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Discussion Starter · #1 ·
I have a basic Murray 20 in mower with a Briggs and Stratton 9D902 3.5 HP 'Classic' L head. It is a 'throttle free' model - ie. no throttle on the handle bar just down on the motor itself with 2 settings.

Lately the RPMs seemed kind of high so I checked it. In the low position it is 2,750 and in the high position it is 3,250. It runs good otherwise, smooth and strong. Just kinda fast - especially the idle. (using my finger I can manually push the vane/butterly closed and idle it down quite a bit)

I've looked at the owners manual for the 9D902 and found no specs on RPM speeds. I checked my old Briggs and Stratton repair manuals and only found 1,750 as a general idle speed for older 92000s. I did NOT find a high or governed or run RPM. I did find an RPM based on 20" blades WRT tip speed.

It's reasonably well maintained past 20 years.
  • Vane moves free and easy.
  • Oil changed annually and air filter cleaned, re-oiled.
  • Plug changed every couple/few years as needed.
  • Air vents blown clear as needed.
  • Several years ago, rebuilt carb and replaced primer bulb

QUESTION: Could someone with better B&S references that I have or can find, confirm what the idle and run RPMs should be for a 9D902 mounted on a 20 in. rotary mower?

Once I have a better idea of the idle and run RPMs - I'll play w/ reconfiguring the vane springs as needed to dial it in a little better. Thanks

62103

62104

62105
 

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briggs isn't recognizing that model number for the spec look ups, however most push mowers are set to 3200 RPM's.
 

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Discussion Starter · #3 ·
3,200 - thanks! ;)

SLOW/IDLE: I need to figure out why it's slow/idle speed is way too high still (without messing w/ the full/run speed). :unsure:
 

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Make sure the linkages aren't binding anywhere, and look for an idle adjust screw.
 

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Discussion Starter · #6 ·
From what I can tell - due to the lack of range of movement in the OE governor lever on the engine - there is no meaningful way to adjust the slow/idle speed separately from the fast/run speed. With the single effective spring - there is a pretty consistent 400-500 RPM gap between slow and fast and they both move up and down in lock-step as you bend the 'tab'. To fully move the RPMs up and down from say 1,700 to 3,200 would require a different governor lever layout and/or different spring(s)/mount point(s). I guess by design it does not have a particularly low idle speed but rather two 'run' speeds of medium and fast. Not sure why but that seems to be the way of it.

So by design I while I could idle it down - but it's fast/run speed would then be too slow. I settled on roughly 2,450 and 2,950 respectively. That's fine for my needs. I don't really ever need to 'idle' it. Just prime it, pull, mow small plot of grass and release handle to turn off and store until next time. Rinse and repeat.

(there is no throttle cable on this mower - the lever is down on the motor and not connected to a cable from the handle)
(the 2nd smaller 'idle' spring does little to nothing - it's mounted to a fixed position, has very modest impact on slow speed and maybe helps stabilize the vane a little)

thanks for the replies and feedback
 
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