O-300A roughness

Tom-D said:
Your chart is a great one, now draw a line for the second plug firing, draw it so it doubles the width of the existing line. see how much the pressure has risen in that time. It does, but is of no significant amount. good theory, doesn't mean a thing in the real world.
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Tom- sounds good but it doesn't answer the question because the other plug didn't fire. The question was whether the other plug can't fire due to the pressure increase. Your earlier reply only addressed the change in adiabatic compression, which I agree is nearly negligible when the piston is near the top of it's range and moving slowly, but not the change in pressure due to the burning fuel.

Tom-D said:
In a normally running engine like the 0-300 when you run on one mag, ignition starts at 1 plug and the flame front must progress all the way across the combustion chamber, switch to the other mag and it will travel in the opposite direction, switch to both and the flame front will meet in the center.
Varying the timing by 10 degrees on one mag will not vary the meeting point and significant amount.
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Maybe, maybe not. Which mag, and 10 degrees which direction? 10° advanced or 10° retarded? Even so, the problem was that the plug that was set to ignite 2° later wasn't firing, so it doesn't matter what the timing difference is, other than it was 2° later.

Tom-D said:
Even if it did, it would make no difference because all the fuel still gets burned and the pressure in the cylinder will stay the same. Thus the RPM will change only when 1 mag is off, simply because all the fuel will not be burned when the flame front must travel all the way across the combustion chamber. So the pressure will be reduced. RPM will change.
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Again, all that matters it the pressure build-up from the first plug that fires. Is it enough to stop the second plug from firing? The fuel must burn in about 1/4 of a revolution, or about 2 milliseconds. Using your numbers for the time it takes to move 2°, that is roughly 4% of the burn time.

A reasonable question is whether the relatively small pressure increase from combustion during that time is enough to stop the other plug from firing.
 
my point was the pressure rise is not enough to make any difference in wether the second plug will fire.
If the second plug is not firing there is a totally different reason.
 
Cap'n Jack said:
Tom- sounds good but it doesn't answer the question because the other plug didn't fire.
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I am not clairvoyant, and quit trouble shooting on line a long time ago.
 
Tom-D said:
my point was the pressure rise is not enough to make any difference in wether the second plug will fire.
If the second plug is not firing there is a totally different reason.
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You sure had a funny way of making that point, using non sequiturs such as the response below:

Tom-D said:
In a normally running engine like the 0-300 when you run on one mag, ignition starts at 1 plug and the flame front must progress all the way across the combustion chamber, switch to the other mag and it will travel in the opposite direction, switch to both and the flame front will meet in the center.
Varying the timing by 10 degrees on one mag will not vary the meeting point and significant amount.
Even if it did, it would make no difference because all the fuel still gets burned and the pressure in the cylinder will stay the same. Thus the RPM will change only when 1 mag is off, simply because all the fuel will not be burned when the flame front must travel all the way across the combustion chamber. So the pressure will be reduced. RPM will change.
Click to expand...

Tom-D said:
I am not clairvoyant, and quit trouble shooting on line a long time ago.
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Actually, my question was answered, whether the increase in pressure due to ignition was sufficient to stop a second plug from firing...through my own efforts. @Checkout_my_Six provided more support when he reminded us that the charge is still burning when running on one plug, so the increase in pressure is even lower than my estimate, since the fuel is burning slower than I thought.

To summarize: As you stated, the change in pressure over 2° of rotation is insufficient to prevent a second plug from firing, although you you only looked at adiabatic compression. It was questioned whether the pressure from ignition also contributed to the pressure in the cylinder, but your replies didn't address that situation. Looking at the timing, I can conclude that the ignition from the first spark plug also doesn't contribute much to a pressure change over 2° near TDC. Given that you claimed that ammonia was acidic, I really don't simply accept your statements anymore.
 
Plugs were 400 hrs old, champions. Oil change was due so I did that and went to tempest all new plugs. By and large the roughness went away. Interesting that despite the plugs the mag check was okay through this whole time. 50ish rpm drop and no difference between the two mags.
 
3 in the green said:
Plugs were 400 hrs old, champions. Oil change was due so I did that and went to tempest all new plugs. By and large the roughness went away. Interesting that despite the plugs the mag check was okay through this whole time. 50ish rpm drop and no difference between the two mags.
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got pictures of old plugs?
 
Here’s a few of the champions.


5f475b0b89024e06b538c2f210a4ced1.jpg


0d05ccd2bf2a20c9db3486a8063ee566.jpg



Sent from my iPhone using Tapatalk
 
First picture, looks great, I'd suggest it was a top plug.
Second picture, that plug is beyond allowable limits, but still running good.
 
