High mag drop

[willirw]

My mag drop on my left mag has gone from its usual 75 rpm drop to 125. ( at 1750, at 2000 it drops 90 rpm, the right mag drop is 35, I have a digital tac) I have tried the following corrective actions and still no change. Cleaned gaped tested and rotated plugs. Pulled left mag and had IA open it up. Everything looked good to him, reset the E gap and reinstalled mag and timed mags. Replaced left harness with new, tested harness before installation. Removed plugs and turned engine over with starter, strong spark ( jumped 1/4 inch or better to ground). I find no fuel staining on the intakes, all the primer lines are good. The engine has 250 SMOH and the mags were purchased new and have the same time on them. I am beginning to think I have a cylinder problem? Thought I would remove the rocker covers tomorrow for a look. I'm not using any oil, less than 1/2 qt in 25 hrs and the belly stays clean. Any ideas I could discuss with my IA tomorrow would be most welcome.

Thanks, Randy

 

[Grant]

A buddy is chasing the same problem. They thought it was a bad plug but after swapping plugs the problem stayed on the same mag. They are going to just rebuild the mags because they have 20 years on them and they have checked all the usual suspects like you have.

 

[dstr59]

well.... just cause the mag switch SAYS left, does that mean its really operating on the left mag? maybe a check of the p-leads for orientation is in order (gee nobodys ever made that mistake before). remember,if you use your mag timing box, the light goes out when the switch grounds the lead.
hey ! is the left mag got an impulse coupling? that could be hanging up, leaving the timing retarded.
that mag drop is it a smooth drop or a rough one? rough ,would indicate a problem on the secondary side of the system (anything downstream of the coil) which i think you've fairly well adressed. a smooth drop, would indicate something on the primary side, points etc.

my bet, would be either the impulse has hung up or your working on the wrong mag.
good luck

 

[mike mcs repair]

well.... just cause the mag switch SAYS left, does that mean its really operating on the left mag? maybe a check of the p-leads for orientation is in order (gee nobodys ever made that mistake before).

Yes check......

remember,if you use your mag timing box, the light goes out when the switch grounds the lead.

Depends on version of timing tool, induction ones are opposite of a buzz box style .....



Start, run on bad mag only for 30 seconds, shut off, open cowl, spit on finger touch each exhaust, repeat.. Find cooler one..
If all are evenly hot it is inside mag issue like a trace on distributor block or bad coating

if one is colder take plugs out of that cylinder and swap with one other cylinder.. Repeat this test, if problem followed plugs, you have a bad plug

if problem stayed with that cylinder, you have a mag,harness, block, plead or switch issue...

Also try disconnecting switch and manually grounding and ungrounding, to see if you have a trace of conductive stuff in mag switch...

Check for broken plead insulation inside plead nut, that can be quite intermittent....

Mag noise filter flaking out, swap from mag to other mag see if problem follows ...

 

[Steve Pierce]

How does it check in flight? when leaned?

 

[willirw]

mag drop

Thanks all for the replys. I will review them with my IA tomorrow. We did check and the mag switches are on the correct mag. Steve in cruise 2400 rpm the mag drop is 85 on each mag when leaned. If all else fails I have a friends cub in the hangar being annualed. I may steal his mag and see if it makes a difference.

Thanks Randy

 

[OLDCROWE]

I went through having a possessed Mag. last year... have fun. We chased it for days; ended up 100% rebuild on both Mags (corrected a past sin in the process) only to finally find one of the RF filters was shorting out through the clamp causing the Mag to go South!