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Just got an older 36" Scag walk behind. Previous owner put a new carburetor on it because it was surging at idle. It runs fine under load. After installing the new carb, he says it still does it.
I messed with it a little yesterday and found that when I remove the air filter cover, it runs MUCH better. As I tighten down the air filter cover screws, it gets worse and begins to surge hard.
Any help or suggestions are appreciated.

Three things come to mind very quickly for you to check. 1. Was it an OEM carb or cheap fleabay clone? I never trust or use clones unless I’ve got no other solutions. 2. Are you using OEM filters? Same reason. 3. Are you over tightening the air filter housing causing and air leak? See this more than once.

Rivets said:

Three things come to mind very quickly for you to check. 1. Was it an OEM carb or cheap fleabay clone? I never trust or use clones unless I’ve got no other solutions. 2. Are you using OEM filters? Same reason. 3. Are you over tightening the air filter housing causing and air leak? See this more than once.

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Thanks Rivets.
1. I'm not sure about the carb. I seriously doubt he used an OEM. I'm going to look at it to see what brand it is.
2. Filter looks new. However; don't know brand. Will check.
3. If I leave the filter nuts loose enough for it to not surge, they are pretty much ineffective. They just float and wobble. As I put the filter housing top cover on, it begins surging a little bit. As the nuts get tighter, surge gets worse. When tightened to a normal point... surge is pretty heavy.

Got more questions. 1. Are you sure the model number is FD460V and not an FB460V? Can not find the engine you listed. 2. Have you tried running the engine with no filter, just the cover fastened securely. This could indicate wrong filter. I’ve looked at an FB460V and the way the air filter is mounted a warped housing would not cause an air leak to cause surging. Have you checked to see if the governor is operating properly and the engine is running at the proper idle speed?

Rivets said:

Got more questions. 1. Are you sure the model number is FD460V and not an FB460V? Can not find the engine you listed. 2. Have you tried running the engine with no filter, just the cover fastened securely. This could indicate wrong filter. I’ve looked at an FB460V and the way the air filter is mounted a warped housing would not cause an air leak to cause surging. Have you checked to see if the governor is operating properly and the engine is running at the proper idle speed?

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Yes Rivets. You are correct. It is FB460V. Had someone read the number over the phone and I thought he said D. Apologies.
Engine seems to run really well with no filter or cover. I have not pulled the filter out and ran it with the filter-less housing and cover. My guess is that it would probably run well.
Governor seems to be operable, as it moves back and forth with the surging. Idle speed seems normal compared to other mowers I've had and currently own.
Thanks

Governor “seems” operable and operating at the proper speed are two different things. An engine not running at the proper speed, especially idle, will surge the same as a lean fuel mixture. Engine speed and fuel mixture need each other and must play nice to eliminate surging.

Should also mention that this is a gravity fed system. No fuel pump. Any help appreciated. Thanks

the obvious must be asked. Is this a new oem or equal filter? Other than the screws warping something that causes a vacuum leak, an air filter restriction would cause a rich running condition. I had one on a mower once that had gotten wet. It looked very clean but the water had changed the nature of the fabric and sealed the pores. Once the filter was replaced the engine ran normal again.

I'm just going to ask this since i've seen it before too many times....
Did you remove any plastic shrinkwrap from the filter from the factory?

sgkent said:

the obvious must be asked. Is this a new oem or equal filter? Other than the screws warping something that causes a vacuum leak, an air filter restriction would cause a rich running condition. I had one on a mower once that had gotten wet. It looked very clean but the water had changed the nature of the fabric and sealed the pores. Once the filter was replaced the engine ran normal again.

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sgkent said:

the obvious must be asked. Is this a new oem or equal filter? Other than the screws warping something that causes a vacuum leak, an air filter restriction would cause a rich running condition. I had one on a mower once that had gotten wet. It looked very clean but the water had changed the nature of the fabric and sealed the pores. Once the filter was replaced the engine ran normal again.

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The filter is an aftermarket filter bought on Amazon. It was with the mower when I got it. Seems to be in good shape, but I’m going to replace it with an OEM.

Scrubcadet10 said:

I'm just going to ask this since i've seen it before too many times....
Did you remove any plastic shrinkwrap from the filter from the factory?

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I didn’t actually buy the filter, but there is no plastic or shrink wrap on it.

BrockSolid said:

I didn’t actually buy the filter, but there is no plastic or shrink wrap on it.

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take it out then tighten the cover on as someone else suggested. If the engine runs right it is the filter, if not then the screws are tweaking something that causes a vacuum leak etc.. I seem to recall some conversations here where some engines have a fuel pump diaphragm built into the base of the air cleaner housing, and when it warps the engines don't run right. If the filter has a lip on it that is part of installing the cover correctly, find an old one and cut the membrane out so you have just that rubber lip.