You are right. The vent is plugged.
The cap is hard rubber with a rubber washer that has 2 small holes
in it. Behind the 2 holes is another flat thin rubber washer that has
deterioated and the deteriorated rubber is actually protruding
partially thru the tiny holes. I suspect it completely stopped up
the vent holes.
I tried removing the deteriorated rubber, but can't really get to it.
If she comes in bitching about the mower dying again, I'll add a new
vent hole thru the cap.
Sounds like a valve clearance problem to me. Either that, or the fuel line
is too close to the engine. But more likely valve clearance.
I took a small engine repair course years ago. To fix this, you have to take
off the motor cover, the cylinder head, and then remove both valves. Grind a
little off the bottom, ch eck the clearance, and put it back together.
Requires some specialized tools, and some replacment gaskets.
That sure wouldn't be the first place I'd look....more likely electrical
as someone else noted w/ thermal expansion...besides the magneto, it
could also be the plug ceramic...(look, look)...oh, I see he did change
the plug so it's upstream of there...
No, the valve seat sinks into the block. There's supposed to be a
little clearance between the lifter and the tip of the stem, so you
grind the end off.
That's for the more common valves in block type., not on an OHV..
This is a push mower Craftsman about a 4 hp.
I removed the gas filter bowl and it was clean. No change after
cleaning and reinstalling. Started right up and ran for 10 minutes..
After it dies hot, it will not start or even try to start. The pull
compression feels about the same hot or cold. But I can't measure the
vacuum (or can I?)
Has to cool down and at some point it cools enough to start on first
pull and runs good for 10 minutes.
I will check for spark next time it is dead.
Actually, I lied. My spouse does the mowing and she is getting
angry at me because the mower is dying. Don't get me wrong, I will
mow when it needs it, but in 30 years, she has never let it get long
enough to need it.
I need to get this thing diagnosed and fixed pronto.
I'm thinking I could use one of those piercing shroeder valves on the
intake manifold to measure for vacuum while I yank the rope. Wonder
if it would work. Or I could test compression thinking maybe the
compression would be different if the suction was off due to valve not
Ok, I'm also thinking I could pull the valve cover and observe the
valves when hot or cold. What am I looking for? As the crank lets
the valve close, it should become loose and I should be able to grab
the stem and lift and let go and hear the valve seating - right? It
would probably be a different sound if it was not seating.....
Forget the damn valves, that's not the problem. You've been given sound advice
by several people: check to see if you have spark when it's hot and not
working. I'm betting you don't, because the magneto has failed.
Doug Miller (alphageek at milmac dot com)
This is an outside possibility, but I had a lawnmower with a leaking
head gasket. Problem was it would leak only when the engine was hot.
The problem was getting worse and worse and I could not figure out
what was wrong. The gasket finall degraded to the point that that
when I happened to brush my hand ove the head, I could feel the hot
gases leaking from the head gasket.
Radio Control Aircraft/Paintball Physics/Paintball for 40+
Many, many air-cooled engines develop this problem. The odds are
overwhelming it's your ignition module, and they're a bastard to
troubleshoot [except, of course, by replacing the module].
As soon as the engine starts to cool -- long enough to check for spark
-- the module cools down enough to check as OK. It's very possible
you'll check the spark and it'll be OK.
They're easy to change, but shop carefully. The price for replacement
can vary radically.
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