Welcome to JUST ANSWER small engines, let me try to help you !!
Do you have the full model number of the unit? example 304567X92a
The tube is the vent tube for the transaxle.
Tractor motor runs fine, good rpm. Belts are fairly new and routed correctly. Blade speed is fine. Adjusting the clutch link to idler arm will make it go faster forward/slower reverse....or....adjusting the opposite way..... slower forward/faster reverse. But can not get forward speed even close to what it used to be. It's very slow at best. Idler pulley rotates freely. Brake not locked.
Is this transaxle "sealed"? I've never seen any evidence of leakage, but maybe it needs oil?
Balck rubber tube sticking up looks like it has a bolt in the top.
You could try adding a little 20W50 through the tube. Have you removed the linkage at the transmission and checked to see you are getting full travel with the foot control?
Working on it now in the garage. I will re-check foot control travel. Will get back to you with any change.
If I add 20W50 oil to transaxle, can it be "overfilled" and cause damage or further problems?
Foot control (speed, forward and reverse) linkage to transmission appears to be allowing full travel. I did not "remove" the linkage, but observed as I operated the foot pedal.
I have the deck off and will leave it off for now.
I have backed off the brake, probably too far, but that should insure that that isn't the problem.
I need to re-install the battery, then try moving/driving and re-adjust the linkage to see if there has been any improvement.
I have other chores to do this morning, but will get back to you later.