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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

3,200 rpm's top no load speed

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).

Don't work on many push mowers just the riders and zero turns. Move the throttle control and see what stops it and see if you can adjust the travel on it so you can set it slower. Maybe some one else here can tell you.

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

Actually the speed isn't designed to have an idle down position. the tab that is underneath is designed to be bent down to hold the engine at one speed. When they did away with the throttle control everybody kept bending the tab up so they could adjust the engine speed, and then complained that the engine ran poorly. The catch was when they went to that design the also removed the idle circuit from the carb, since it was now designed to run on speed set by the springs, This was done to meet EPA guidelines. If the engine is a fixed one speed it only has to meet standards at the speed, but if it has an adjustable throttle then has to meet standards through out the entire speed range.

The springs are supposed to be set by removing the tension on the small spring and setting the tension on the large spring to 200 rpm under desired speed and then retention the small spring to bring it up the last 200 rpm.

Interesting - thanks. The engine above has a lever with a 'detent' at each extreme so I guess I'd call it a 'two speed'. But definitely no 'idle'. Those old L-heads weren't the cleanest I guess and this must have been a way to milk a little more productive life out of the line before retiring it. (when did they finally stop making this line of engines?)

pgtr said:

Interesting - thanks. The engine above has a lever with a 'detent' at each extreme so I guess I'd call it a 'two speed'. But definitely no 'idle'. Those old L-heads weren't the cleanest I guess and this must have been a way to milk a little more productive life out of the line before retiring it. (when did they finally stop making this line of engines?)

Click to expand...

2014 Briggs went entirely OHV in 2015. And just single speed with a fixed throttle. the detents were a holdover from a previous version that had multiple speeds. Looks like the tab that makes it a fixed throttle has been straightened out in the picture you provided.

The tab to the right of the governor spring and hanging off the edge of the fuel tank is supposed to be bent straight down to make the engine a fixed throttle setup

Tab on lever hasn't been modified - it's just the way it was when it left WalMart 20 years ago when new.

Good memory - the owners manual states "On *some* models the governor lever is bent into position and is not adjustable" and points out the "Governor Control Lever (if equipped)" in an illustration.

Looks like they come either way back in '01. Thanks,