B

BillC

Well Known Member
:(Problem defies rational thinking

Here is the situation and hopefully someone out there has the answer.
When I started up engine last Sunday it didn?t feel just right so did quick Mag check. #2 cylinder was not firing on left Mag. Tried to clear the fouled plug but no joy. Shut down and pulled lower plug (left Mag) looked fine but rather than risk it I replaced it with a new one. Started her up and again failed on left mag #2 cylinder. Again replaced lower plug with another new plug. Same results. Ok so I swapped plugs top for bottom. Now right mag fails (top plug) Looks like bad plug so replace it with another new but this time different manufacturer. Right Mag still fails. Swap top #2 and #4 plugs and right mag still fails. Swap top to bottom plugs and now left Mag (lower) fails. Thinking that perhaps my 10 year old harness could be the problem I ordered a harness set which arrived today. Replaced harness and left Mag fails on #2 cylinder. New plug in bottom, left still fails. Swap plugs top for bottom and Right Mag (top ) fails.
I?m thinking that my next step is to tear down one of the Mags or just order two new ones (big $$$ that I would rather spend on fuel).
Background:
Engine O-320, 1100 hrs since new and 110 hours since bottom rebuild, compression #2 cylinder 76/80. Both Mags were new last year and have ~95 hrs.

ANY HELP OR INSIGHT WOULD BE GREATLT APPRECIATED
 
When did you check the compression? Recently?

Carb for FI?

How do you know #2 cylinder is not firing? EGT'S?

You are right, that hurts my head, and I've never been rational. ;)
 
Last edited:
Is it possible that your instrumentation is mis-wired and (say) the #1 cylinder is wired to the readout for #2, therefore nothing you do to #2 fixes the problem that actually exists on #1?
 
When you say "fails", do you mean the egt of that cylinder is cold?

In the middle of your description, you say "Swap top #2 and #4 plugs and right mag still fails." Which cylinder's egt showed cold then?

Is it running quite rough during these "failures"?

You do have a mystery...
 
Follow up answers to questions.
Engine is carb not FI. Checked compression Monday as part of trouble shooting process. Have Dynon engine monitor and verified that it was reading correct cylinder, pulled #2 CHT and indeed Dynon showed outside air temp and not higher (warm) cylinder Temp. When I say Mag failed I mean that both CHT and EGT for # 2 cylinder start dropping when switched to that mag. In the one test I swapped the plug known to be firing in #4 cylinger Top for the non firing plug #2 top. These are both fired from the same mag.
Any other thoughts out there????
 
Last edited:
Does the plug fire when it is OUT of the cylinder and connected to the mag?

ie: Pull one plug from each cylinder. Connect pulled plugs to harness and ground plugs. Crank engine while someone watches spark plugs to see if they fire.
 
BillC said:
Follow up answers to questions.
Engine is carb not FI. Checked compression Monday as part of trouble shooting process. Have Dynon engine monitor and verified that it was reading correct cylinder, pulled #2 CHT and indeed Dynon showed outside air temp and not higher (warm) cylinder Temp. When I say Mag failed I mean that both CHT and EGT for # 2 cylinder start dropping when switched to that mag. In the one test I swapped the plug known to be firing in #4 cylinger Top for the non firing plug #2 top. These are both fired from the same mag.
Any other thoughts out there????
Click to expand...

In short, it is always cylinder #2, right? (the outcome of the #2 and #4 switch is still not clear to me - did #2 or #4 then misfire?) During all the messing around, no other cylinder has misbehaved? If this is true, you have had #2 misfiring on two different mags, multiple spark plugs, two different harness sets. Seems to not be ignition.
 
Good idea Gary, I'll try the plug removal/firing later today. Alex, Yes it is always #2 cylinder that has the problem.

Thanks for the input guys and keep the ideas comming.
 
Start looking elsewhere

AlexPeterson said:
In short, it is always cylinder #2, right? (the outcome of the #2 and #4 switch is still not clear to me - did #2 or #4 then misfire?) During all the messing around, no other cylinder has misbehaved? If this is true, you have had #2 misfiring on two different mags, multiple spark plugs, two different harness sets. Seems to not be ignition.
Click to expand...

