Low compression cylinder...contributes to vibrations?

alaskan9974

alaskan9974

Pre-takeoff checklist
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alaskan9974
Got the plane in the hangar the other day, was getting a slight vibration that wasn’t present before.

6 point cht/egt didn’t show much abnormal, but it was vibrating enough, some friends asked why it was shaking.

Checked compression and every cylinder was at 70+ except mid left #4 was at 27/80. Scoped it and was burnt with blowby under the cover so I pulled the jug.

Bought an new harness also, waiting for my a&p to stop by tomorrow to install it and the reman cylinder.

Would the low compression of one cylinder be enough to introduce vibrations? Not to the point of roughness and shaking stuff apart but a vibration nonetheless.
 
yes in a 4, I lack that experience in a 6.
 
I had a 20 in a 6 and didn't notice it at all and I'm very sensitive to monitoring that.
 
run it for a hour and recheck.
 
Domenick said:
What does this mean?
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Exhaust valve was coloring with a lot of blowby under valve cover.

Pilot Steve said:
I had a 20 in a 6 and didn't notice it at all and I'm very sensitive to monitoring that.
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It’s a IO520, was the center left or #4.

I did pull the mags also for him to inspect, Bendix 1200s. One was replaced about 30 hours ago but the harness didn’t arrive in time so was changing that as well.

Vibration wasn’t centered to a specific mag though it did become a little more apparent on one from L to R.
 
A sticky valve would cause vibration but then your cht/egt would probably have shown that.
 
Witmo said:
A sticky valve would cause vibration but then your cht/egt would probably have shown that.
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Sticking valves usually have audible misfiring as a symptom that will gradually improve as the power advances and engine temps rise, rather than just vibration. EGTs will be very low and MP will be high and it’s more severe too, as it will eventually bend the pushrod(s).
 
Ryanb said:
Sticking valves usually have audible misfiring as a symptom that will gradually improve as the power advances and engine temps rise, rather than just vibration. EGTs will be very low and MP will be high and it’s more severe too, as it will eventually bend the pushrod(s).
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Maybe usually, but not always. I've seen a couple hung valves that follow the exact trend that Witmo and the OP are describing.
 
Snowmass said:
I do not really know but I hope you will tell us after replacing that cylinder. Then we will all know.
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Helped a little, but still a little vibe on the R mag and a little on L at high power. Nothing on ground run ups.

Mechanic thinks condenser is bad on R. I checked logs, it’s about 10 hours since I got it from spruce a few months ago so will see if there is a warranty.

Thanks for the replies will see if a new mag helps.
 
0-470, right? Add some isopropyl to the fuel. A little jug per 10 gallons is approved by Continental and Lycoming and won't hurt fuel bladders. Getting a few drops of water accumulated in the carb bowl will interfere with normal fuel pickup through the main jet.
 
Confusing. If vibration on both mags then why replace condenser on right mag. Please keep us updated. Also, are you saying that the very low compression had very little effect on vibration? However, fixing that cylinder still needed to be done.
 
Snowmass said:
Confusing. If vibration on both mags then why replace condenser on right mag. Please keep us updated. Also, are you saying that the very low compression had very little effect on vibration? However, fixing that cylinder still needed to be done.
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It feels like a misfire, timing is spot on. L mag doesn't run butter smooth on its own but it is very noticeable on R. Worse at high power.

Bought a Surefly mag for the left, and a new mag for the right. Will send this one back to Kelly and see if it was the cause. They told us not to open it up, or remove any bolts due to warranty so can't check inside.
 
Stewartb said:
0-470, right? Add some isopropyl to the fuel. A little jug per 10 gallons is approved by Continental and Lycoming and won't hurt fuel bladders. Getting a few drops of water accumulated in the carb bowl will interfere with normal fuel pickup through the main jet.
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It is a IO-520, have done the following;
  • Replaced #4 Jug rest at 68+
  • New Harness
  • New Tempest plugs on bottom and cleaned and tested top plugs (less then 20 hours on tempest)
  • R Mag is overhauled with less then 15 hours, old one had arcing inside so replaced it
  • L Mag is same hours, so replacing it with a surefly
 
Don't forget to look for other possible causes of vibration: motor mounts, cracked engine mount, prop imbalance, fuel injection. Also, a good static compression test does not always mean the cylinders are producing the desired compression. I had a Cessna 150 with an O-200 that had a moderate vibration with perfect static compression. It turned out to have camshaft lobe wear.
 
Some small vibration increase on single mag is often normal.

However, if I understand you even very low cylinder compression had little effect on vibration.

However, there is a difference between vibration and roughness so I am not sure which you mean. Vibration is a very regular shaking whereas roughness is more irregular.
 
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lsaway said:
Don't forget to look for other possible causes of vibration: motor mounts, cracked engine mount, prop imbalance, fuel injection. Also, a good static compression test does not always mean the cylinders are producing the desired compression. I had a Cessna 150 with an O-200 that had a moderate vibration with perfect static compression. It turned out to have camshaft lobe wear.
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We installed the surefly mag, but it wouldn't run, disconnected the P lead and it started firing, there may have been a short on the wire or switch that didn't shut the regular magneto off, so we are replacing the wires and ignition switch, hope this solves it.
 
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