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Discussion Starter #1 (Edited by Moderator)
I rebuilt my Lawn Boy S21BSN mower two years ago. At that time, I replaced the drive belt. Last summer I noticed that it would only propel when I engaged the drive bar about half way. After the mower warmed up, I could engage the bar completely and it was fine. This year, it hardly propels at all. I put my foot on the drive flywheel and it appears that the belt is slipping on the blade shaft. I checked the pulley on the blade shaft and it is tight.

Having said that, it appears that the belt is loose or too big. The adjustment appears to be fully adjusted. Besides replacing the belt (A PAIN ) any recommendations?

Thank you,

Doc Doug
 

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The self-propel power take-off from the engine is the rather small pulley directly under the engine's bottom crankshaft seal. Bottom seal leakage which would probably go unnoticed on non-self-propel machines can deposit enough oil mist onto that drive pulley to greatly reduce belt driving friction.

I know that saying any design strategy by LawnBoy was a poor choice can upset people who have become ego attached to those machines. But this is one of those cases where I think they should have done better.

Check to see if that belt has become slightly oiled. If it has, you probably need to replace the bottom crank seal and use something to remove the oil from your belt. Actually, Dawn dish washing detergent is a great oil remover.
John
 

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Also, check to make sure that the pulley mentioned by LoveLearn hasn't had the set screw holding it to the crankshaft hasn't either backed out or snapped off.

It's happened to me. My S21ZSR has the same SP setup and it's worked flawlessly for me.

Charlie
 

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Discussion Starter #4
Excellent points.

I did check to ensure that the pulley is solid to the shaft and not slipping. It is truly the belt slipping on the pulley and not the pulley slipping on the shaft.

With respect to the bottom seal, that is something that I overlooked. I replaced the seal when I rebuilt the engine... but you know how that goes. I will check the bottom seal in a few days and post the update.

Thanks for both of your suggestions! Good stuff.:thThumbsU

Doc Doug
 

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there isnt much adjustment for the belt tension on this model. this is not the best sp system to say the least. the toro era models are far better . i have one of these and prob would not buy another. ground speed is too fast for commercial use . even with all the washers out
 

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i am looking into that echo. but due to the fact that the drive wheels have plastic gears and are no longer sold-i prob shouldnt be using it commercially at all. this mower has such a sweet f engine -its a shame not to use it. i use it sparingly and only on decent yards that are smooth. my goal is to exercise every one of my lawnboys as last year i didnt use but one(22261)
 

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Discussion Starter #9
I see some build up of dirt at the base of the engine. (See photos) As a result, it is possible that gas/oil spray could be on the belt, causing the slippage. Where you see the base is clean, I wiped the dirt with my finger.

Please review the photos and advise your thoughts. Replacing the bottom gasket is such a PINTA on this engine. The three gaskets on the muffler assembly can be expensive too, if they fail to stay intact. Ughhhhhh

Thanks, Doc Doug
 

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I'm reluctant to post this as my "takes" on so many issues don't align well with most other discussion group participants. But I'll risk censure.

Air flow through ducted fans is well known. Many pulleys simultaneously serve as self-contained ducted fans. For instance, pulleys driving air compressors often shape their ring-containment spokes as fan blades. They push air from one side of the pulley-fan to its other side. If OMC had configured their engine-mounted pulley as a pulley-fan, pushing air from above the pulley to below the pulley, air flow from any leakage passing through the engine's bottom seal would be pulled through the pulley center rather than being driven around the pulley. Relatively clean air flowing into the low-air-pressure region above this self-contained ducted fan would prevent oil mist that might escape past the bottom engine seal from flowing upstream to reach the drive belt.

I know this may seem like a trivial issue, but I'll bet that this bottom engine pulley self-propel slippage you have experienced is fairly common over the service lives of S21BSN and similarly-driven machines. OMC was NOT the only company to drive self-propel systems with this same configuration, but I have never seen even one of them "do it right", which in my not so humble opinion, would include configuring their engine-mounted bottom pulley as a self-contained ducted fan for the reasons I've just explained. Even if an engine bottom seal were leaking pretty badly, that oil flow should be driven away from the clean belt rather than onto it by using the better design configuration I just described.

