O-300A roughness

3 in the green

3 in the green

Line Up and Wait
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c170b
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?
 
I'd guess the new carb and magneto are starting points, since the engine ran good before they were installed.

Tom-D will have comments about the carburetor.
 
With the good mag checks the plugs may ‘seem’ O.K.? Did you ferry home across higher elevations?

With the recent annual, I’m sure the plugs were looked at, age, hours of plugs?
 
my first things to look at....timing and fuel flow.

Because the mag drops are normal.....I'd lean more towards a timing adjustment.
 
Thanks for the suggestions - will be starting with the plugs to look for anything obvious.
 
First thing I'd do is pull the plugs and see if they are fouled.

Why was the carb changed? what is the part number of the carb? it is on the data tag.

how many hours have you run 100LL?
 
3393RP said:
I'd guess the new carb and magneto are starting points, since the engine ran good before they were installed.

Tom-D will have comments about the carburetor.
Click to expand...

Engine also ran good for awhile after the carb and mag (at least a couple dozen hours). But those are the most recent major items that were done.

Ferry went through higher elevations (NM/AZ) without a hitch.
 
Tell us how you lean.
 
3 in the green said:
Engine also ran good for awhile after the carb and mag (at least a couple dozen hours). But those are the most recent major items that were done.

Ferry went through higher elevations (NM/AZ) without a hitch.
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sounds to me like the typical fouled plug due to running too rich.
I ask again, why was the carb changed?
 
Sorry I don't think carb was changed, but overhauled. Previous owner was trying to diagnose an issue and ended up being intake leak, that was resolved and ran perfect since.

Plugs are about 400ish hours old, change to 100% 100LL was about 15 hours ago. I lean aggressively on the ground, to stumble and then in from there. But, I haven't flown it enough to have any build-up.

Will pull the plugs first this weekend (doing an oil change anyway) and will see what we see.
 
3393RP said:
Tom-D will have comments about the carburetor.
Click to expand...
Last two yellow tag carbs from Kelly rebuilt, that I have installed ran OK. although they do run richer, the old carbs ran 6.5 to 7.5 gallons per hour leaned. the new yellow tagged Kelly re-builds run about 7.5 to 8.5 gallons per hour.
 
Will get back to you on the carb tonight, Tom. Don't have it in front of me as everything is at home, and I don't want to guess. Appreciate you taking the time!
 
3 in the green said:
Will get back to you on the carb tonight, Tom. Don't have it in front of me as everything is at home, and I don't want to guess. Appreciate you taking the time!
Click to expand...
When you do tell me what color your pipes are running.
and you're welcome.
 
I like to start with the easy stuff, that the owner/pilot can handle. That often is the plugs themselves. Besides any changing, inspection itself may yield some clues.

I learned my lesson back in my formative years, changed a starter on a Chevy when all I had was a bad battery.
 
3 in the green said:
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.
Click to expand...
This is very typical of a loose Venturi.
 
sourdough44 said:
I like to start with the easy stuff, that the owner/pilot can handle. That often is the plugs themselves. Besides any changing, inspection itself may yield some clues.

I learned my lesson back in my formative years, changed a starter on a Chevy when all I had was a bad battery.
Click to expand...

A lot of batteries get replaced. Then the starters. Then the $20 starter contactor, which was the whole problem in the first place. In airplanes, the master contactor has to be suspected as well. Starter current runs through it.

Champion plugs (the old style with the screw-held resistor and spring) with 400 hours on them are very likely the problem. That resistor setup was a first-class pain, and often made plugs fail the spark test at the first cleaning. I've had Champs fail at 50 hours. Had one fail right out of the box. And a mag check doesn't always find the trouble, either. And with some engines, like the O-300, that have the mags set at two different timings, the first one fires and by the time the second one fires the combustion pressure is already coming up a bit, and the second plug, if it's weak, can't fire. You get vibration. Checking the mags means one mag isn't firing, so that phenomenon doesn't happen and everything seems OK. I have fixed vibration like that once or twice by pressure-testing the plugs and replacing whichever one doesn't spark reliably at 140 psi or so.
 
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Dan Thomas said:
A lot of batteries get replaced. Then the starters. Then the $20 starter contactor, which was the whole problem in the first place. In airplanes, the master contactor has to be suspected as well. Starter current runs through it.

