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.
replying to Sherman, Marc Belanger wrote:
The lawn mower is probably losing spark. It will get good spark if it is cold,
but once hot, one of 3 things could be happening. First, the spark plug could be
filled with crud, and when it is cold, the crud has a lot of non conductive
grease in there. This will turn to a liquid when hot and become conductive
enough to short out the coil pack and kill the spark. I would replace it if it
looks like there is crud between the edge of the steel and the porcelain on the
inside of the spark plug. Look between the metal electrode and follow the
porcelain where you can't see it anymore.. Yeah, thats the place, either clean
it out and get the bl;ack stuff out, eliminating the short and it will probably
work again. if not, the spark plug wire may be cracked and work while cold, but
once it warms up, the break is separated enough not to work. Same thing with the
coil. Or, the coil could be hust loose. It needs a ground to work and if the
coil is loose, when cold it may be connected electrically then when it warms up
it may disconnect. The coil and spark plug wire are one piece on a tecumseh or
briggs and stratton and are less than 25 bucks normally. Get one, screw it on,
wire it and rreplace the spark plug. Then, check to be sure the shut off wire
isn't shorted to metal anywhere, it it is, the mower will still not start. tape
it or reconnect it correctly to fix. The coil is under the crankcase cover. Oh,
btw, the coil needs to be mounted the correct distance from the flywheel if you
never seen one before. Try to line up or mark the old one before loosening it.
Some coild only mount in one location so this may not matter.
Less than 1 chance in 10,000,000 of THAT being the problem. Spark
plugs and infewrnal combustion engines don't quite work that way.
Again you are picking at straws - with something perhaps in the
nieghborhood of a 1 in 5,000,000 chance.
Now you are gettind somewhere close to the realm of possibility - and
even probability. The coils have solid state components in them that
can fail from heat and vibration.
The FIRST thing to do is remove the shroud and blow all the chaff and
weed seeds out of the cooling fins to make sure the engine is
cooling..Re assemble the engine - run it 'till it quits, pull the plug
wire and hold the end of the wire while someone gives the rope a quick
pull. If your eyes don't light up, you KNOW it is a spark problem.
Generally, if the coil is loose it will hit the flywheel and it is
more likely to have a problem starting cold than hot.
Why not check that first, and quite possibly save yourself 25
And in many cases (all older tecumsehs ) under the flywheel too.
On a Briggs (external mounted coil) a peice of cigarette package
cardboard or a business card is "close enough for government work".
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