Still Running Rough! Go Figure!

[Tom G]

Some of you may recall that I've been having some problems with mainly my right magneto. I get a good ground runup followed by the mag going rough under load in the air. Well now its occasionally running rough on both mags and I'm beginning to think its a fuel problem.

I went to fly this morning and the right mag was very rough although as the engine warmed up it seemed to get better. But it never got good enough to fly. Occasionally it would be rough on both mags.

We wanted to look at the carburetor so we dropped the lower cowl. I noticed that the intake manifold was dripping wet with water. I figured it might have been carb ice even though we used a good deal of carb heat during the runup.

Since the right mag was rough we figured it might be a bad plug (they have just been replaced. We put the old plugs back in and it ran up perfectly. We figured the problem was solved (as we have with a number of things).

It ran up 100% on the ground so I took off. During the climbout I switched to the right mag and it was missing badly again. I turned downwind and did another mag check. This time even when I went back to both mags the engine continued to miss badly. I increased the throttle and it barely responded. It got better after that and on final I had normal throttle response again although the right mag was still rough.

Back on the ground and it ran up poorly again.

This problem has us totally baffled. It has now run the gamit of occuring on one or both mags (although 98% of the time its the right mag), high or low rpm, under a load or at idle and normal runup.

I'm thinking now that its not ignition at all but a fuel problem. Several times we have made changes like cleaning the plugs, changing the plugs, removing the washers that were mistakenly put in the ercoupe cups on the top plugs and every time we did these things it responded and ran fine only to go bad again.

The plane gets flown regularly and the gas drains without a problem or trace of contamination. Could my carb be bad?

Its on the ground now until this is resolved for sure.

Any ideas would sure be appreciated.

Thanks guys.

[flyguy]

Re: Still Running Rough! Go Figure!

Wish it was a carburated small bock Chevy, I could tell exactly what to look for to fix the problem. They seem to make airplane engine systems to be difficult to troubleshoot.

I will tell you what happened to me just the other day.
I have Eisemann mags and C26 plugs on top and RM41's (or something like that) on the bottom. The 41's are recently new.
Both top and bottom were running rough, but the bottom was worse. Would get better sometimes. Had just had the bottom plugs out and cleaned to get all the storage oil out after putting struts on. Finally bottoms got too rough so I took them out, little black, but way down beside the insulator, in one plug, there was just a few tiny lead (oxide?) chunks. Removed them with my trusty dental tool and put the plugs back in. Engine really wound up good and ran especially smooth. Been fine since. Drop on BOTH top and bottom mags is much less. I have a lot of trouble believing that a few tiny lead chunks at the BOTTOM of the insulator, on one plug could make that much difference, and somehow make both mags run rough, but it sure did. Mags, at best, are marginal ignitions and the little things matter.
I'm sure you already know to check the appearance of the plugs to give you an idea of what is going on. It really helps to have a mixture control when trouble shooting a carburator problem.
I suppose it goes without saying that the carbs on these engines are pretty crude devices and like the talking dog, it is not so much amazing that it does not talk well, but rather that it talks at all.
A few specifics: worst for ignition to work correctly is wide open throttle and lower rpm (maximum cylinder pressure), as in a climb. Try to get as much information as possible from your plug condition, that is color, deposits, ect. If you get a bad mag check, that is when you want to immediately pull the plugs and look at them. Sometimes if you find something funny on a bottom one, check the top one on that cylinder too. It might give you an additional clue to the problem. Check for a way rich idle adjustment.
If engine is running rough on the ground right now it is the best situation for trouble-shooting: pull and check the plugs carefully now.
Darryl

[Guest]

Re: Still Running Rough! Go Figure!

Hi Tom,

Has the harness been tested?

Since your problems seems to become apparent when you switch to the right mag it doesn't see like a fuel issue BUT, did you switch back to both when you had the throttle response problems?

If so then fuel does seem like a possibility.

I had a problem once where two gaskets on alower plug allowed it to get oil fouls and run rough after sitting but got better as it warmed up, seems like you already fixed that issue, where you also getting bottom oil fouled/wet plugs?

Any chance you have auto fuel with ethanol in it?

Dave

[Tom G]

Re: Still Running Rough! Go Figure!

Dave, yes it was back on both when I had the throttle response problems thats whats making me think its fuel. The plugs are not fowling and I'm running 100LL.

[Guest]

Re: Still Running Rough! Go Figure!

Tom,

has the harness been tested with a high voltage tester?

How old is the harness?

Dave

[NY86]

Re: Still Running Rough! Go Figure!

Which plugs are connected to the right mag? Are you sure it's not a plug fouling issue? Seems every time you mess with the plugs it gets better...

Typically the left mag fires the lower plugs and they are more succeptable to fouling, but I've seen plenty of cases where the mags were wired to the wrong plugs. Also, when you go to the right mag the left plugs could foul from not firing.

Or not...

[Tom G]

Re: Still Running Rough! Go Figure!

Dave, my wiring harness is less than six months old.

NY86, the right mag fires the top plugs and that is the mag that has the problem. The plugs are not fowled. Sometimes when I switch back to both after checking the right mag the engine smooths out. Occasionally it does not. I'm thinking that maybe the bottom plugs load up when I'm checking the right mag but it doesn't happen all the time. Actually most of the time it doesn't. Since the problem has occurred under all conditions I'm thinking my carb may be bad.