Champion plugs (the old style with the screw-held resistor and spring) with 400 hours on them are very likely the problem. That resistor setup was a first-class pain, and often made plugs fail the spark test at the first cleaning. I've had Champs fail at 50 hours. Had one fail right out of the box. And a mag check doesn't always find the trouble, either. And with some engines, like the O-300, that have the mags set at two different timings, the first one fires and by the time the second one fires the combustion pressure is already coming up a bit, and the second plug, if it's weak, can't fire. You get vibration. Checking the mags means one mag isn't firing, so that phenomenon doesn't happen and everything seems OK. I have fixed vibration like that once or twice by pressure-testing the plugs and replacing whichever one doesn't spark reliably at 140 psi or so.
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Tell me how much time it requires an engine to travel 2 degrees @2700 RPM. then tell us how much the pressure in the cylinder can rise in that time.

Remember the timing is 26 +or- 1 and 28 +or- 1 so any one that knows the 0-300 series times the engine at 27 degrees. both lights go out half way between the two
 
Tom-D said:
Tell me how much time it requires an engine to travel 2 degrees @2700 RPM. then tell us how much the pressure in the cylinder can rise in that time.
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Just enough. I clued into it when I read about it in some engine manufacturer's publication a long time ago. And I found it once or twice.
 
Dan Thomas said:
Just enough. I clued into it when I read about it in some engine manufacturer's publication a long time ago. And I found it once or twice.
Click to expand...

So if you'd simply swapped the two plugs it would've run fine (because now the weak plug isn't firing in the higher pressure). Interesting.
 
Dan Thomas said:
Just enough. I clued into it when I read about it in some engine manufacturer's publication a long time ago. And I found it once or twice.
Click to expand...
Do the math, the prop travels 360 degrees in one revolution. X 2700 in one minute. that's 972,000 degrees in one minutes. so 2 degrees would take how long?
So we are talking same syndrome as dynamic compression, not enough time to make a difference.
I've seen 0-300s out of time by as much as 10 degrees on one mag and not effect the mag drop, enough to worry a pilot.
I've been known to play with mag timing on the test cell, 2 degrees is probably closer than most A&Ps can get when timing the 0-300.
Many theories don't work in practical daily operation.
Have you thought why TCM put the timing 2 degree apart on the 0-300?
 
the time it requires to do two degrees at 2700 RPM. .000123456790 of a second
 
and that can mean the difference between a fire breathing combustion event out the exhaust vs. a fire consuming cylinder "heating" event.....
 
Back to the original post, I did have an occasional sticking valve on an O-300 years ago.
 
Tom-D said:
Do the math, the prop travels 360 degrees in one revolution. X 2700 in one minute. that's 972,000 degrees in one minutes. so 2 degrees would take how long?
So we are talking same syndrome as dynamic compression, not enough time to make a difference.
I've seen 0-300s out of time by as much as 10 degrees on one mag and not effect the mag drop, enough to worry a pilot.
I've been known to play with mag timing on the test cell, 2 degrees is probably closer than most A&Ps can get when timing the 0-300.
Many theories don't work in practical daily operation.
Have you thought why TCM put the timing 2 degree apart on the 0-300?
Click to expand...
Tom-D said:
the time it requires to do two degrees at 2700 RPM. .000123456790 of a second
Click to expand...

If one plug has fired, that means the fuel-air mixture is also burning. How much is the pressure going up during this time? The image below suggests that cylinder pressure increases rapidly over a small angular range.

Start-of-combustion-and-combustion-duration-definition-indicating-net-accumulated-heat.png

Dan Thomas said:
And with some engines, like the O-300, that have the mags set at two different timings, the first one fires and by the time the second one fires the combustion pressure is already coming up a bit, and the second plug, if it's weak, can't fire.
Click to expand...
Adding in the pressure increase from the burning fuel, it seems the theory isn't implausible.
 
Cap'n Jack said:
If one plug has fired, that means the fuel-air mixture is also burning. How much is the pressure going up during this time? The image below suggests that cylinder pressure increases rapidly over a small angular range.

Start-of-combustion-and-combustion-duration-definition-indicating-net-accumulated-heat.png


Adding in the pressure increase from the burning fuel, it seems the theory isn't implausible.
Click to expand...
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.
 
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.
 
if there was an engine monitor to see the temps....in Tom's scenario the EGTs would rise with one plug firing. A partially combusted charge continues to burn in the exhaust chamber.
 
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