I agree with Alex. Unless you've gotten confused during all this testing (easy to do!) then it's not an ignition problem.

I'm leaning toward an intake leak. Did you have the intake pipes off?
The intake gasket can get skewed and cause a leak.

Also I'd pull both valve covers on that side and watch the valves while turning the prop. Just to be sure the valves are moving (opening) about the same amount.

Good Luck! And I hope you find the problem soon.

Mark
 
Mark Burns said:
Also I'd pull both valve covers on that side and watch the valves while turning the prop. Just to be sure the valves are moving (opening) about the same amount.
Click to expand...

And while you're there make sure that the valves are completely CLOSING. A stuck (open) valve could be your problem.
 
Bill, if that was my plane, I'd pull the left mag and check the cap carefully for cracks or bad terminal for #2. I've seen this happen before.
 
Yes Alex, when switched to to offending plug/mag the engine runs very rough but when switched to both or good plug/mag engine runs smooth (slightly smoother on both). This happens at all RPMs from ~800 to 1800(normal runup speed). If it were an intake leakor valve problem wouldn't it happen all of the time and on both plugs/mags? Also if it wer a bad valve wouldn't that have shown up on compression CK? This is the strange part in that it runs fine on one plug/mag and not the other and will switch when I swap plugs. Raining now so willhave to wait on some of the testing until better weather, but still open to ideas.
 
Last edited:
Problem Solved

:D:DOk so I started working on the various suggestions offered by you kind folks when may friend Ken, the super engine expert shows up. He has me start the engine and switch mags until the bad one was found, which was the left (lower plug) one at this point. We remove the upper and lower plugs and he examins them closely. Then we swap the upper and lower plugs and tried the engine again. Sure enough it's now the upper plug that is not firing. We again remove both plugs and after some more close examination Ken suggests that we increase the gap. The gaps were factory set at~0.019 so we increased the gap to something just over 0.02. Tried the engine again and sure enough ran fine on either Mag. His explanation was that the cylinder may be running a little leaner in the colder weather and the smaller gap was not producing a long enough spark. So in payment for his great help we did a few laps around the patern and since the weather was only marginal VFR we had the patern to ourselves. The tower approved early turnouts and short approachs without even have to ask. Even got to do my first high speed fly-by. Great fun, too bad I lost almost a week of great flying weather for a simple thing like too small a gap.
Anyway, A big thanks to everyone for your help and suggestions!
 
BillC said:
:D:DOk so I started working on the various suggestions offered by you kind folks when may friend Ken, the super engine expert shows up. He has me start the engine and switch mags until the bad one was found, which was the left (lower plug) one at this point. We remove the upper and lower plugs and he examins them closely. Then we swap the upper and lower plugs and tried the engine again. Sure enough it's now the upper plug that is not firing. We again remove both plugs and after some more close examination Ken suggests that we increase the gap. The gaps were factory set at~0.019 so we increased the gap to something just over 0.02. Tried the engine again and sure enough ran fine on either Mag. His explanation was that the cylinder may be running a little leaner in the colder weather and the smaller gap was not producing a long enough spark. So in payment for his great help we did a few laps around the patern and since the weather was only marginal VFR we had the patern to ourselves. The tower approved early turnouts and short approachs without even have to ask. Even got to do my first high speed fly-by. Great fun, too bad I lost almost a week of great flying weather for a simple thing like too small a gap.
Anyway, A big thanks to everyone for your help and suggestions!
Click to expand...

That doesn't sound right. Check your mags, if they're Slicks and fall into the serial # range listed in SB3-08A, open them up and have a look. I just OHed a set on a 540 that were in bad shape from bad carbon brushes as outlined in the SB. BOTH were on the borderline of failure.
 
Lean Mixture may be the clue

Bill- I know: "If it ain't broke, don't fix it." But having been there with you and having seen the rough running on sometimes the left, and sometimes the right mag, I am wondering if you might have an intake leak that is causing a lean mixture, as mentioned in a previous post. There are some relatively easy checks to do to see if you have an intake leak. Give me a call and we can continue troubleshooting, if you want.

Pete
PS: Before it's too late, you could always swap planes with me, straight across! ;) Mine doesn't have any mag problems (at the moment).:rolleyes:
 
You must log in or register to reply here.