Engine: Briggs & Stratton 3.5HP on rotary mower.
Symptoms: This engine is about 10 years old. I got it going after a
neighbour had abandoned it. It has run reasonably well but never very
fast and almost no throttle control. (i.e. Either its running or
Recently the engine has been running very fast. This was followed by
white smoke billowing from the oil filler cap while running. I topped
up the oil but may have overdone it. Quite a bit of it got spewed out
of a joint in the oil filler tube onto the deck. I notice that the
compression is low when turning by hand. Tonight it would start run
for a few seconds and then stop.
(I've taken the motor apart before and put it back together but I'm
more of an electronics guy than a motor man.)
From the symptoms I reckon that the problem might be worn piston
Q1. Is there anything else I should consider before replacing these?
Q2. Any simple way of determining the wear on the rings?
i dont know if it works on two cycle engines, but for a car this will work.
get a compression gauge and crank the motor (be very careful not to cut your
head off while doing this) to get a compression reading. now squirt some
oil in the plug hole. dont flood it, but more than just one little squirt.
what you are trying to do is see if the compression goes up by putting oil
in there thus sealing leaky rings. if the compression goes up when you do
this, its the rings. if its stays the same (low) its something else.
Briggs 3.5 HP engines take a total of 16 to 20 ounces of oil. Remove the
fill cap, tip the entire mower over, and drain out the old oil. Put in one
pint (that's half a quart) of oil and see if that runs better. Use good name
brand oil, not the store brand. Penzoil, Mobil, Quaker State. All good.
Cheap oil doens't run very well in air cooled engines.
B&S often will run when old but by then the valves, cam, rings ,
bearings etc etc are worn making repair worthless from my point of view.
Do a compression test, have the correct amount of oil and live with it
or junk it, unless you like to tinker.
I opened it up and got a local repair shop to look at it. The cylinder
walls are badly scored. Hence low compression and exhaust gas in the
oil sump causing heating and spewing. The engine isn't worth spending
Meanwhile the grass is growing longer and longer ...
firstname.lastname@example.org (DaddyMonkey) wrote in message
I have a lawnmower with a BS 3.5 HP engine. If I overfill the oil or if it
gets tilted too far to the side where the crankcase breather is oil will get
onto the air filter, into the carburator and cause white smoke. It usually
clears up after running it a few minutes.
As far as replacing the piston rings goes in my opinion it is not worth
repairing. By the time the piston rings are worn the crankshaft is probably
also worn and the case would need to be replaced because there are no
bearings. It is likely that the piston rod would need replacing too.
I have seen BS lawnmower engines on eBay recently for $66 Buy-it-now with
reasonable shipping (smallenginedeals was the seller, I think--and yes, I
have bought an engine from them and was happy).
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