Mower decks why so many models

[p38js 444]

So I have a 416h with the onan and a C120 special with a 14hp kohler,  I have a 36in rear discharge deck that I mow with on a regular basis on my C120/ 14hp and it cuts great the grass is level no high spots or low spots every once in a while it will leave a long weed,  I mowed with the 416h that has a 42in side discharge and it mowed great same thing the grass is level no high spots or scalping.  So the engine is hurt on the 416h another story for later looking like valve seats (just add it to the list).  So I took the 42in off and put it on the c120/14hp and now the cut is different the deck is setup as per the instructions for the deck and set the tire pressure per the instructions using a calibrated tire gage (airplane mechanic sorry).  I was able to get the cut ok, the deck is level side to side and down in the front about an 1/8 of and inch out of level per instructions.  Since my rear discharge cut so nice and I really liked the width of the 42in side discharge I thought a 42in rear discharge deck would be a wise choice.  So someone is going to say why not just use the side discharge, good question I have some very large windows on the back of my house and when my wife or kids mow they are used to the rear discharge and they threw grass and some rocks at the windows I would rather not replace a window and I’m very thankful for the help and if a rear discharge is available why not avoid having to remember which way to throw the clippings.  So I found a nice 42in rear discharge deck with no baffles, ordered new belts greased her up really well oiled the idler belt bearing and while I was waiting on the belts to show up I found a mulching insert for the deck.  Installed mulching insert installed new belts and put the deck on the c120/14hp and started to mow what a train wreck the left side was scalping the grass the right side is not cutting all the way to the edge of the deck and the middle would lay down and not lift to get cut.  Made sure deck was level from side to side and tried to adjust for and aft I could only make level the adjustment would not let me go past level, I also could not adjust the height past the third notch on the high adjustment.  Checked everything I could anti scalping wheels look to be the correct part number as per the parts manual the height adjustment wheels look to be correct per the parts manual.  I put my old 42 in deck on the c120/14hp and it cut good.  Put the 36in back on and it cut great. 
 

My question is why did they make so many model decks are they for specific models even though they fit all tractors with the attach o matic.  Is my tractor the problem is the deck bad if the 416h wasn’t hurt I would put the 42in rear discharge on to see how it cut on that tractor.  If someone out there has a good 36in rear discharge deck that they want to trade for a 42in rear discharge deck with a mulching insert I could be persuaded to trade.  My 36in is in pretty bad shape she is starting to rust out pretty bad.   I don’t think it is a hp problem the 14 was rebuilt and machined with all Kohler genuine parts took me a while to track down Oem parts and paid through the nose.  
 

if anyone has some special secrets that the owners manual is not saying I’m all ears and willing to try different setups

[lynnmor 8,967]

I think that different blades were used with the recycler deck.  Are you running the engine at maximum speed and is the belt on the larger PTO pulley?