Air pressure crankcase TC96
Seasoned HDF Member
Joined: Mar 2021
Posts: 18,496
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From: backwoods
I really don't know. Vacuum has potential to suck in containments. Don't those old bikes of yours blow oil?
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Tourer
Joined: Feb 2008
Posts: 267
Likes: 13
From: Philadelphia
Well I went ahead and replaced the breather assemblies both heads with Harley kit 17025-03A and everything fine now. No air at all evident when removing oil cap. Bike runs great. Thanks to all for your input and support.
Jim
Jim
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From: Honah Lee
@jim70. Did you pull the breathers apart you removed to see the issue.
The one way poppet valves are one way out. They function on the downstroke of the two pistons.
The two pistons share a common crank pin, so they share the same direction except briefly at the bottom and top of stroke.
If your engine rings are sealing well, there is going to be a brief time of low vacuum in the crank case when both pistons are on the uptroke.
Good chance, depending on where you push or key the engine off depends on what you will hear as you break the seal on the dipstick.
The one way poppet valves are one way out. They function on the downstroke of the two pistons.
The two pistons share a common crank pin, so they share the same direction except briefly at the bottom and top of stroke.
If your engine rings are sealing well, there is going to be a brief time of low vacuum in the crank case when both pistons are on the uptroke.
Good chance, depending on where you push or key the engine off depends on what you will hear as you break the seal on the dipstick.
Last edited by Jackie Paper; May 31, 2026 at 08:13 AM.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,944
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From: poway
Was the bike running poorly before?
Last edited by Max Headflow; May 31, 2026 at 08:01 AM.
Stellar HDF Member
Joined: Sep 2021
Posts: 2,068
Likes: 1,491
From: Black Hills of SD
Good job. Nice to have it corrected.
I knew it had to be a vent issue. No engine should hold vacuum after shut down over a few seconds.
I was asked to help on an issue with a TPE 331 Turbo Prop aircraft engine. It had a vacuum similar to what you descried [ I know, not apples to apples ] after shut-down and the oil fill cap would gasp when opened. Normal operation the engine was supposed to have a negative pressure of 0.00 to 5.00 HC. It was in spec running, but at shut down would hold the pressure. I found the Air-Oil accumulator on the oil tank had the inner material coming apart and blocked the internals baffles. The nylon mesh spiral material just came apart and block it. Pretty wild and tough to see inside the A/O accumulator. Little tiny piece of what is similar to teflon tape, the size of string.
So, the turbine engine would pull the required vacuum but could not release it. First time I had ever run into that one.
Any way - You got it - maybe help others that ever run into that situation in the future.
I knew it had to be a vent issue. No engine should hold vacuum after shut down over a few seconds.
I was asked to help on an issue with a TPE 331 Turbo Prop aircraft engine. It had a vacuum similar to what you descried [ I know, not apples to apples ] after shut-down and the oil fill cap would gasp when opened. Normal operation the engine was supposed to have a negative pressure of 0.00 to 5.00 HC. It was in spec running, but at shut down would hold the pressure. I found the Air-Oil accumulator on the oil tank had the inner material coming apart and blocked the internals baffles. The nylon mesh spiral material just came apart and block it. Pretty wild and tough to see inside the A/O accumulator. Little tiny piece of what is similar to teflon tape, the size of string.
So, the turbine engine would pull the required vacuum but could not release it. First time I had ever run into that one.
Any way - You got it - maybe help others that ever run into that situation in the future.
Seasoned HDF Member
Joined: Dec 2005
Posts: 19,944
Likes: 8,038
From: poway
Good job. Nice to have it corrected.
I knew it had to be a vent issue. No engine should hold vacuum after shut down over a few seconds.
I was asked to help on an issue with a TPE 331 Turbo Prop aircraft engine. It had a vacuum similar to what you descried [ I know, not apples to apples ] after shut-down and the oil fill cap would gasp when opened. Normal operation the engine was supposed to have a negative pressure of 0.00 to 5.00 HC. It was in spec running, but at shut down would hold the pressure. I found the Air-Oil accumulator on the oil tank had the inner material coming apart and blocked the internals baffles. The nylon mesh spiral material just came apart and block it. Pretty wild and tough to see inside the A/O accumulator. Little tiny piece of what is similar to teflon tape, the size of string.
So, the turbine engine would pull the required vacuum but could not release it. First time I had ever run into that one.
Any way - You got it - maybe help others that ever run into that situation in the future.
I knew it had to be a vent issue. No engine should hold vacuum after shut down over a few seconds.
I was asked to help on an issue with a TPE 331 Turbo Prop aircraft engine. It had a vacuum similar to what you descried [ I know, not apples to apples ] after shut-down and the oil fill cap would gasp when opened. Normal operation the engine was supposed to have a negative pressure of 0.00 to 5.00 HC. It was in spec running, but at shut down would hold the pressure. I found the Air-Oil accumulator on the oil tank had the inner material coming apart and blocked the internals baffles. The nylon mesh spiral material just came apart and block it. Pretty wild and tough to see inside the A/O accumulator. Little tiny piece of what is similar to teflon tape, the size of string.
So, the turbine engine would pull the required vacuum but could not release it. First time I had ever run into that one.
Any way - You got it - maybe help others that ever run into that situation in the future.
What makes it bad for HD motors? The vacuum wasn't bad for the turbo prop, it simply showed you were the issue was with that particular system. What failure caused the vacuum to hold on HD?
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Tourer
Joined: Feb 2008
Posts: 267
Likes: 13
From: Philadelphia
Dear all,
I had a look at both removed umbrellas valves and difficult to see or feel and major anomalies, but in any case a fresh new set along with their foam and new gaskets restored the bike to normal operation. No further issues of any air hisses, be it pressure or vacuum. Bike runs great and has 30K on it despite 16 years old.
I imagine over time, wear does exist.
My gratitude to the help and support on this forum.
Jim
I had a look at both removed umbrellas valves and difficult to see or feel and major anomalies, but in any case a fresh new set along with their foam and new gaskets restored the bike to normal operation. No further issues of any air hisses, be it pressure or vacuum. Bike runs great and has 30K on it despite 16 years old.
I imagine over time, wear does exist.
My gratitude to the help and support on this forum.
Jim
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