Small Engines (Lawn Mowers, etc.): Briggs and stratton 4HP, briggs and stratton, horizontal shaft

Question
Hi. I have just purchased a new engine. I have attached it to the frame, checked the oil, put fuel in and then started it. It started first go only it is revving up and down. I noticed the choke vane swinging back and forth. I checked the springs and they are all there. However I have not yet attached the blade. Do you have any ideas please?
98902 2063 B1 06080351
Thank you

Answer
Jason
This sounds like the governor is not adjusted correctly.Attached the blade to put a load on the engine,if there is no load it may not run correctly.Then adjust the governor according to the specs. To adjust the governor on a Briggs and Stratton horizontal shaft engine, you would loosen the nut on the governor arm and push the governor arm down, clockwise. Then you would turn the governor shaft clockwise also and tighten the nut. Generally Governors should never need to be adjusted, unless the governor arm has been removed or tampered with. In the event that they do need adjusted, here is how to do it.

One of the most important parts of your engine is the engine governor. An improperly adjusted governor can shorten your engine's life. If  engine speed is not controlled, the engine will destroy itself. Never run an engine over speed, especially without a load attached to it. Even for a short period of time it can cause catastrophic failures, like blowing the magneto magnet into 100's of little pieces. Over speeding is something that needs to be taken care of immediately, because it can be a real hazard with metal coming loose at high speed. This page will explain how to adjust the mechanical governor on Briggs and Stratton and Tecumseh Engines.

First, let's understand a little more about governors and why they may not be working right:

Most complaints about governors fall into two categories:

  1. Engine runs too fast or too slow.
  2. Engine surges or hunts.

You should do an inspection of the governor linkage and spring before trying to solve either of these problems. Make sure everything is operating freely and the spring is not damaged or stretched. Also, check the governor static adjustment to see that all free play has been removed between the spindle and carburetor. The best way to do this is to move the throttle from idle to full open and note the way the governor shaft moves. If it goes clockwise, then loosen the clamp screw and with the throttle wide open turn the shaft all the way clockwise and re-tighten the nut. Make sure the throttle moves from idle to full open freely after making the adjustment. Refer to the engine repair manual for exact adjustment procedures.

After you have made the above check, restart the engine and see if the governor now operates correctly. With the engine at idle, move the governor lever with your finger to open the throttle and it should push the arm back toward idle if working properly. One way to do this test is with the governor spring removed. If it still over speeds or has no push toward idle, you probably will need to have the internal parts checked inside the motor or recheck the static adjustment. See your local repair shop for this!

The other problem that often occurs is governor hunting or surging up and down. Most of the time this is not the problem of the governor, but a partially plugged carburetor, usually a plugged idle circuit or worn linkage. If you can rule out these causes and have checked the adjustments I talked about above, then you may have a governor that is set too sensitive. In other words it reacts too quickly and over compensates for the speed. Many governor arms have several holes in the arm to change sensitivity. By moving the governor spring to a hole further away from the governor shaft you will make it less sensitive. Before playing with this, check the repair manual for the engine you have and see if it has a way of adjusting sensitivity. Sometimes you may have to change governor springs, etc.

Many of the smaller lawnmowers used what is called an "air vane governor". This type of system uses the air flowing through the engine to operate a vane back and forth against a spring, just like the mechanical governor does. The big problem with air vane governors is debris sometimes collects in the system blocking air flow. When this happens the engine will over speed. So, with an air vane system it is very important to keep the cooling fins clean so the proper amount of air can move through the engine. The governor spring is precisely calibrated for an engine that has full air flow going through it. If the air flow is blocked it can over speed the engine.