Tom-D said:
First picture, looks great, I'd suggest it was a top plug.
Second picture, that plug is beyond allowable limits, but still running good.
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Tom, look closer. Aren't those little lead balls I see between the center conductor and the ground lugs?

Jim
 
weirdjim said:
Tom, look closer. Aren't those little lead balls I see between the center conductor and the ground lugs?

Jim
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Doesn't look like that to me.
it's the shoulder of the ceramic to me
 
All you need to know is that they're old Champions. Those had a spring-loaded resistor in them that was phenomenally unreliable. An awful lot of POAers have had Champ trouble, until Champion rather quietly changed the resistor design to the integral type. They bought Auburn, the best aircraft sparkplug ever, around 12 years ago, and shut them down. They would have been smarter to replace their line with the Auburns.

Read this and educate yourself: https://blog.aopa.org/aopa/2015/03/19/champion-from-denial-to-acceptance/
 
Tom-D said:
No way in hell to know that.
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Did you read the article? It's easy to separate the old from the new. And unless he put 400 hours on them since the new ones came out four years ago, they'll be the old ones. And I bet there are still old ones in many parts rooms.
 
Dan Thomas said:
Did you read the article? It's easy to separate the old from the new. And unless he put 400 hours on them since the new ones came out four years ago, they'll be the old ones. And I bet there are still old ones in many parts rooms.
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By looking at the pictures there is no way you can tell what brand they are.

"All you need to know is that they're old Champions".
 
Tom-D said:
By looking at the pictures there is no way you can tell what brand they are.

"All you need to know is that they're old Champions".
Click to expand...

In post #12 of this thread the OP said the plugs had 400 hours on them. In post #28 I suggested that if they were Champions with 400 hours on them, that was likely his problem. And in post #48 he said this:

Plugs were 400 hrs old, champions. Oil change was due so I did that and went to tempest all new plugs. By and large the roughness went away. Interesting that despite the plugs the mag check was okay through this whole time. 50ish rpm drop and no difference between the two mags.

No guessing. It was all there the whole time.
 
Tom-D said:
By looking at the pictures there is no way you can tell what brand they are.

"All you need to know is that they're old Champions".
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Do I need to repeat my self?
 
3 in the green said:
I'll get right to it...

New/first owner here. '53 C170B with an O-300A and a 7653 prop. Overhauled early 2017, ~270 hrs SMOH. Prebuy and ~16 hour ferry over and ran smooth like an O-300 should.

First flight at new home and takeoff is anemic, climb-out less than stellar, and noticeable (but not terrible vibration). Not stumbling or coughing, but vibration and less than full power. Static runup not too far off, about 2,200, on the low side. At first, vibrations only present themselves at higher (1,500+) rpm, then as engine warmed up the vibrations worsened to a degree and also were noticeable at lower rpm. Mag checks are perfect, 50-75 rpm drop, no difference between the two.

A few other facts (clues?):
-previous owner ran mogas a lot, or a blend with 100LL
-now since being ferried it's 100% 100LL
-fuel selector rebuilt on the ferry flight over as it started leaking
-annual was completed early March as part of my pre-buy and new carb, left mag, timing set, intake gaskets as there was a leak there

I'm thinking maybe a slightly sticking valve? Would a switch to 100LL cause this so soon?
Click to expand...
 
Go to the: cessna170.org website and ask where all the 170 experts are except rather than here,
 
3 in the green said:
I'll get right to it...

New/first owner here. '53 C170B with an O-300A and a 7653 prop. Overhauled early 2017, ~270 hrs SMOH. Prebuy and ~16 hour ferry over and ran smooth like an O-300 should.

First flight at new home and takeoff is anemic, climb-out less than stellar, and noticeable (but not terrible vibration). Not stumbling or coughing, but vibration and less than full power. Static runup not too far off, about 2,200, on the low side. At first, vibrations only present themselves at higher (1,500+) rpm, then as engine warmed up the vibrations worsened to a degree and also were noticeable at lower rpm. Mag checks are perfect, 50-75 rpm drop, no difference between the two.

A few other facts (clues?):
-previous owner ran mogas a lot, or a blend with 100LL
-now since being ferried it's 100% 100LL
-fuel selector rebuilt on the ferry flight over as it started leaking
-annual was completed early March as part of my pre-buy and new carb, left mag, timing set, intake gaskets as there was a leak there

I'm thinking maybe a slightly sticking valve? Would a switch to 100LL cause this so soon?
Click to expand...
 
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