I removed/cleaned the carb and tank and cleaned the air filter and it has a new plug. New fuel for this season. It runs great for about five seconds then gradually starts to seek/hunt. It's very pronounced. The engine dies while slowing down, then revs back up. I have a video where you can see the little dealie, behind the carb, moving back and forth.
It will gradually run worse and worse over about two minutes before dying. It seems to like running best when I lift it up, by pushing down on the handle.
If this site will let me post a video, I will give you the link. Audio is terrible... cell phone. Otherwise, I will post it on YouTube and tell you what to search for, to find it easily. This is really annoying because my lawn is getting huge/out-of-control/embarrassing.
Search for these tag words, on YouTube: ASDFG B&S mower surging/seeking/hunting
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Re: Briggs&Stratton 4.5hp seeking/hunting
Did you replace the pump diaphragm and gasket between the carb and fuel tank? There is also a screen on the carb and a secondary pickup 'hole' on the fuel tank that needs to be cleaned between them too. You'll see that when you take the carb off the tank.
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I believe the condition your describing is called "hunting" this sometimes happens when engine has a problem with it's air/fuel mix usually due to a carb problem
id start by cleaning out the fuel tank and filters and then moving on to the carb
I am pretty sure this is a 4.5 Hp engine with the carb on top of the tank. Plastic carb 4 or 5 mounting screws mating carb to tank. Remove the air cleaner... and tighten the carb to fuel tank mating screws. It is drawing air between the tank and the carb. This should stop the surging. If is has been a long period that you used the mower with the surging problem the diaphragm between the tank and the carb may be either out of position or damaged. The diaphragm is not expensive. Usually the first time this symptom presents itself you can resolve it by tightening the carb to tank scresw. IF you have the primer bulb carb..mash the rubber of the bulb against the carb body. If the surging stops... the primber bulb has a pin hole in it.
You passed the mower model number and not the engine model number. When we use the 11A number you passed all we get is the base and handle parts. If you need more help I need the model number off the engine just above the spark plug wire on the air shroud.
Your Briggs & Stratton Classic engine uses a diaphragm to pump fuel. Yours has deformed and needs to be replaced. It is a very easy repair. Remove the air filter. There is a bolt on the end of the fuel tank nearest the spark plug. Remove this bolt. There is another bolt just behind the carburetor that holds the tank to the engine. Remove this bolt also. Now you should be able to gently pull the tank away from the engine. After the carb clears the intake tube, you can tilt the tank enough to remove the governor linkage from the carb butterfly shaft. Now the tank/carb assembly is loose and in your hand. Go to the workbench. There are 5 screws that hold the carb to the tank. Remove these 5 screws. When you lift the carb from the tank, you will see the diaphragm gaskets. Remove the old and install the new set. The 2 part diaphragm set consists of a rubber diaphragm and a paper gasket. The rubber part goes against the tank, then the paper gasket, then the carb. Leave the screws in the carb as you reinstall it, and the screw tips will align all parts during reassembly.
The diaphragm set is readily available at any Briggs parts dealer or online. Make sure you take the Model, Type, & Code numbers with you when you go for parts. These numbers are stamped into the metal fan shroud just above the spark plug.
Your Briggs & Stratton has a Sprint or Classic motor on it. Every few years, the diaphragm in the carburetor must be replaced.
Briggs part #495770
1. Remove the air filter assembly. This is a good time to clean and re-oil the filter.
2. Remove the 3/8" head bolt from the front of the fuel tank.
3. Remove the 1/2" head bolt from the side of the fuel tank.
4. Gently pull the fuel tank assembly toward you and tilt it away from you. While doing this, you will need to unhook the governor wire from the throttle plate.
5. Drain the gasoline (you can use it again later)
6. Remove the 5 screws that hold the carburetor on top of the fuel tank.
7. Pull the carb straight up. Be careful not to lose the spring or strainer on the bottom of the carb.
8. Remove the diaphragm.
9. Clean the carb and tank surfaces with carb cleaner spray.
10. Open the package with the new diaphragm in it. You will find 2 pieces. One is rubber, the other is paper.
11. Place the rubber diaphragm onto the fuel tank, then the paper one. Gently reinstall the carb onto the tank.
12. Push the 5 screws back into the carb. Before you tighten any of the screws, have all 5 pushed through and wiggle the carb a little, this will allow the diaphragm to align properly. Tighten the screws.
13. Reinstall the tank/carb assembly. Make sure you hook up the governor linkage.
14. Inspect the governor spring(s). They should be connected on both ends. You should not disconnect them for this repair.
15. Reinstall the air filter, gas, prime, and GO!
With the air cleaner removed, see if the engine will run for even a short time when you give a couple of quick squirts of starter fluid into the carb.
If it runs with that, the orifices inside the carb are dirty and need the gunk cleaned out.
Before extended storage, always drain/use up all the gas in the tank and run the engine til the carb runs out of gas.
That being the case...your miss is fuel related. Your mix is too rich..(too much gas...lack of air)...as the tank empties, your mix will lean out, basically telling you what the problem is. I recommend that you first clean the air filter, as this can cause the engine to **** too much gas. After cleaning, if you can tell no difference, the next step would be to adust the car...(air bleed). If this doesn't work, remove the carb soak in carb cleaner, blow out with compressed air, and install a new carb kit, (seals gaskets, and new seat if equipt with a needle valve and float. Good Luck! I hope you find this to be very helpful!