Briggs & Stratton lawnmower: smoke from oil filler

[BikeSpark]

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 stopped.)

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 rings.

Q1. Is there anything else I should consider before replacing these? Q2. Any simple way of determining the wear on the rings?

Thank you.

[xrongor]

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.

randy

[SJF]

Somewhere, I've heard that burned exhaust valve is also a common cause of lost compression on these engines.

SJF

[xrongor]

if its the valve, the oil test wont raise the compression. if this is the case its most almost certainly the valve.

randy

[DaddyMonkey]

could you have over-filled the crankcase with oil?

draining the oil and adding the proper amount should be the first (and lest expensive) step.

[Stormin Mormon]

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.

[m Ransley]

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.

[BikeSpark]

Thanks, everyone. 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 time on.

Meanwhile the grass is growing longer and longer ...

[Ulysses]

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).

This might be debris around the throttle linkage.