Briggs & Stratton engine "hunting"

Briggs & Stratton are a US manufacturer of small engines, used in many items of garden equipment as well as for outboard motors, generators and the like.

Hunting
The Briggs & Stratton vertical-shaft mower engine is a pretty decent engine, it has a long service life and requires little maintenance. The occasional clean spark plug and change of oil is probably about it.

I recently inspected the engine on my ten-year-old lawnmower. There was no evidence of carbonisation, no gum in the carburettor and all the linkages and pivots moved freely.

But...

B&S mower engines have a tendency to hunt. That is, the engine speed surges up and down when the throttle is at a constant position. Plenty of people have documented this, and the cause is nearly always the mixture control. A small adjustment taking a few minutes will fix the problem.

First find your mixture screw
The mixture screw is on the side of the carburettor. It's not alone. Nearby you will also find end-stop screws for the various bits of the carburettor linkage. Adjusting these will not help.

You can tell which is the mixture screw because it has a spring around the shaft, it goes into the throttle body itself, not through a projecting lug, and you can't see the business end of it. If you tweak a screw and the linkages don't move, it might still not be the right screw. There are screws to regulate maximum throttle position, and these don't come into play unless the engine is running at full speed.

You typically don't need to remove any covers to adjust the engine as there are access holes provided, but if the engine is hunting it's probably worth removing at least enough of the covers to drop a little light oil on the various pivots and spring hangers, and perhaps give it a clean if it needs it.

The adjustment
Mostly hunting is caused by too weak a mixture. What I have found is that usually you have to unscrew the screw, that is, turn it anticlockwise. Virtually every mower these days will have some kind of safety interlock, so you will either need to faff around a lot or get an assistant.

First, warm up the engine to normal operating temperature. In other words, run it until it starts hunting. Then, unscrew the mixture screw a quarter-turn at a time until it stops. I found that screwing it back in a quarter turn does not reintroduce hunting, so I do that. If the engine starts misfiring, missing the occasional beat, it's probably too rich so screw the screw in a quarter turn. It's an empirical process: when it sounds right, it is right. Actually there is a procedure in the manual which is more exact, but for an engine that has some hours on it and the carburettor spindles are a little worn, this is good enough I reckon.

Links

 * Briggs & Stratton website, has a lovely movie of how to find your engine model number, also has downloads of most owners' manuals (but not service manuals).