On Wed, 21 Dec 2011 14:53:05 -0500, Bob Engelhardt
wrote:

Bob Engelhardt wrote:
The engine on my leaf vac wouldn't start & I diagnosed low compression.
It's a 5HP B&S. Here's a picture of the exhaust valve seat (after
lapping):
http://home.comcast.net/~bobengelhardt/ValveSeat.jpg

Does it (the seat) need to be replaced? The B&S Repair Manual says that
after replacing a seat, it needs to be cut. How important is the
cutting? I'm not about to buy a cutter & I'll take the engine to a
repairman instead.

Well, I adjusted the valves, put it back together and tried it. The
compression was much better, but it still didn't start. It fact, it had
a terrible kick-back. Which was due to the flywheel key being sheared
and the timing being off by 90 degrees! Which was very likely the only
problem all along, even though the compression did test poorly.

The key replaced, it still doesn't start! Arrrgh! It has spark & won't
start even with gas squirted in the cylinder.

Well, maybe it's the valve clearances - that's the only thing that's
changed, right? They're close: the exhaust should be .009 - .011 & it's
11 (12 is nogo). The intake should be .005 - .007 & it's 8+ (9 is
tight). Is the clearance that important? Or is it likely something
else entirely? Although I don't know what that could be.

Befuddled,
Bob
Have you tried a new spark plug, by chance??? They can look good and
STILL cause difficult starting.