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Intermittant Slick Mag check issue

BH1166

BH1166

Well Known Member
I have reviewed the SLICK sticky, learned lots, but nothing about my issue. An example of the issue is, first mag check of the day all is good, second likely same, third for the day not so much, 150-200 drop on left impulse coupled mag. The drop is not correct, but the engine runs as if someone pulled a spark lead or two off...horrible roughness. I can leave it at mag check RPM and no change, regardless how long I wait, increasing RPM only increases the shaking and roughness. I go back to both and move up to cruise RPM and it clears. I can then do a normal mag check and all is good, 50-80 rpm drop, nearly same left to right. I went nearly two weeks and no flights and pulled all the plugs, thinking maybe oil, all were clean and grayish brown, dry. Not finding a pattern, has happened first mag check of the day, or the fourth, sometime multiple times in a day, sometimes like yesterday, 4 checks over 7 hours, one issue. Issue has been going on for 6 months. Mags are 355 hours since new, along with M2989LH M2990RH harness and all new UREM40E plugs gapped .018. I also have SLICK START. O320/160/FP

Thanks
 
Ok, guess I will change plugs

50 miles from AS so will go grab 4 new UREM40E plugs and replace all on left mag. QUESTION..... Are new plugs generally gapped within range? I will check, but wondering should I buy gapping tool as well. Thanks
 
`Experience suggests new plugs are gapped correctly for something - but rarely for the airplane on which i am installing them....
 
I suspect you are on the right track; you could try checking internal resistance in the old plugs. Next likely culprit here is the harness == either a deteriorating wire, or distributor cap/ contact.
 
Rough mag

I had similar issue last week at a field after 2 hr flight mag check 3 times before departing retained same results decided to fly 15 minute leg home landed checked mags again on ground had same results pulled all plugs out #3 plugs fouled so bad I thought plugs were gonna have to be replaced, #4 plug top not as bad, took to my local mechanic he said I need to practice better leaning management on taxing. Cleaned plugs glass blast checked spark back in action mag drops no more than 50 left right 3 hrs since.
 
Had similar issue with mine. Turned out to be coil failing in mag. Mine would clear itself when cool and then acts up after thoroughly heat soaked.
 
Plugs with high resistance or a bad coil in the mag will have these symptoms, especially when hot.


Vic
 
Well I have 4 new plugs...and a new tool. If coil, the mag will need to be sent off, not a field repair? Plugs are over due to be replaced on age alone. Thanks
 
Mags can be repaired in the field but you'll need the appropriate tools and knowledge. Plugs with high resistance (> 4000 ohms) can cause the coil to go bad. Age of plugs is not a problem, hours of operation result in an increased gap, which must be corrected every 100 hrs or so. Too large a gap = higher resistance and potential mag damage.
 
BH1166 said:
I have reviewed the SLICK sticky, learned lots, but nothing about my issue. An example of the issue is, first mag check of the day all is good, second likely same, third for the day not so much, 150-200 drop on left impulse coupled mag. The drop is not correct, but the engine runs as if someone pulled a spark lead or two off...horrible roughness. I can leave it at mag check RPM and no change, regardless how long I wait, increasing RPM only increases the shaking and roughness. I go back to both and move up to cruise RPM and it clears. I can then do a normal mag check and all is good, 50-80 rpm drop, nearly same left to right. I went nearly two weeks and no flights and pulled all the plugs, thinking maybe oil, all were clean and grayish brown, dry. Not finding a pattern, has happened first mag check of the day, or the fourth, sometime multiple times in a day, sometimes like yesterday, 4 checks over 7 hours, one issue. Issue has been going on for 6 months. Mags are 355 hours since new, along with M2989LH M2990RH harness and all new UREM40E plugs gapped .018. I also have SLICK START. O320/160/FP

Thanks
Click to expand...

I see no mention of whether you have EGT on all cylinders?

If so, it can be a valuable troubleshooting tool.
Look for a pattern when the problem is occurring.
If it is always an abnormal EGT on one or two cyl, focus on that.
It it is random EGT variation on the entire engine it would point more to something common in the mag like coil, points, condenser, etc.
 
Thanks Ray, I have neither, good to know it is possible. I have over the last few days read several times about resistance harming mags. Learning!


Scott I do have EGT and appreciate the insight. Issue has only been evident during mag check, and disappears once it clears. I have not seen anything in cruise abnormal.
 
BH1166 said:
Thanks Ray, I have neither, good to know it is possible. I have over the last few days read several times about resistance harming mags. Learning!


Scott I do have EGT and appreciate the insight. Issue has only been evident during mag check, and disappears once it clears. I have not seen anything in cruise abnormal.
Click to expand...

Observing EGTs during a bad mag check can sometimes provide major clues.
 
In addition to the other suggestions a failing mag condenser can cause intermittent issues.
 
rvbuilder2002 said:
Observing EGTs during a bad mag check can sometimes provide major clues.
Click to expand...

Focused on RPM roughness.....will redirect focus to EGTs next occurrence. Thanks for heads up Scott!

Thanks Walt for another potential problem/solution.

New plugs going in today. The removed plugs will get a closeup inspection tonight for discrepancies/visible issues, and I will check resistance on each. Life/weather has kept me away for last few days.

Thanks !
 
Installed new Tempest Plugs

New plugs were verified to be gapped .018. Installed. Plugs removed looked near new from a wear perspective. Still .018 gap, electrode still round, not football shape. Looking under magnification shows very little lead deposits. Though log book said Tempest plugs were installed, not the case, the four (1/3 top, 2/4 bottom) , all Champion REM40E. My multimeter is self adjusting for range, so unsure exactly what it is telling me other than the resistance of 2 are waaay different than the other two. Per my meter, multiple times, I got the following: .910 / .201 / 1.202 / 1.322. I can say, I have flown multiple times, one flight had 4 mag checks , 2 were with 30 minute leg flights back to back with landings and subsequent mag checks, no rough left mag check ! So, the left mag check roughness is not appearing in 3 flights. What do others see as to the resistance readings I got. Thanks
 
Walt said:
In addition to the other suggestions a failing mag condenser can cause intermittent issues.
Click to expand...

You might also look to see if it?s a little loose. I once experienced one that wasn?t tight and it caused intermittent fowling and roughness.
 
I have had more than a few mags go bad before their 500 hour overhaul is due- Probably 50% of them. You say all of your plugs look good, so I would stop looking there. Start with the most obvious (to me anyway) is the magnetos. You are close enough to 400 hours that if it were me I would have them overhauled.
 
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