Surging Lawnmower

I have a 4.5HP Yard Machines lawnmower that ran without issues up until a couple weeks ago when I changed the oil and the spark plug. I put in the recommended 20 oz of oil, checked the dipstick to make sure I didn't overfill and when I started it up it's been surging ever since. The spark plug is the right part # and the oil is the correct weight. I can't figure out what's wrong with it.

Any ideas?

 

Did you tip it over on its side to get the oil out of it?

 

No, it has a drain plug on the bottom so I used that.

 

If it wasn't surging before, put the old plug back in it. If that makes no difference, get a little Sea Foam from an auto parts store or Walmart and add that to the gas. Changing the oil wouldn't make any difference in itself. Sometimes people tip them over to get the oil out and upset things in the carburetor.

 

I did tip the mower over maybe to a 30* anglle when I was looking where the oil drain was. I'll reinstall the old spark plug tomorrow and see if that did it. If not, I'll try the sea foam. Hopefully one of these does the trick. If not, any other suggestions?

Thanks for the help so far.

 

What engine is on it?_________________________

 

Briggs and Stratton. I'm going to try the sea foam in a few. Will report back shortly.

 

I checked the spark plug and it looks pretty bad and it's only been run about 30 mins since I changed it. So I tried installing the old one and the engine didn't surge until after about 10-15 seconds. I then took out the spark plug and it was more dirty than when I put it in. Here's a pic:


I tried the sea foam after the spark plug, added about 2 oz to the fuel tank and ran it for about 5 mins. It's still surging. I'm at a loss...

 

The Sea Foam works fast, but not in five minutes. Put 4 ounces in the tank and take the mower out and mow with it through a tank full.

 

I added approx 2 more ounces of the sea foam and mowed my back yard (~30'x30'). It still surges. Could it be the carb? I've been reading the manual on how to adjust the carb, but can't figure it out for the life of me.

I'm getting pretty frustrated with the thing. Thinking about taking it to a local shop to have them mess with it.

 

It could be the governor, but likely the carb if nothing was disturbed in the oil change. But really, the carb shouldn't have been affected either.

Depending on the engine/carb, you would normally have a couple adjusting screws on it. One would work to hold the throttle open at idle. That doesn't change the mixture. The other would change the mixture. The latter is the one you want. While it's running turn that screw out slightly until it stops surging.

If you can send us a picture of the carb, do that.

 

Still don't know just what engine you have, but being a 4.5 briggs, chances are it's the classic version which tells me that you either have a clogged air filter or you need to replace the carburetor diaphragm.

 

If it is the carb on tank briggs;
You need to install a new diaphram kit, it is Briggs part # 795083 or old # 495770 also Lowe's has a kit #5083 which has the same parts.
Here we go;

Remove air cleaner, there are 2 bolts that hold things together, one on the front of the tank 3/8" and one into the block 1/2", remove these bolts, now "slowly" slide the carb/tank off the intake tube and breather tube, now tilt the tank in to release the throttle linkage and waalaa the carb/tank is removed. Check the intake tube to see if the “O” ring seal and plastic retainer are still on the intake tube, if so remove them and re-install them into the carb. Remove the 5 screws from the carb/tank remove carb(don't loose the spring) now spray all holes, cracks and crevases in both the carb and tank surface with brake parts cleaner, remove the main screen(looks like a thimble), now with a small screwdriver pry out the main jet(carefully) and clean it, check the “O” ring on the main jet for damage, if it is damaged it must be replaced, reinstall the jet, it can be difficult some times to get it to snap back in place(I use the rounded end of a screwdriver handle). Remove and clean the fuel pickup stem(not necessary if the fuel tank was clean). Clean any junk/rust from the tank. Install the diaphram on the tank then the gasket(no goo or sealer) now carefully replace the carb(the spring will try to misalign the diaphram), tighten all screws a little at a time so as not to crimp the diaphram until they are all tight. Install the carb/tank in reverse order and you are done. If I missed something one of the real mechanics will correct me. Have a good one. Geo

 

I would try geogrubb's advice "Remove and clean the fuel pickup stem" as your first step. I had a tiller doing the same thing, and after many hours of cleaning and buying/installing new carb parts, I found a plugged strainer in the fuel tank. If only I had checked that first...