Valve Question

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  #1  
Old 12-31-02, 11:46 AM
J A Boggan
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Question B & S valves

The engine I'm talking about is a Briggs & Stratton, model - 92502, type - 3101-03, code - 87081951. I was just doing a general rebuild, rings, valve job, carburator rebuild ect. When I got it rebuilt, it would not "hit a lick". I checked the spark...which was fine. So I decided to do a leakdown test. Zero pressure on the second gauge. Also, could hear air coming from the exhaust. I have replaced both valve guides, refaced both valve seats, lapped the valves in (also had both valves ground to the appropriate angle), exhaust - 45 Degrees. Upon examining the valve faces, they each had a beautiful grey uniform stripe on them. I went ahead and refaced and lapped them again (adjusted the valve gap at 0.008 - exhaust, 0.006 - intake). Put the engine back together...the same result. Does someone have an idea about what's happening? Any feedback would be greatly appreciated.
 
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Old 12-31-02, 12:07 PM
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Do you have good compression? Are you sure the camshaft didn't get out of time when installing it? Are you sure it is getting fuel? Sometimes we run into things like this, and have to simplify our thinking to discover the problem. Remember it has to be one of these basic things: fuel, spark, air, or compression...all at the right time. Is there assembly lube in the cylinder, on the rings? If not, you may want to squirt a bit of oil in the cylinder to help seal the rings until it starts up and seats the rings.
 
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Old 12-31-02, 03:11 PM
J A Boggan
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Cheese,
When I cut the valve seats, I noticed it was slightly off center. Even if this happens, it shouldn't make that much difference, should it. Where else could all that pressure go that quickly? I will try squirting oil in the cylinder to help it to seal better.
 
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Old 01-01-03, 12:52 AM
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The seat is not centered with the valve guide? That could be a problem. One fairly simple way to tell if you have a bad seal at the valves is to put the engine at TDC on the compression stroke, remove the spark plug, and using a rubber tipped air blower nozzle, fill the cylinder with compressed air. Listen for air escaping out of the carb (intake valve leaking), or out of the muffler (exhaust valve leaking), or into the crankcase (rings leaking). There will, of course be a certain amount going past the rings, but shouldn't be a whole lot.
 
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