- How to do-it-yourself instructional
discussing troubleshooting an ignition with a fouled spark plug and
demonstrating both dry and wet compression tests on a four stroke (four
cycle) Tecumseh lawn mower engine.
- This sparkplug is a little bit blackened, a
little bit fouled, you would say. Now the most likely cause of this is
overfuelling or a rich fuel condition. Let
me compare it here to a sparkplug out of my car, which is a very clean
burning machine, especially alongside of my lawn mower. However a machine
like a lawn mower sees such dirty service and has kind of a crude air
filtration system. So youíre going to have problems like that.
- Reasons for a blackened or carbon deposited
spark plug would include things like too rich a fuel mixture or a sticking
choke or a clogged air cleaner, you know, anything thatís going to
increase the fuel in the fuel : air ratio. Also a good possibility is oil
seeping past the rings on the compression piston and getting up into the
combustion chamber and fouling it. So what weíll do is a quick compression
test and have a look and see what kind of numbers weíre pulling.
so check it out, on the left we have my low-range compression tester, on the
right my high-range compression tester. The high range of course is for high
compression diesel engines and the low range one is for a wide range of
vehicles and machines including this tiny little four stroke engine weíre
going to give a test to. OK you just push on this spot right here and that
releases the lidÖ just remove these two screws. Thereís another one in
behind here. Thatís why you need the long extension on your socket wrench.
And as that one comes out it just falls away like that.
- OK so weíll have a look in here. Thereís
your money shot right there showing the throttle plate so the procedure here
is to put a screwdriver in this hole to block that open. Whatever you use
make sure that it canít fall in and it wonít break apart and pieces of
it fall in. Put my screwdriver on in there past the throttle plate and make
sure itís blocked open before I pull on the cord. Thread it into the
spark-plug hole, just do it about like that: hand tight. Itís a good idea
to just put it down somewhere safe out of the way.
- OK so I looked around online to see if I
could find some technical specifications for this engine. A new machine like
this will make about eighty psi in pressure. Sixty psi on the other hand is
generally agreed to be acceptable. Twenty psi is only a quarter of eighty
psi so thatís twenty five percent. Most mechanics Iíve worked with agree
on a range of acceptability of around fifteen to twenty percent. So twenty
five percent is quite generous, now keep in mind that it would depend on the
type of machine. Consensus seems to be that itíll still cut grass
acceptably under normal conditions even with a twenty-five percent loss in
- [Engine turns over]
- Sorry about the bumpy ride.
- OK Iíve done a dry compression test and
the results are in weíve got eighty psi and its been holding eighty psi
for five minutes now.
- OK weíre just going to put a drop of
thirty weight oil in there. Maybe two shots. Put the compression tester back
- OK so now with a drop of oil in the
combustion chamber weíll do a wet compression test.
- OK now the wet compression test is eighty
psi. This ten year old lawn mower of mine is making the same compression it
did the day it came out of the factory.
- [Mower starts]
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