Mag drops with new engine

How to keep the Cessna 170 flying and airworthy.

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GAHorn
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Re: Mag drops with new engine

Post by GAHorn »

n2582d wrote:Might want to check the magneto capacitor.
That is an astute suggestion. A bad capacitor will act up with roughness, backfiring, and intermittent operation.
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Iurii S.
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Joined: Tue Jun 16, 2020 11:16 am

Re: Mag drops with new engine

Post by Iurii S. »

Can a severe drop be due to the fact that you are switching off the upper plugs, and the lower plugs are fouled with oil due to new rings?
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mmcmillan2
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Re: Mag drops with new engine

Post by mmcmillan2 »

Joe Moilanen wrote:
mmcmillan2 wrote:Mine do have the Bendex mags. Type S6LN-21. Overhauled 8/22.
I've never had my timing drift that much from new or rebuilt mags breaking in before. I don't think I've ever experienced more than 1 or 2 degrees drift for any reason, YMMV. I think I'd monitor the points gap if the trend continues?
I don’t think they drifted, I think they were never set right. I should have left the plane at my mechanic (1.5 hr drive from home) when I found the big mag drop and the engine not idling below 1160rpm. Mechanic said it was ready to pickup, but he wasn’t there when I got it. I called him and he said to run it 5 hrs then tune it.

I was in a hurry to fly it home then head to work so I took it. The next day I found the throttle arm on carb was set incorrectly, and full range stop to stop wasn’t available. I fixed that with help of a local mechanic. Then I got a local guy to help retime left mag but he hadn’t done one in a coon’s age and it took 4 hours. 8O
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mmcmillan2
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Re: Mag drops with new engine

Post by mmcmillan2 »

Iurii S. wrote:Can a severe drop be due to the fact that you are switching off the upper plugs, and the lower plugs are fouled with oil due to new rings?
That’s what my installer mechanic thought, but after cleaning plugs it was still dropping excessive. Had to adjust timing.
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mmcmillan2
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Re: Mag drops with new engine

Post by mmcmillan2 »

n2582d wrote:Might want to check the magneto capacitor.
Thanks, will look into that.
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mmcmillan2
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Re: Mag drops with new engine

Post by mmcmillan2 »

As of yesterday, my mag drops were 75 left and 50 right. Better than 150 left / 15 right for sure. Engine sounds smooth.

Mechanic A was a friend that helped me out. He spent 4 hours on the left mag. He hadn’t done one in a long time, and didn’t have the Bendix P-lead adapters.

Mechanic B was a local one, he set right mag in less than an hour.

For anyone going down the road of mag timing, buy a pair of new gaskets first. Both of mine split while adjusting.

Mechanic B noticed I didn’t have any excess wire hanging out of my mixture cable attachment. It seems that cable should have been replaced when mounting new engine. Further reading in the books showed me that the clamps for the cable housing shouldn’t be aluminum construction, nor or rubber dampened. Mine is poor looking aluminum with some electrical tape. Parts on order from Spruce.
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170B owner, KCFD, CFI(I), ATP Multi
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