On 23 Feb 2007 10:28:59 -0800, "Jay Chan"
wrote:

I have an old John Deere 522 snow thrower that has a 5hp 4-stroke
engine. When I received it from my father-in-law, I noticed that it
occasionally had backfire (or afterfire) from the exhaust. But

I guess you mean afterfire, because backfire is firing through the
carburetor. A lot of people have been misusing the word lately.

lately, the problem seems to be more apparent -- either it happens
more often or I start paying more attention to this problem.

Seem like the backfire occurs as soon as I start the engine regardless

I don't really know the term afterfire, but I think that's the term
you want.

the engine is under load or not. Sometimes, I even see flame throwing
out from the exhaust. Consequently, the exhaust is blackened.

Online source says that the air/fuel mixture probably is too rich in
fuel and the incompletely burnt fuel ends up getting burnt in the
exhaust. According to the operator-manual, the problem must have
something to do with the carburetor not adjusted correctly or the
carburetor leaks air.

Leaking air would normally lean the mixture, not enrichen it. But
that still leaves "not adjusted correctly". Someone else who knows
more about small engines will have to help here, but I thought there
was generally only an idle mixture or speed adjustment, and you'r not
talking about when it is idling, or are you?

Another cause of afterfire would I think be a retarded spark, that
fires too late so the gas is still burning when the exhaust valve
opens. Does the thing still have the proper amount of power? I
would think that quite a bit of power would be lost if this is the
problem. I don't relaly think it is the spark timing because every
small engine I know, that can't be adjusted. (Although it could be
retarded if you hit something that stopped the shaft from turning and
partially sheared the key that holds the flywheel to the crankshaft.
(The flywheel usually causes the spark as it passes the coil) You
should be able to take off a cover or two and see the top of the
crankshaft and the flywheel, and you should be able to see the key
that way and be able to tell if it is still rectangular in cross
section. If it is shaped like a B or 8 but the bottom half is offset
from the top half, it is partially sheared. Ask again how to replace
it. Getting the flywheel off takes some instruction.

Either case, the manual suggests me to bring it
to the dealer. But I really don't want to bring it to the dealer in
the middle of winter when I need to use the snow thrower. I would
much prefer to bring it in in late winter or spring when I don't need
to use it any more.

My questions a

o Can I continue using the snow thrower in this winter? Then I can
bring it to the dealer in the spring.

Ask your fil how long it has been doing this. Maybe for years. Is it
getting worse?

How much more snow will you have, how many more hours does it have to
run?

o Is this something that I can fix if I can find a service-manual and
a parts list? I am reasonably handy (home improvement...etc) despite
the fact that I have never opened up an engine before. Note that the
operator-manual doesn't say anything about fixing / checking /
adjusting the carburetor. I hope that the service manual may have the
necessary info.

If it is the carburetor, you don't have to open the engine. Two bolts
and maybe a hose or a couple wires and the carb is off. The lawnmower
guy wanted to see me about every replaceable part, for a total of
maybe 25 dollars. iirc. If he were around the corner, I would have
replaced the parts one at a time, in order to learn which one was the
problem, but I needed to mow the lawn and he was several miles away,
so I bought almost all of them. The jet(s) and the needle valve seemed
most important. A needle valve that doesn't shut will definitely
cause too rich a mixture, if it will start, because the height of the
gas in the bowl will be too much. Jets I guess don't get bigger, they
get smaller when gas hardens in them over the summer (in the case ofa
snow blower) but a lot of people would just replace everything.


Thanks in advance for any info.

Jay Chan