behoof

mind dose the same if I leave it for to long I take spark plug out dribble a bit of fuel down the hole replace plug then away we go its a bit of a ball ace but I just think thank god its started lol

Another thing to remember with these rear mounted gas tanks is not to let it run dry.. They can be a nightmare to get the fuel flowing again!

almost certainly the pump - mine is lazy. Leave it longer than a day and it struggles to draw fuel through

Hi behoof. As you say, it could be the pump or even the fuel tap not sealing properly.
 
 I had the same problem with my C-125. I've fitted a priming bulb of a boat in the fuel line. Just one squeeze and it's away.

Welcome to MOM behoof, I can't add anymore to what's been said above... Your C-161 looks a nice machine, those 16hp Kohlers do have some power

Just read your description and viewed your pic of the problem again and I believe I now understand where the issue is?, but I can also be mistaken.
Suspect you could have a problem with the PTO Bearing shaft. It's the short stubby shaft that carries the Quick Release Pin.
On that shaft, there is what you will call a 'Snap Ring' in UK it is also known as a Circlip.
That Snap Ring carries All the Pressure that the PTO Control Lever applies to the PTO Clutch and pressure Pad friction material.
This is the only example I have on mine that is similar to show the Snap Ring in place (on an older 72 model)-
 

 
That Snap Ring sits in a shallow groove in the Shaft and wears out the square edges of the groove and so the ring slips out when pressure is applied !.  
If this is your issue, you will need to replace the Shaft and Snap Ring. I presume your machine is about 1978 which will have a Model number like 81-16KS01?. Either way, the part numbers remained the same for the next several years. Here's a snap shot of Toro's IPL covering the component parts- 
 
  
Parts #33 & #34 are the suspects and the Toro Part nbrs are- 33  =  #102872 & 34 = 936125 if you need them.
 
It is a weak spot in the design, but ok as long as spare parts are available , like in the U.S. where you should have no problem obtaining them.
To overcome this design issue, I for one have modified it, by producing a shaft with a Shoulder formed where the Snap Ring goes and carries all the pressure-

The inside carries a thick washer (not shown) and retained by a Socket Head Bolt to carry the pressure when releasing the PTO away from the Pressure Plate.
  
Edit - also noticed you need to fix the Roll Pin arrowed in this copied image as it is part of your problem-

Hey, welcome to mom. Great machines - I've got one, nowhere near as tidy as that!

welcome to MOM behoof, i like your avatar

Hello All, 
 
New here and wanted to say Hi from beautiful Yale, Mi.
 
I was wondering if anyone had any knowledge regarding the clutch <I think it's the clutch> on the WH C-161? 
 
Just got it yesterday and it runs and drives great and the mower deck seems to spin up just fine but as soon as it hits more than about 3 or 4 larges blades of grass the belt and deck slow right down to about nothing in rpm. As soon as clear from the grass and I re-engage it it spins up again but again the same process in the grass?
 
I've looked at what I think is the clutch and nothing obvious pops at me except that the pin I see that goes thru the end of the <what I think is the clutch hub> hub is too long for the opening by the bearing I see just inside it? I was thinking that perhaps I should either replace it with an original if available of redrill this one closer too the place it mounts? I'm just spit-balling here because it's new to me... 
 
I've included a couple of pics of my new <older> WH... 
 
Thanks for taking a read of this,
 
behoof 

silly question your belt is tight isnt it you tighten it by ajusting the black nob at the front end of tractor ??

That pin shouldn't be the problem. As said above, is the deck drive belt tight enough? I tighten up till I can just squeeze the bottom runs together. Between deck pulley and mule.

I won't disagree with checking the Deck Drive Belt Tension, but also still check PTO Clutch pressure adjustment. 
 
Follow these instructions extracted from the Operator's Manual to adjust it and the Brake Pad pressure-
 

 
Hope this helps and welcome.
Regards