5.3hp mower engine runs fine when at low throttle but idles up and down at high throt

[kippstakes]

I've been doing some work on my mower and cant seem to smooth out the idle. At low throttle the engine purrs, but when it's throttle up/set to high the idle is all over the place. I don't think it's the governor since I can hold the throttle valve open with a driver and it still goes up and down. The carbs are clean, the airbox is clean, it's getting a good spark and the compression is 90.
Any ideas?

Thank you

 

[mechanic mark]

http://www4.briggsandstratton.com/m...on_Troubleshooting_Detail_Reference_Guide.pdf

see table 12, hunting & surging.

 

[kippstakes]

Thank you

 

[DaveTN]

Hello and Welcome to LMF! Sounds like not getting enough fuel on the high
speed circuit. Take a twist tie off a loaf of bread, skin it back and take the wire
and bend it at 90 degrees. About 3/8 inch will do. Clean out each tiny hole
near the throttle plate, usually 3, and spray it with carb cleaner and that should
help a lot. Sometimes varnish gets into the tiny holes and interferes. Hope this
helps. :thumbsup: