Small Engines (Lawn Mowers, etc.): B&S engine surge, stop screw, mechanical governor

Question
I have a Murray (B&S 4HP) mower that all off a sudden started surging on me. It will stay running but surges really bad. i changed the air filter, took apart the carb and cleaned it, replaced the spark plug and gave it new gas- still the same thing. Once the plug is pulled, it is wet with gasoline. Curiously enough, the mower runs great when it is running out of gas- the motor stops surging and rev's up properly for about 5 seconds before it completely runs out of fuel. Wondering if this is something familiar to you?

Answer
Hi Neil,

Here are some steps to check the problem you outlined:

1.  Check your springs   linkages move freely. Also check governor spring is correctly inserted to governor lever arm. If you have a mechanical  governor preform a governor adjustment.

2,  Run engine at each of the speed settings. This will highlight when surge occurs.  If you find the problem happening while it's idling then the air/fuel mix is likely the cause of your problems. (although you have already cleaned your carb.)

3.  If surge occurs at light load .  Look for an idle speed screw, a stop screw on top of the carb.  Hold throttle lever against the idle speed screw and increase governor idle speed by slowly turning the screw clock-wise. If surging stops, replace the idle spring and linkage and reset the governor idle speed.  If surging continues to happen then you might try re-checking the carb for problems.

I hope this information is helpful to fixing your problem.

Regards,
Howard