Hi all,
New guy.  I have a Kohler courage SV590 and a Dosens carb off a same-size B&S engine.  The engine starts right up and runs at higher speeds but dies at about half speed when I try to idle it.  I've cleaned the carb out with cleaner and wired out all the little holes but no help.  I assume it's a carb issue but I'm no expert.  Has anyone seen this before?  Thank you.
 
Rick A

What's a "Dosens carb"?
 
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I can't tell you much about it;  it's the only one I've seen.  Here it is on Amazon:

I tried to post a link to the ad which apparently violated some guideline so you'll have to look on Amazon under Briggs & Stratton 19.5 hp model 42E707 to see a picture and description.  The rectangular structure on the bottom is a built-in fuel pump.  Apparently they're not very common.
 
Rick A
 

Ok, found it... Never saw that one/style before..
 
Does it look very similar to what you had?
 
Anyway, you can try closing the choke most of the way and see if it'll idle down. 
Your richening the mixture as it sounds like your not getting fuel at lower RPM's 
 
Try that and report back..
 
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Ok, found it...
 
Is it very similar to what you had?  The one I found, stock for the Kohler, is VERY different 
from what you have now..  Exactly what brand/ type machine is it on?  The model posted fits
MANY different machines and different HP's..
 
In any event, try closing the choke most of the way and see if it'll idle down.
It sounds like its running lean on the lower RPM range...
 
 
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It's the same carb I have.  It's on a Toro 4200 zero turn which didn't have a carb on it so I used this one from a B & S,  same size engine.  
I'll try your suggestion,  though it seems that if it's leaning as I throttle down it would start running rough or backfire,  but it runs smooth and suddenly cuts off.  Thank you.
 
Rick A

Well you have compression (for high speed running).
 
Your either loosing spark, (not likely) or fuel.  
 
Spraying some carb cleaner, starting fluid when the rpms will also add fuel (sprayed just a little bit) to try and keep running.  
 
We need to narrow down the issue.. 
 
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I tried partially choking it as I throttled down but no help.  Adding ether would have the same effect I think.  As a last resort I guess I'll have to take a carb off another mower and try it;  was hoping to avoid that since all the linkage will have to be adapted.  Incidentally,  I'm doing this while bypassing the governor-  throttle straight to the carb.
 
Rick A

I also have a 4200 Toro Zero turn (2005) with an 18 HP Briggs.
 
I've never seen/heard of that carb you have before this post.  
 
The Briggs either had a Nikki or Walbro carb.  (The Nikki is very problematic).