B&S 28M707-1127-E1 surging

Hi.
 
I have an old Toro ride-on mower with a B&S 28M707-1127-E1 engine.
 
I have it running and can use it but it surges terribly if the throttle is set anywhere other than idle or full. If I hold the governor arm it will run smoothly. It still surges at full throttle, but not as badly as mid throttle where it surges to a stall.
 
It sat for years before I got it and I gave it a service: Cleaned tank, carb, new main jet main jet and gaskets (kit), new air filter, spark plug, changed oil, new battery, etc.
 
I've done the static governor adjustment. The springs look ok but I cannot stop the very erratic surging. I tried bending the tag on the throttle assembly to add more tension to the main governor spring, but it still surges.
 
Any help would be much appreciated.
 
Regards,
Greg.
 
 

Thank you for your reply.
 
I have cleaned the carb a number of times with no improvement.
 
I mentioned above that if I hold the governor arm it will run smoothly. Adding to that, no matter which position I manually hold the governor arm (so it doesn't govern), it runs smoothly, from idle to mid to full throttle.
 
Based on that, should I still be looking at the carb?

My apologies for the slow reply, but I put up with this for a while before removing the carb again.
 
I thoroughly cleaned the carb again and the problem still persists.
 
If I operate the choke while it's running as suggested above, it dies.
 
It runs fine at full throttle, and if I quickly drop the throttle to idle it runs OK. But anything in between it surges immediately and dies.
 
If I hold the throttle still (by the governor link) it runs OK in any position (mid throttle for example). I have checked the linkages, governor spring, etc and it all seems fine.
 
I can use the mower because it runs at full throttle but I'd still like to get it running properly. I might try and find another carb to try but even old ones seem to be expensive.

Do you have a Nikki or Walbro carb?
 
https://www.jackssmallengines.com/jacks-parts-lookup/manufacturer/briggs-stratton/briggs-stratton-engine/200000-399999-series/28m700-to-28m799/28m707-1127-e1/carburetor-gasket-kits 
 
If a Walbro, part #142, nozzle, with VERY TINY holes must be spotless.

It's a Walbro and the carby kit I used had a new #142 jet.
 
I used the mower a few days ago and I noticed it was running differently - high revs at idle. Moving the throttle made no difference but it wasn't surging.
 
So I ripped off the air filter and lowered the idle screw and adjusted the mixture and it's running ok now right through the throttle range with no surging. Maybe something cleared itself in the carb?
 
It sort of makes sense because I had to initially adjust the idle screw in to get it to idle.
 
I hope it continues to run well. If not, I think I'll know the problem is the carb (as topic responders suggested).
 
Thanks to everybody for your help.