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  #1  
Old 02-05-2022, 07:42 PM
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ShookieRV7A ShookieRV7A is offline
 
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Location: Gilbert, AZ
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Default Question for you A&P's on my O-360

I have an O-360 parallel valve with dual P-mags and I've been chasing a CHT issue.

One thing I did was re-time the the PMags pet the people at emagair. After re-timing, I did a pull-through test to make sure the mags were timed together. All was well.

I also on the suggestion of a friend, did a pull through test in the REVERSE to see the results. And that's where I saw my left Pmag was firing first before the right.

Understanding that our engines only run in one direction, what could be the cause for this lag on only the right mag when doing a pull-through test in reverse?


I'm thinking of swapping the Pmags and see if the lag is still on the right side (doing the reverse pull-through test), which to me would suggest an issue (if it is an issue) in the accessory case. If the issue moves to the left, then that is inside the pmag.

Thoughts?
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  #2  
Old 02-05-2022, 08:14 PM
Scott Hersha Scott Hersha is offline
 
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Location: Cincinnati, OH
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Iím wondering why you would pull it through in reverse? Thereís a certain amount of lash in the gear train, and the engine is timed in one direction only, which takes into account the gear lash inherent in your engine. It seems normal that you would see a difference in timing when pulling the engine backwards.
If you are suffering from higher than normal CHTís, you could Ďfoolí your engine timing to a certain degree by adjusting the start timing on your eMags. Set your engine to 5* after TDC for engine start. The engine will transition to run after about 400 RPM after engine start. The advance curve will be retarded by 5* going forward from there. Instead of seeing 35-38* max advance (I donít recall what it is), you would actually be experiencing 5* less, which may result in lower CHTís. The engine will probably start better too with no chance of a kick back.
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  #3  
Old 02-05-2022, 08:51 PM
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ShookieRV7A ShookieRV7A is offline
 
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Quote:
Originally Posted by Scott Hersha View Post
Iím wondering why you would pull it through in reverse? Thereís a certain amount of lash in the gear train, and the engine is timed in one direction only, which takes into account the gear lash inherent in your engine. It seems normal that you would see a difference in timing when pulling the engine backwards.
If you are suffering from higher than normal CHTís, you could Ďfoolí your engine timing to a certain degree by adjusting the start timing on your eMags. Set your engine to 5* after TDC for engine start. The engine will transition to run after about 400 RPM after engine start. The advance curve will be retarded by 5* going forward from there. Instead of seeing 35-38* max advance (I donít recall what it is), you would actually be experiencing 5* less, which may result in lower CHTís. The engine will probably start better too with no chance of a kick back.
Yeah, that was the purpose of the most recent timing I did. I set it to +5 after TDC per the folks at Emagair.

The suggestion from a pilot friend to do the pull-through in reverse was to see if there was a difference in the forward timing versus the reverse. I understand that there will be some lash/slop in the gears - it just is suspect to me that the right mag lags behind the left in reverse.
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  #4  
Old 02-06-2022, 03:51 AM
sailvi767 sailvi767 is offline
 
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Is it a old wives tail or does it have some basis in fact? I was taught not to pull a engine through in reverse?
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  #5  
Old 02-06-2022, 06:33 AM
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Internally the PMag uses an optical encoder ring with an index pulse. Since they can be timed arbitrarily with as little as one pulse in the normal direction relative to the index pulse, there could be many more counted in the opposite direction. Timing in reverse being off would be expected. Checking to see if the timing is the same in reverse wont tell you anything even if you had mags and they were mechanically timed. Even with Slick mags there's a small variation in timing on the back side of the magneto cam relative to #3 firing position.
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  #6  
Old 02-06-2022, 08:21 AM
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ShookieRV7A ShookieRV7A is offline
 
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Quote:
Originally Posted by rocketbob View Post
Internally the PMag uses an optical encoder ring with an index pulse. Since they can be timed arbitrarily with as little as one pulse in the normal direction relative to the index pulse, there could be many more counted in the opposite direction. Timing in reverse being off would be expected. Checking to see if the timing is the same in reverse wont tell you anything even if you had mags and they were mechanically timed. Even with Slick mags there's a small variation in timing on the back side of the magneto cam relative to #3 firing position.
That's what I was thinking. But my hangar neighbor started rambling about "grounding the plane" and "engine rebuilds" if the Pmags are off on the reverse pull through test versus being correct on the forward pull-through test.
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  #7  
Old 02-06-2022, 09:03 AM
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Quote:
Originally Posted by ShookieRV7A View Post
That's what I was thinking. But my hangar neighbor started rambling about "grounding the plane" and "engine rebuilds" if the Pmags are off on the reverse pull through test versus being correct on the forward pull-through test.
Ignore him.
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  #8  
Old 02-06-2022, 09:27 AM
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sahrens sahrens is offline
 
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Default CHT issue

PMag timing is only one of many issues that could have an effect on CHTs. What is your specific CHT issue? Is related to all your cylinders or just one or two? Just in climb or during cruise?
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  #9  
Old 02-06-2022, 09:44 AM
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Do you still have an impulse coupler in there?
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  #10  
Old 02-06-2022, 09:46 AM
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ShookieRV7A ShookieRV7A is offline
 
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Quote:
Originally Posted by sahrens View Post
PMag timing is only one of many issues that could have an effect on CHTs. What is your specific CHT issue? Is related to all your cylinders or just one or two? Just in climb or during cruise?
So I've chased out all of the baffling leaks over the last two years, and have done the #3 cylinder mod to get cooling air to the bottom. All of my CHT's seemed to settle into my comfortable range under 380.

My #4 runs hotter by 30-40 degrees.

On a recent cross-country flight out to CA, we got to talking about the cooling and I tried a check of a single mag at cruise power and altitude.

Left Mag: All CHT's settled down to 350-370 - EGT's went up slightly as expected.

Right Mag: Same results except #4 remained hotter.

This is what prompted all of the discussion about he right mag timing and "reverse pull through test".

Although, After watching this video (https://www.youtube.com/watch?v=df1DwFNYFLs) I'm thinking I am moving to a too cool situation.
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