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-   -   Spark plug misfire (https://vansairforce.net/community/showthread.php?t=185545)

SMRacer 08-06-2020 05:50 PM

Spark plug misfire
 
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.

scsmith 08-06-2020 06:20 PM

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.

sailvi767 08-06-2020 06:24 PM

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

SMRacer 08-06-2020 06:32 PM

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.

VA Maule 08-06-2020 07:54 PM

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.

cajunwings 08-06-2020 09:48 PM

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

lr172 08-06-2020 10:06 PM

Quote:

Originally Posted by scsmith (Post 1452414)
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

SMRacer 08-06-2020 10:15 PM

Quote:

Originally Posted by lr172 (Post 1452464)

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.

Scott Hersha 08-07-2020 07:29 PM

Sh..can the mag, and get another LSI and dedicated 4ah battery.

Captain Avgas 08-07-2020 08:22 PM

Quote:

Originally Posted by SMRacer (Post 1452466)
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 ???


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