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I have a Z445 with Briggs & Stratton engine. It recently had a surging issue, but I took apart and cleaned the carb, which fixed that problem. Now the engine sounds fine in neutral but when moving, it seems to run more slowly than it should. Has new spark plugs, filters (air, fuel & oil), oil changed, new trans belt, fresh gas. I also followed the procedure to purge air from the trans system. Not sure what else to do.

StarTech said:

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The traction belt has been replaced, arm seems fine. The engine actually seems to struggle when in drive. Sounds fine in neutral though.

Sounds like a governor issue
Stand behind the mower where your finger can touch the governor arm
Get some one to start the engine and while you have your finger touching the governor arm get them to engage the blades.
If the governor is working properly you should feel the arm move to open the throttle and the engie get louder

If not you have messed up the linkages when you returned the carb to the mower .

You may have a dead cylinder. Disconnect plug wire from one cylinder and see if it starts. Then try the other cylinder.

Hammermechanicman said:

You may have a dead cylinder. Disconnect plug wire from one cylinder and see if it starts. Then try the other cylinder.

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So I did this and it ran with the left side unplugged, not with the right. Pulled the plugs and the right one is black, left one still looks new.

You have a dead cylinder. Check for spark on dead cylinder. If spark is good it probably has a blown dead gasket.