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Old 05-03-2019, 05:27 PM   #1
flatheadmatt
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Default Valve cover air leak

Well I decided to do a bubble check on my intake after heat cycling my new top end (37u motor) I apply 10-12 psi and air escapes through my rear cylinder valve cover. I closed it and sprayed it and sure enough it bubbles from the middle of the valve cover. Also the hissing quiets down considerably when I close the valve cover. What is going on here? Anyone had this issue before? Any input appreciated before I yank the cylinder
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Old 05-03-2019, 06:18 PM   #2
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Default Re: Valve cover air leak

Presuming your valve seals and guides are good, let's start with something easy: are your valves adjusted correctly? If the clearance is too tight, this would be a potential symptom.
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Old 05-03-2019, 06:51 PM   #3
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Triple checked that. I even set em a couple thousandths loose just for initial start up. I just did a compression test. WoT and no oil. Front at 90 rear at 89
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Old 05-03-2019, 06:55 PM   #4
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Default Re: Valve cover air leak

Sounds like a small leak... maybe a little kiss with some compound is in order. Do you have a three-angle valve or just a 45 single cut?
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Old 05-03-2019, 07:01 PM   #5
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Not sure on the angle. I got em assembled from ACF and bolted em on. In hindsight I should a checked.
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Old 05-03-2019, 07:01 PM   #6
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I'm still wondering about the air coming out the valve cover. Weird....
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Old 05-03-2019, 07:15 PM   #7
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Default Re: Valve cover air leak

What is the weird part? You pressurize the intake, and it comes out the valve cover.

Ergo, the valve is leaking at its seat if you have it on the heel of the cam.
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Old 05-03-2019, 07:32 PM   #8
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Default Re: Valve cover air leak

If I get this, your using a plate were the carb goes on the manifold and putting 12 psi in with the intake valves closed. By the way you only need 3 to 5 psi to check for leaks, more could damage the manifold seals, this is not very likely but is possible. The air is coming past the valve and guide, the rear has more clearance. Not likely, but if for some reason you have guide seals that one is leaking more.
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Old 05-03-2019, 09:01 PM   #9
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Yes, I've got a plate on the manifold and am using air with soapy water to make sure it's all sealed up. (I will use less psi going forward) Air is escaping out the rear intake valve cover and making a hissing sound. (37U motor). All the lifters are adjusted correctly. So would that mean to much clearance between the stem and guide? I dont think there are any guide seal on flatties.
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Old 05-04-2019, 01:57 AM   #10
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Default Re: Valve cover air leak

Yes, between the valve stem and guide. No guide seals are required, just mentioned it as there is no telling what you have till you look. A lot can happen in 81 years.
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Old 05-04-2019, 11:03 AM   #11
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Default Re: Valve cover air leak

I believe Paul uses three angles when he does a port and flow job.

I also know for a fact he's allergic to testing intakes with air.

My money is on a valve to seat leaking. A stem and guide leaking air isn't concerning to me... it's a flathead. Oil ain't gonna climb up the valve to go burn, IMO.
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Old 05-04-2019, 11:29 AM   #12
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Default Re: Valve cover air leak

Air passing the seat would be going into the cylinder, that would be the opposite direction from were the air leak is. Also 90 and 89 psi (compression) isn't saying leaking valve.
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Old 05-04-2019, 11:53 AM   #13
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I reassembled everything and am gonna put some miles on it today. I'll report back how things go.
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Old 05-04-2019, 12:00 PM   #14
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Default Re: Valve cover air leak

Quote:
Originally Posted by joe49 View Post
Air passing the seat would be going into the cylinder, that would be the opposite direction from were the air leak is. Also 90 and 89 psi (compression) isn't saying leaking valve.
Fuck, you're right. I'm sending poor Matt on a wild goose chase.

Perhaps at 12psi he blew apart the sealing cork?

Subquestion, is the valve guide and valve and oil between them going to hold 12psi? Genuinely curious; that sounds like a lot to ask.
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Old 05-04-2019, 07:44 PM   #15
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After doing a couple short rides and cool downs I put about 60 miles on the U today. Ran great! I pulled both plugs and they looked good. Good to go!
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Old 05-04-2019, 09:43 PM   #16
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Awful scenery.
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Old 05-05-2019, 12:15 PM   #17
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Default Re: Valve cover air leak

Glad it's sorted, I agree with Liam, awful view.
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Old 05-05-2019, 12:37 PM   #18
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Default Re: Valve cover air leak

So, what was the fix?
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Old 05-05-2019, 01:01 PM   #19
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These bikes will run
I think everyone maybe being a little too anal
Hubble bubble toil and trouble
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Old 05-05-2019, 01:14 PM   #20
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Default Re: Valve cover air leak

We can't all have good scenery.

What kind of comfortable cruising speed can that pretty, funky machine achieve?
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