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Valve cover air leak

3K views 31 replies 9 participants last post by  JAWS 
#1 ·
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
 
#2 ·
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.
 
#4 ·
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?
 
#7 ·
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.
 
#8 ·
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.
 
#9 ·
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.
 
#11 ·
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.
 
#14 ·
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.
 
#21 ·
I think I agree with this. Joe's right, the air is slipping past the guide. I don't think it matters, because I think when there is a reasonable amount of oil between the valve and guide, it seals strongly enough.

I been thinking about this for a few days. And overthinking it, too, I guess. :)
 
#24 ·
Judging by the way it ran and the color of the plugs I dont think the leak matters. When I pumped up the manifold with a hand pump to 5 psi it held fine. I was using too.much pressure.

I've got a 24 tooth tranny sprocket. The bike can do 70-75 for cruising. It's all highways out here with no traffic (or people) so I keep it at a comfortable 65-70. Its taken a year of working the bugs out but I'm finally able to put some miles on it.
 
#26 ·
I thought about that, too.

I think the butterfly valve would pop off long before a cork passed air. I was wondering if his was frozen; I've seen a bundle that are.
 
#27 ·
The compression was 90 and 89 psi. So air is not going into a cylinder, past a bad valve seal. So not going to get past the rings. This does assume no valve was off seat. But to add the easiest path for air pressure would be to leave the cam chest threw the case vent.
 
#29 ·
Open the pushrod tubes on a bike and do a leakdown... you’ll hear air from the tubes.

Same thing happens when rolling one over to do valve adjustment with the plugs in on a bike that was just running. Shit girgles through the oil if there is any present.
 
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