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Spark plug misfire

SMRacer

Well Known Member
Patron
Excessive RPM drop on mag check (Mag on left, LSI on right) reveals #3 plug not firing based on EGT drop when LSI is shut down.

Numerous swapping of components have me concluding that it is indeed #3 mag plug not firing, but that the spark plug is NOT the problem.

I then swapped out the harness to determine if the problem was in the harness. No change, still excessive RPM drop and corresponding EGT drop on #3 when LSI shut down.

That leads me to believe the problem is in the mag.

Any suggestions. LSI ignition working properly. All other cylinders appear normal.
 
If it were a car, I would say replace the distributor cap. You must have a arc trace to ground in the mag that is shorting that plug? I'm not familiar enough with the guts of mags to know if that is a separate part that can be replaced.
 
I assume you have done the obvious and swapped both leads and plugs to other cylinders. I had a similar issue and the plug looked fine. Swapped it later to find the problem moved with it. Should have been step one but I am learning!
G
 
Troubleshooting

Yes, I changed to a known good plug. And then I swapped two plugs from different cylinders. I also confirmed that my engine analyzer showing a problem with #3 was actually #3.

Do the internal workings of a mag allow a fault on a single wire as opposed to a general degradation of performance on all wires?

Also the original harness is only about 6 months old. Swapping to an older but serviceable harness seems to prove that it is not the wire or the mag cap.

Unless someone comes up with some alternative fix, I'll swap out the mag tomorrow.
 
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Cap

The distributor cap that Steve Smith is referring to is not the external metal cap that the leads are fixed into. The cap that maybe suspect is internal to the mag case , it has receiver holes for the springs that attaches to each lead. To inspect the Bakelite cap the mag will have to be taken apart, to see if it's carbon tracking inside where the roter button might be grounding out in the carbon suit deposits that tend to accumulate on the inside wall of the disturber cap.

Edit: yes carbon tracking can cause fault on only one spark plug.
 
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Misfire

I would start by taking the cap with the wires off the back of the mag to look for obvious problems like burning, arcing or loose springs ect. If that looks good it’s time to look inside the mag or get it to someone who can test & repair.

Don Broussard
RV9 Rebuild in Progress
57 Pacer
 
If it were a car, I would say replace the distributor cap. You must have a arc trace to ground in the mag that is shorting that plug? I'm not familiar enough with the guts of mags to know if that is a separate part that can be replaced.

I also would guess excessive wear on the #3 distributor post or possibly a carbon trace. The inside of the top of the mag is a distributor cap and rotor. Both are replaceable, but expensive. Guts look just like a 70's vintage distributor cap on the inside. Key difference is the the 2:1 reduction is done inside the mag; just under the rotor. Auto distributors run at 1/2 crank speed, but a 4 cyl mag runs at crank speed to keep the power production at reasonable levels. Reduction is done at the rotor.

How do you KNOW the plug is not the problem? Did you confirm 1K resistance?

Larry
 
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How do you KNOW the plug is not the problem? Did you confirm 1K resistance?

Larry

Thanks everyone for the advice.
So to recap: Actions thus far,
Test run showed an EGT drop in #3 cylinder when LSI turned off.
I removed the #3 plug and installed a new plug. No change.
I switched the #1 and #3 plugs (I know the #1 was functional) No change. So I know the #3 plug was functional when swapped to the #1 cylinder.
I proved the engine monitor's #3 reading was indeed relating information from the #3 cylinder.
I switched out the harness to a known functional harness. No change.

Fortunately I have a spare mag on the shelf. I'll install it tomorrow.

Considering the steps I've taken, I've got to conclude that it's an internal malfunction in the mag.

Additional suggestions are still welcome. I'll post the results of the mag switch tomorrow after the work is done.
 
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Thanks everyone for the advice.
So to recap: Actions thus far,
Test run showed an EGT drop in #3 cylinder when LSI turned off.
I removed the #3 plug and installed a new plug. No change.
I switched the #1 and #3 plugs (I know the #1 was functional) No change. So I know the #3 plug was functional when swapped to the #1 cylinder.
I proved the engine monitor's #3 reading was indeed relating information from the #3 cylinder.
I switched out the harness to a known functional harness. No change.

Fortunately I have a spare mag on the shelf. I'll install it tomorrow.

Considering the steps I've taken, I've got to conclude that it's an internal malfunction in the mag.

Additional suggestions are still welcome. I'll post the results of the mag switch tomorrow after the work is done.

How many hours on the mag ???
 
Update

Installed a NEW mag today. Fortunately I had one on the shelf
The problem appears to be resolved. I will know for sure when I test run the engine tomorrow.

Yeah, I hate mags. Would love to switch the left side to electronic, but I didn't build the airplane and I'm reluctant to undertake that project.

The mag had about 1000 Hobbs hours, and was serviced at about 550.

Thanks for the help everyone. When I have the mag examined by someone knowledgeable, I'll report back the specific failure.
 
Lessons learned

So when you don't have a hangar full of diagnostic equipment, you take the next best path. You start swapping parts to see what resolves the problem.

So we had a dead plug or very weak spark on the left mag, plug #3.
Naturally we checked plugs first. All kinds of new plugs and swaps with no change. We eliminated plugs as the culprit.

Next we changed the harness. I used the former harness that I had replaced after 20 years because performance was degrading, but still acceptable. No change. Not the harness.

Installed a NEW mag. No change. Not the mag.

What? It has got to be one of those three things.

We decided that the least likely problems were the mag and the plugs. So what's the odds that we have TWO bad harnesses with the SAME lead defective (#3). I share my hanger with another RV builder. He had just hung his engine. (Read: spare parts) Installed his never run harness and our problem was solved.

An autopsy of the TWO failed harnesses revealed the SAME problem. Both had a breach in the inner insulation right at the last bend of the wire prior to the plug.

So what did we learn?
1. Don't be quick to assume spare parts are viable.
2. Be careful of how severely you bend the plug wire. I'm not sure what the manual specifies as minimum bend radius for plug wires, but these bends were about a 1 1/2 inch radius. Apparently that is too severe.

The whole episode made me feel dumb and smart at the same time.
 
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