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Discussion Starter #1
So, this isn't an issue with my 750, but I trust the mechanical know-how here more than the other forums I've perused while researching my issue.

I have an overhead cam engine that won't start, and is spitting air and fuel out the air intake on the compression stroke. I've pulled the spark plug (which was wet) and held my finger over the hole to get a rough estimate of what the compression is doing (they don't sell a leak-down tester at my auto store?!?), and I get absolutely no pressure buildup on the power stroke, but an air hiss out of the intake, and a puff out of the crank case vent hose at the cam cover. I would take that to mean that the intake valve isn't seating properly, and that air is back-flowing out the intake. The air out the valve cover has me stumped though, unless there's also serious ring damage, there shouldn't be much air flow from the cylinder to the crank case...

Also adding to the mystery is that during the power stroke, the cylinder holds a vacuum against my finger. No air leaks from the intake to the cylinder, or from the valve cover to the cylinder. That seems like it rules out both a bad valve AND bad rings. I checked the lifters, and there's enough lash that the springs are able to close the valves completely on both the compression and power stroke.

I've pulled the head and am going to test for leaks with water, and lap the valve/seat if there is a leak, but ... the whole thing seems totally weird. Any thoughts?
 

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Timing chain or belt jumped?

Find out if this is an interference engine. Meaning valves can contact pistons if the valve timing is off, and bend valves.

Year, make, model?
 

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Discussion Starter #3
I was trying to keep it purely theoretical so that MAYBE it could help with a vulcan motor down the road ...

Its a Kohler Courage sv610 off a riding mower. It's not an over head cam, so all the timing is on the bottom end, possibly gear driven. The leak is evident the whole way through the power stroke, and the valves open and close properly compared to the piston stroke, so I don' think timing is the issue.

I've pulled the head, and there's been no impacts between the valves and the piston, and the rods are all straight. Still need to pull the springs.
 

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You might pull the flywheel and see if the key is sheared.

None of the valves were leaking?

The springs aren't real strong on their best day.
 

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Also, routine valve adjustment is important on these small engines, the valves become tighter and don't close.
 

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Discussion Starter #6
I checked before disassembly, and the rockers weren't pushing on the valve stems when the leak was happening.
The flywheel is secure.

I don't get how there could be a good enough seal to be air-tight during the power stroke but have a leak during the compression stroke. There was no movement in the rockers. Seems like if it's blowing air out the intake during compression, it would also be sucking it in during power.

I'll lap the valve, and scrap the engine it if that doesn't fix the problem.
 

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Not sure what you mean about no movement on the rockers...

It can get air on the intake stroke.

I didn't have any luck lapping mine and neither did the machine shop for $50, on a Briggs v-twin. Got new heads from eBay.

Did the valves hold liquid or leak any?
 

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Make sure the intake valve spring isn't broken or really weak. Compression can pop the valve open with a bad spring.

The cam is gear drive with followers and pushrods. One of my cam lobes was rounded off, had to eBay a new cam also.
 

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Discussion Starter #9
I meant that during both the compression and power stroke the rockers had just a tiny bit of play in them (stems not being pushed on at all), and that the rockers stayed motionless during those strokes until it was time to open the exhaust valve.

"It can get air on the intake stroke" Yeah, but on both the compression and power strokes both valves are closed. Nothing else changes in the combustion chamber except the direction the piston is moving, so how could air leak during one but not the other? It's as if the intake valve is acting like a one-way valve flowing towards the intake manifold when it's completely closed.

I've now pulled the head and filled the intake with gas, which immediately leaked past the valve. Spring seems strong, so I'll move forward on lapping to see if it will seal up.
 

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Yup, that leaking valve is where you're getting the pop.

I think the only time you notice the problem is on the power stroke, even though it leaks all the time. Engine fires and some of the combustion comes back through the leaking intake valve.

If you're not seeing a normal amount of lift at one of the rockers, it may have a worn cam lobe.

Good luck with the lapping, I even put a drill on the valve stem with no luck. I think the valve faces were just cupped too much, too deep. Hand lapping wouldn't have done it in ten years of spinning.
 
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