It's not sticking, I did the rope trick, it didn't feel like it was stuck, but then I pulled the valve cover off, just to double-check.
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C90 excessive rpm drop...
- Thread starter 1934A
- Start date
skywagon8a
MEMBER
SE Mass MA6
That would solve the harness question.
DENNY, What do you mean by "rope trick"?
It does seem strange for it to be only the right mag if it is a valve. The high 350 rpm drop is high for just one lead or plug. Even though the engine only has 250 smoh, it would do no harm to check E-gap on the right mag. That's an easy job. If you can't do it, your A&P can. Don't let him tell you to send it out.
The rope trick is to feed a rope into the cylinder through the spark plug hole, then rotate the engine, compressing the wadded up rope into the top of the cylinder, popping the stuck valve loose.
I was pretty well certain that it wasn't a valve problem, or any type of issue with the cylinder, since it runs fine on the left mag. I've swapped plugs, tested plugs for resistance, tested harness leads, can't come up with anything conclusive. I'm leaning towards it being a mag issue.
I was pretty well certain that it wasn't a valve problem, or any type of issue with the cylinder, since it runs fine on the left mag. I've swapped plugs, tested plugs for resistance, tested harness leads, can't come up with anything conclusive. I'm leaning towards it being a mag issue.
skywagon8a
MEMBER
SE Mass MA6
OK I understand what is meant by "rope trick". Never had occasion to do it.
At this point, look at the points in the mag. They may not have been tight and moved slightly. You need to take the mag off to accurately set the points to the proper E-gap. I can talk you through it if you don't know how.
At this point, look at the points in the mag. They may not have been tight and moved slightly. You need to take the mag off to accurately set the points to the proper E-gap. I can talk you through it if you don't know how.
I'd appreciate if you could talk me through it!
BC12D-4-85
SPONSOR
Fairbanks, AK.
Wouldn't the internal E-gap affect all leads?
Gary
Gary
skywagon8a
MEMBER
SE Mass MA6
Take the mag off the engine.
Place a piece of paper between the points.
While holding in your hand, rotate the drive gear in the direction of rotation.
As you rotate, the magnet strength will get stronger.
At the peak strength there will be a slight balance point where it will remain still on it's own.
If you continue the rotation the strength will slightly increase again before dropping off.
So, you are wanting the points to just start to open coming up on the first maximum strength peak.
Set the points so you can just pull the paper at this first strongest point of magnetism.
Tighten the two point screws alternatively little by little. If you tighten one down too much before the other, the point position can change.
Double check after the point screws are tight.
If this mag has an impulse, make certain the pawls are not engaged.
Place a piece of paper between the points.
While holding in your hand, rotate the drive gear in the direction of rotation.
As you rotate, the magnet strength will get stronger.
At the peak strength there will be a slight balance point where it will remain still on it's own.
If you continue the rotation the strength will slightly increase again before dropping off.
So, you are wanting the points to just start to open coming up on the first maximum strength peak.
Set the points so you can just pull the paper at this first strongest point of magnetism.
Tighten the two point screws alternatively little by little. If you tighten one down too much before the other, the point position can change.
Double check after the point screws are tight.
If this mag has an impulse, make certain the pawls are not engaged.
skywagon8a
MEMBER
SE Mass MA6
It should. At this point checking the E-gap is to just eliminate that possibility. The 350 rpm drop is high for just one plug or lead. The fact that the issue is just on one mag is puzzling.
BC12D-4-85
SPONSOR
Fairbanks, AK.
I asked earlier if aircraft lead centers are made from resistance wiring? Or plain "0" Ohm wire? I assume 0.
Gary
Gary
I tested resistance in all 4 leads on the right mag, and got low readings on all 4, so I think they're all ok
BC12D-4-85
SPONSOR
Fairbanks, AK.
Here's one way to test the leads:
Gary
Gary
Another thought.... If the mags were both timed to 28⁰, could that cause the problem I'm having? The reason I ask is that I guess I wasn't paying very close attention when my A&P timed the second mag. I knew he timed the first one to 28⁰, but not sure where he set the second one at. From what I've read, one should be at 28⁰, and one at 26⁰. Thoughts?
BC12D-4-85
SPONSOR
Fairbanks, AK.
From earlier Post #32
How about doing that follow the harness deal?
Gary
That would solve the harness question.
How about doing that follow the harness deal?
Gary
skywagon8a
MEMBER
SE Mass MA6
A couple of degrees difference in mag timing to the engine wouldn't make for a 350 rpm drop.
Last edited:
I'll try this later today.
I stepped away for a few hours to get my mind on something else, and when I came back to it, I realized I had never double checked that the leads were going to the correct cylinders. The firing order for a C90 should be 1,4,2,3. The way my leads were routed, pictured below, the left mag is correct, right mag was firing 3,4,2,1, right? Or am I missing something?
Attachments
skywagon8a
MEMBER
SE Mass MA6
They should both be the same.
Just finished a mag check after swapping #1 & 3 leads, all good now. 50 rpm drop on each mag. Apparently, when the engine was taken off the plane it was on, leads 1 & 3 were mis-labeled. I didn't give it a second thought as I hooked everything up. Pretty embarrassed that it took me two days to figure it out, but maybe someone else will learn from it. Thank you for all the help and suggestions, what a great forum!
