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Question for you A&P's on my O-360

ShookieRV7A

Well Known Member
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?
 
I’m wondering why you would pull it through in reverse? There’s 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 CHT’s, you could ‘fool’ 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 don’t recall what it is), you would actually be experiencing 5* less, which may result in lower CHT’s. The engine will probably start better too with no chance of a kick back.
 
I’m wondering why you would pull it through in reverse? There’s 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 CHT’s, you could ‘fool’ 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 don’t recall what it is), you would actually be experiencing 5* less, which may result in lower CHT’s. 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.
 
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?
 
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.
 
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.
 
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.
 
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?
 
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.
 
Find new friends.

Cooling issues are as simple as "no fire" in engine runs. Almost always one of three things:

Insufficient fuel
Insuficient Delta P
Incorrect ignition timing.
 
800+ hours on my unairworthy (?) RV.

Listen to to Dan & Scott H above. I have 4 A&Ps in my area. They know absolutely NOTHING about Pmags. They’ll admit it. The E-Mag Air installation manual and their troubleshooting manual have the info to follow. Brad is helpful if needed.
 
troubles we have

I've never ran my engine in reverse rotation. But, common sense tells me to time the ignition system in a forward rotation.

I have had so many smart mechanics tell me the weirdest stuff in my life. I just ignore them and find somebody else if it doesn't pass my common sense test.
Ooh, the stories I could tell.
Art
 
A&P

I learned decades ago how little practical knowledge most A&P's have about engines. I cringe every time I see a post " I assembled my engine with help from an A&P."
 
I learned decades ago how little practical knowledge most A&P's have about engines. I cringe every time I see a post " I assembled my engine with help from an A&P."

Yeah. My friend/hangar neighbor is a smart guy, but he's not an A&P which is why I was asking here. He's just a bit melodramatic at times.

Why I would check the timing in reverse left me scratching my head. But he did bring up a good point about the lag we observed in right mag vs. left.

So when he started going on about 'grounding the plane' and 'tear-down of the engine' I figured I would pump the brakes and start asking for other opinions.

It seems logical to me that a bit of lash is to be expected. And since this is a rotating part only in one direction I wasn't overly concerned. Just wanted to check with the group and see what if anything folks had to say.
 
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?

That is true of an engine with a vacuum pump using graphite vanes - they take a "set" in the direction of rotation and can crack/break if rotated backwards and they hang up. That's the only component on the engine that cares about direction of rotation, and if you have an electronic EFIS and no vacuum system, then even that is gone.
 
Pmags

I have an O-360 with a PMag and a Bendix mag. I changed to running the PMag on the B curve which helped with cooling. I chased the cooling leaks with RTV to make sure I wasn’t loosing any air. My CHTs run in the 350 range except on extended climb, if I lean aggressively they will peak around 380. I’m not comfortable running at that temperature so I just don’t lean that much. Due to the inefficient air/fuel movement inherent in O versus IO I can not run lean of peak. I have read Mike Busch comments on leaning but he would agree that a carb engine may not get to lean of peak.

You may find some relief changing to the B curve. If you are not sure how to do that find someone close by that can help. Some folks have reported better cooling switching to the BR9 series of plugs. I didn’t but you might.

I do know checking timing it the reverse direction buys you nothing except confusion. Fortunately the A&P program I attended expected you to be able to rebuild an engine so it would and did run. A good foundation for going into the maintenance world
 
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Not trying to Dis your hangar buddy, sorry about that. Just a lot of things around that folks get hung up on and think are gospel. Maybe if we knew the whole story ...
 
Find new friends.

Cooling issues are as simple as "no fire" in engine runs. Almost always one of three things:

Insufficient fuel
Insuficient Delta P
Incorrect ignition timing.

Depending on where you are running the engine (i.e., ROP or LOP) the hot cylinder could also be getting too much fuel. Mixture has a huge impact on CHT's.

It would be helpful if you could get a graph of CHT vs fuel flow, starting well rich of peak and slowly leaning it out over a couple minutes' time. Lean it until it really is losing power and (likely) running rough. Do this at some power at or less than 75%. This will tell you how evenly distributed the mixtures are among the cylinders. Look up "GAMI spread" to learn more.
 
Not trying to Dis your hangar buddy, sorry about that. Just a lot of things around that folks get hung up on and think are gospel. Maybe if we knew the whole story ...

It's all good. I just needed to do a sanity check here on VAF. After a ton of research, like reading the entire Lycoming and Superior rebuild manuals, I found out that the idler gear and magneto gear allow for .004 to .015 gear lash. If I do the math correctly, that means that by the time you measure something in the reverse at the magneto gear, you can be up to .030 difference from the crank gear.

So I was able to resolve the "accessory case rebuild" and "grounding" talk.
 
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?

Nothing to do with OP, but...

I was told that some A&Ps warn not to pull the engine through in reverse because the older style vacuum pumps were designed to turn in only one direction. Pulling the engine through backwards could cause the vanes inside the vacuum pump to crack and fail.
 
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Nothing to do with OP, but...

I was told that some A&Ps warn not to pull the engine through in reverse because the older style vacuum pumps were designed to turn in only one direction. Pulling the engine through backwards could cause the vanes inside the vacuum pump to crack and fail.

Yeah. And I knew that part, but since I'm vacuum-less in my plane, I wasn't worried about that.
 
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