[Jim Doody]

Re: Still Running Rough! Go Figure!

http://www.sacskyranch.com/eng37.htm Sticking valve? Jim

[flyguy]

Re: Still Running Rough! Go Figure!

If you haven't done it already you might want to check your P wire connector and shield part of the cable. Check for frayed wires sticking out where it connects to the switch, and check the switch. You would be looking for an intermitten short, not an open, of course. If you are sure the fuel valve will shut the engine down you can check all that at one time by disconnecting the p-wires at the mags, start the engine and check if problem goes away.
Again do all the top plugs look about the same when the mag is cutting out?
Do a ground run and keep it on the "bad" mag, probably lower rpms, until the engine quits or at least runs for a while on that mag only. Then check the plugs for that mag. Check the cylinder temperatures at that time. That is for harness failure. Not likely for all 4 to go bad at one time. Maybe shutting it down with the fuel valve to cutoff without taking the switch off the offending position would be a good idea. Somewhere in there may be a clue.

Carb: what color is the inside of your exhaust pipe, in daylight if possible. Does the engine run equally rough at all rpm's? Do you have a black exhaust inside or get some black smoke? When it is running rough give it a small squirt of prime and see what happens. Rev up, or choke down worse? Do you have an EGT or a mixture control? If you do have to trouble shoot the carb a non-TSO EGT is pretty cheap and you can do a temporary installation for ground runs only.
DC

[fearofpavement]

Re: Still Running Rough! Go Figure!

It may be worth checking your point gaps. Close points will make a weak spark. Usually when the points get close it advances the mag timing but if the points are close and the mags were retimed without resolving that issue... May be worth revisiting even if you already hoed that ground.

A weak spark can cause lots of problems and won't fire a plug that ordinarily would fire. So when you clean the plugs and things work well, it may not actually be the plugs that are the problem but inadequate punch to fire them. (for whatever reason)

One other thing. You stated that your right mag fires the top plugs. Have you done anything to confirm that the mag switch is actually controlling the correct mag? This is a common mixup and in many cases wouldn't really matter but when you are trying to troubleshoot a persnickity system it becomes crucial.

[Garry Crookham]

Re: Still Running Rough! Go Figure!

Tom
From reading the previous thread on this problem you said you had Bendix mags. Are they the SF4 or S4RN series. If the mag is an SF4 I would suspect the coil might be defective, a common problem on those mags. I've got a coil and will be happy to send it to you, the only problem is that all the parts for those mags are ancient and even a new coil can be defective.

Garry Crookham
N5112M
Tulsa

[dennispip]

Re: Still Running Rough! Go Figure!

For what it is worth, I had a "dying" mag that acted just as yours. It hung in there for a year or so, with normal runups and mag checks, and was only demonstrating intermittent and very brief symptoms for a few seconds. Then I noticed a siginificant drop in performance and power on take off, that following a totally normal run-up and mag check. I went back and landed and did a mag check and found a totally dead left mag. After replacement, the engine ran better than ever and was completely smooth. No brief perceptions of power drops or roughness that only lasted a few seconds, etc.

My bet is that your right mag is not "right" at all and if you replace it or get it fixed back to normal operating standards, your plane will be your "Lil Darling" again.

Best of luck no matter what it proves to be and may you safely return her to the skies sooner than later!

[NY86]

Re: Still Running Rough! Go Figure!

I'm not sure how a carb issue ccould interact with the mag check.

Here's a long shot, though. Say the carb is running almost rich enough to cause a miss fire. The top plugs fire in a richer environment than the lower plugs (which are closer to the exhaust). When you switch to the right mag you get a little rich missfire. Since the overall mixture is too rich, while the left mag is off the lower plugs ocasionally foul. The left mag usually tests good because those plugs fire in a leaner environment.

Or not...

Switch the left and right leads and see if the problem follows the mag or the plugs...

[Tom G]

Re: Still Running Rough! Go Figure!

Thanks everyone for all of your comments.

The reason that I'm talking a fuel issue here is that I have now seen the engine run rough on both mags even though the right mag is the one that checks out bad on the runup sometimes. I've never seen the left mag rough when the right checks out good.

When the engine runs rough on both mags it always does it after I've switched from the rough running right one back to both.

So it does make sense that the bottom plugs (left) could be loading up while its running on the right.

[Len Petterson]

Re: Still Running Rough! Go Figure!

I remember an outboard I hade many moons ago. It hade metal cover over the pluggs similar too the Ercupe plugg covers for shieldings.
It ran badly and we removed the motor ( wheigt 200 lbs) broth it home installed new pluggs and it ran great. Without the covers! I reinstaled the covers and back to the boat we went.(;f
It ran bad again!
The ignition cables had cracks in them where they went trough the plugg covers and grounded out at what point.
How did I find out?
I was upset and said some not so nice thing about old motors. I grabbed the plugg cover as dad was pulling the rope. I got a rude awakning to say the least!
Ckeck your plugg wires for cracks! Hope it helps!
What about the mag switch! They can brake up inside?
Len