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Lawn-Boy S21ZSR runs slow


Locarno's Avatar
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08-23-09, 01:36 PM   #1  
Lawn-Boy S21ZSR runs slow

Hi all--thanks for these helpful tips on this forum.

My Lawn-Boy S21ZSR runs too slow. More specifically, it starts fine; when set to the "tortoise", it seems to run at a proper speed--it can propel itself while cutting, but thick grass slows it down. (I have the blocking "mulch" insert in, so that the grass doesn't get expelled much.)

When I move the throttle up to medium, it gets a bit slower, and then on the "hare", it is about the same as the tortoise setting. In fact, if I switch back and forth between tortoise and hare, there really isn't much difference--the mower has a hard time cutting thick grass.

Any ideas how I might try to troubleshoot this? The last time I rebuilt a carb, I botched it up--I'd like to get better and know my mower, so I'm willing to try the more intermediate repairs. Thanks in advance,

P-Sz

 
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08-23-09, 06:19 PM   #2  
There is a coil type spring under the black ""Thumbwheel"" kinda thing on the top of the carb,, This is likley broken.... If not you can adjust the governor with this "thumbwheel" as it'll ratchet & change the tension on the spring.. Since you have no speed control,, to me it sounds like you need a new governor spring,,,, Roger

 
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08-23-09, 06:56 PM   #3  
Great, thanks, I'll look for that.

Does that have a number in this drawing?

Parts and Diagrams for Lawn Boy S21ZSR, Lawnmower, 1991 (SN K00000001-K99999999)

 
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08-23-09, 07:18 PM   #4  
That would be number 6. Have a good one. Geo

 
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08-24-09, 06:53 AM   #5  
The throttle is also controlled by the governor lever, item #4 in the illustration. This can become disconnected to the spring or with age break. Your throttle control cable should move this lever to where more tension is put on the spring. There is a thumb wheel, item #5 in the illustration, that adjusts the tension on the spring. The spring should open the throttle up and the more tension will counter act the governor so you run at a higher speed. The spring can also break an end that hooks into the governor lever or at the thumb wheel slot. Item #23 adjust the fuel air mixture and can also be set too lean to not allow running at the faster speed. Turning the screw out will allow more fuel. I would look at the spring and linkage first to see everything is hooked up and working. Your engine should be running 3000 rpm at high speed if you have a tack to check it.

 
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08-24-09, 06:11 PM   #6  
You might want to remove the muffler and have a look at the exhaust ports in the cylinder head. They could be restricted with carbon and the muffler may have a build up as well. This could cause a loss of operating RPM's and reduced power from the engine as well as a throttle that does not seem to respond.

Best of Luck...

 
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08-24-09, 08:15 PM   #7  
Posted By: Locarno Hi all--thanks for these helpful tips on this forum.

My Lawn-Boy S21ZSR runs too slow. More specifically, it starts fine; when set to the "tortoise", it seems to run at a proper speed--it can propel itself while cutting, but thick grass slows it down. (I have the blocking "mulch" insert in, so that the grass doesn't get expelled much.)

When I move the throttle up to medium, it gets a bit slower, and then on the "hare", it is about the same as the tortoise setting. In fact, if I switch back and forth between tortoise and hare, there really isn't much difference--the mower has a hard time cutting thick grass.

Any ideas how I might try to troubleshoot this? The last time I rebuilt a carb, I botched it up--I'd like to get better and know my mower, so I'm willing to try the more intermediate repairs. Thanks in advance,

P-Sz
I want to say very thank you for this great informations. Now i understand about it.
Thank you.
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08-27-09, 05:52 PM   #8  
Great, thanks! The spring was indeed detached. I struggled for a great long while--turns out that you have to lodge the spring correctly; the "end" with the single angled piece must rest inside the square cutout in the thumbwheel (I had to bend the end to make it a sharper angle so it would stay); the "beginning" with the multiple-angled hook must come out the bottom and hook onto the governor lever. In such an arrangement, the hook of the spring should stay hooked on the governor lever and one can adjust the thumbwheel safely. Any other arrangement for me caused too much of the spring to poke out, which caused erratic running and unhooking of the spring.

Once the hook was secure and the carburetor/air filter assembly reattached, I started it and it ran really slow. I turned the thumbwheel clockwise (right) a few clicks several times, starting after each time, and the engine ran faster and faster until it seems about right.

Thanks, couldn't have done it without you! What a flimsy, annoying spring!

 
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