it on one or the other mag, only when it is on both mags.'

This makes me think that it still might be a mag issue. How much time is on this set of mags? When you replaced the points and coils, did you do a thorough check (i.e. a 500 hr inspection?).

Considering some of the other possible causes such as valve or fuel, swapping out a set of mags might be a somewhat easier way to prove or eliminate the issue being in the ignition system.

Web
 
wireweinie said:
I'm suspicious of the description 'It doesn't happen if I run it on one or the other mag, only when it is on both mags.'

Web
Click to expand...
That makes me wonder how long he left it on just one mag? Most people will not run an engine for an extended period of time on just one mag. The boogy man might get them. ;-)
 
Had the same thing on a stearman a couple of years ago. Ran fine on one mag or the other. Ended up being the carb. Can’t remember exactly, but it loose or a gasket was torn, something like that. Hope it helps


Sent from my iPhone using SuperCub.Org
 
You might try doing a "Lean Mag-Check" in-flight. This technique is taught in the Advanced Pilot Seminar. They recommend it on every flight, because a "weak" mag might appear to be just fine when you are doing the standard mag check pre-take-off (rich mixture, well below cruise RPM), but show VERY different behavior when it's done leaned out, at normal cruise RPM while in flight.

And if you have an all-cylinder digital EGT, make sure you're watching it while you do the leaned-out in-flight mag check. If only one cylinder misbehaves, it's more likely to be a spark plug or wiring issue. But if all of them go wonky at the same time, it's the mag.

PS - you want to do this at altitude somewhere with a decent landing area within range, in case the engine just quits on you.
 
There's no way you could tell this from my username, but please stop calling me 'he'.
 
Yup we assume and then we are the apologists. Hope the problem gets addressed. FWIW gender identity is a fluid subject.

Gary
 
Continental O200, not a Lycoming but same exact symptoms. Likely cause exhaust valve not seating once in a while. Did it on a brand new engine in a brand new Legend Cub after 200 hours. Two cylinders were mucked up with lead. I had not been smart enough to use TCP etc in the beginning.

Also, it couldn’t hurt to check the spec on the valve springs. I just replace them all on an O300 after removing a cylinder to fix a stuck valve. The big spring measured out of spec by about 50% low. Based on that we changed them all. This airplane sat for 13 years, so that could have been a contributing factor.

Rich
 
Alright y'all let's not get distracted :) the engine has just over 400 hours on it, and was torn down about 150 hours ago because it had a prop strike. That was about 20 years ago, though. It wouldn't be surprising if it had a bad cam, however, it makes full power and valve clearances look pretty good. That being said, everywhere I've looked there's been corrosion. Push rod tubes, rocker arms, under the rocker boxes. Wouldn't be shocking if it was also on the cam. I'm thinking I'm going to pull a cylinder soon, but, since it isn't making metal, and it's making full power, I don't want to go through all that trouble just to look like an idiot.
 
The O300 (C-145-2) I spoke of is on my 170B. It was in a hangar in Naperville un touched for 13 years. We did pull a cylinder and took a look at the cam which looked good. At least the part we could see. This engine has 250/SMOH done in 1992. Before 2007 it flew for less than 10 hours a year for several years. It has had two stuck exhaust valves in the last 20 hours, hence the removal of all the springs and replacement. Also, one additional cylinder had a very tight exhaust valve in the guide.

It was ferried to Sandwich, IL where the pre buy and a whole bunch of other work was done. I see you’re in DeKalb , so maybe you know of these airports.

I hope you find your problem. I know how frustrating it can be

Rich
 
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Richgj3 said:
The O300 (C-145-2) I spoke of is on my 170B. It was in a hangar in Naperville un touched for 13 years. We did pull a cylinder and took a look at the cam which looked good. At least the part we could see. This engine has 250/SMOH done in 1992. Before 2007 it flew for less than 10 hours a year for several years. It has had two stuck exhaust valves in the last 20 hours, hence the removal of all the springs and replacement. Also, one additional cylinder had a very tight exhaust valve in the guide.

It was ferried to Sandwich, IL where the pre buy and a whole bunch of other work was done. I see you’re in DeKalb , so maybe you know of these airports.

I hope you find your problem. I know how frustrating it can be

Rich
Click to expand...

Did you buy it recently? was it polished and red? I may know the airplane! I am familiar with all those airports!

As for mine, I never did test the springs. The valves werernt tight at all. But I guess testing the springs would have been a great idea. Might pull em again. I boroscoped the cylinders and the intake valves had some carbon build up on the faces but the exhaust all looked good. The intake didn't look like enough build up to re-lap the valves, but maybe I need to take another look.
 
Yes, that’s probably the one. N2444D. A fellow named Mike Wiseman had it at Naperville for many years. He passed away last December and asked his good friend and the guy who rebuilt it in 1992 to find “a good home” for it. I had new avionics installed from JA Aircenter before I got it home.
These pictures were taken at Sandwich.
 

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Richgj3 said:
Yes, that’s probably the one. N2444D. A fellow named Mike Wiseman had it at Naperville for many years. He passed away last December and asked his good friend and the guy who rebuilt it in 1992 to find “a good home” for it. I had new avionics installed from JA Aircenter before I got it home.
These pictures were taken at Sandwich.
Click to expand...

That is the airplane I was thinking of! It's beautiful.
 
I was going to suggest gripping the airbox and twisting it back and forth while watching for any movement between the the carb top and bottom. Marvel Schebler carburetors can act weirdly if the screws get loose. One had the occasional burp in cruise. Another continued idling with the mixture at idle cut off. A third had a scary flat spot when advancing the throttle. Tightening and resafetying the screws made things normal.

Djfly said:
Had the same thing on a stearman a couple of years ago. Ran fine on one mag or the other. Ended up being the carb. Can’t remember exactly, but it loose or a gasket was torn, something like that. Hope it helps


Sent from my iPhone using SuperCub.Org
Click to expand...
 
Have you looked at the valves with a bore scope? I have had stuck valves after run hard and then powered back. I can't think of any other smoking gun.
 
I had a Pawnee once that started acting similar. Finally stuck a valve enough to bend a pushrod. Fixed that and started again after a few hours. The owner started adding a fuel lube made by Casite. Some use Marvel Mystery Oil. Symptoms went away and he started using it continually. Never saw the problem again.

It might be worth an experiment. Nothing to loose.
 
My low time O-320 with ~7-800 hrs liked to shudder-rough after a long takeoff and reduced power. Deep snow run or floats. Finally the cam-lifters went and required overhaul. I assumed it was from heat causing reduced clearances in the valve train when thinking about it later.

Gary
 
Just a guess but running on one mag can slow combustion and let still burning air/fuel out the exhaust. EGT rises and CHT may decrease over time. Fired with both mags on creates quicker combustion and keeps the heat in the cylinder which can raise CHT. More CHT heat = tighter valves? Maybe if the heat level of the head contributes to the sticking.

Gary
 
Had a mag failure on a charter in a 250 Comanche, flew all the way home (2 plus hours) on one mag with no adverse effects. The only problem with one mag is that we are all used to flying with two. When one goes down we get nervous. How often have you had to pull over to the side of the road when the ignition failed in your car? I have once or twice over the past 70 years but it was long ago, not enough to be concerned in an airplane.
 
Maybe I'm wrong but if you run it on one mag, you run the risk of fouling the other plugs. so, if I do in fact have a bad mag, which I don't believe I do at this point, then if I foul the plugs on the good mag, while I'm running it on the bad mag, I run the risk of having no power. Obviously all of my maintenance flights have been within gliding distance of my airport, but I'd rather not do a dead stick landing.
 
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