Another "trivial" but poorly configured OMC choice when they were designing LawnBoys can be seen by examining the bolt fastener which secures the rewind assembly with a single hex-head bolt located so closely to the engine's reed-valve intake block that servicing the rewind assembly requires first removing the intake assembly which is secured by 4 more hex-head bolts and, during reassembly, realigning the speed-control rod through a small hole. Those familiar with this issue, just think of how much easier serving those pull-start rewind mechanisms would be if OMC had simply used an Allen-head fastener bolt instead of a hex-head fastener bolt to secure that assembly. OMC designers knowingly elected to leave so little space around that hex-head fastener that no socket nor end wrench can lock onto it without removing the intake assembly. Yet Allen-head fasteners were common 50 years ago. Why didn't one of those designers correct this servicing problem? Of course, we can correct that problem by swapping an Allen head fastener bolt for the original hex-head bolt.

Just some of my views. By the way, if someone has an F-engine rod assembly (rod, cap, 2-steel cages) to repair another S21BSN, let me know. I instructed a friend through a rebuild but he failed to tighten the rod cap fasteners. It ran for about 10 hours before it began making warning noises, which were ignored, and then failed.
Thanks for your consideration.
John
 

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I'm reluctant to post this as my "takes" on so many issues don't align well with most other discussion group participants. But I'll risk censure.

Air flow through ducted fans is well known. Many pulleys simultaneously serve as self-contained ducted fans. For instance, pulleys driving air compressors often shape their ring-containment spokes as fan blades. They push air from one side of the pulley-fan to its other side. If OMC had configured their engine-mounted pulley as a pulley-fan, pushing air from above the pulley to below the pulley, air flow from any leakage passing through the engine's bottom seal would be pulled through the pulley center rather than being driven around the pulley. Relatively clean air flowing into the low-air-pressure region above this self-contained ducted fan would prevent oil mist that might escape past the bottom engine seal from flowing upstream to reach the drive belt.

I know this may seem like a trivial issue, but I'll bet that this bottom engine pulley self-propel slippage you have experienced is fairly common over the service lives of S21BSN and similarly-driven machines. OMC was NOT the only company to drive self-propel systems with this same configuration, but I have never seen even one of them "do it right", which in my not so humble opinion, would include configuring their engine-mounted bottom pulley as a self-contained ducted fan for the reasons I've just explained. Even if an engine bottom seal were leaking pretty badly, that oil flow should be driven away from the clean belt rather than onto it by using the better design configuration I just described.

Another "trivial" but poorly configured OMC choice when they were designing LawnBoys can be seen by examining the bolt fastener which secures the rewind assembly with a single hex-head bolt located so closely to the engine's reed-valve intake block that servicing the rewind assembly requires first removing the intake assembly which is secured by 4 more hex-head bolts and, during reassembly, realigning the speed-control rod through a small hole. Those familiar with this issue, just think of how much easier serving those pull-start rewind mechanisms would be if OMC had simply used an Allen-head fastener bolt instead of a hex-head fastener bolt to secure that assembly. OMC designers knowingly elected to leave so little space around that hex-head fastener that no socket nor end wrench can lock onto it without removing the intake assembly. Yet Allen-head fasteners were common 50 years ago. Why didn't one of those designers correct this servicing problem? Of course, we can correct that problem by swapping an Allen head fastener bolt for the original hex-head bolt.

Just some of my views. By the way, if someone has an F-engine rod assembly (rod, cap, 2-steel cages) to repair another S21BSN, let me know. I instructed a friend through a rebuild but he failed to tighten the rod cap fasteners. It ran for about 10 hours before it began making warning noises, which were ignored, and then failed.
Thanks for your consideration.
John
WAYNIEBOY

LoveLearn:
Hope your still around since your 2010 response. Just finished a complete overhaul for my S21BSN. Carb, BBC,
Exhaust, & Tranny. The disc clutch tranny was quite a challenge, had to order a new shift fork. Filled the entire chmaber with Stens 00 grease, works good. Cleaned out the engine with an entire can of sea foam, lot of smoke and liquid carbon pouring out of my rebuilt muffler. Also had to purchase a low torque ( 10-400 in. lbs.) wrench for most of the engine mounting bolts after a stripped one.
Do you have any experience in replacing the drive belt on this engineering nightmare of a mower ??? if so
give me some heads up.

Thanks
 

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Looks like he hasn't been on since 2012. Bill
 

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oh well gave it a try, these old machines need time and patience, I think the younger generation has lost that.
 

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I think to replace the belt you have to loosen the engine from the deck (3 or 4 bolts?) and be able to push the belt up from the bottom one you make enough clearance. Going from memory here!
 

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I recognize this is an old thread but since its been revived, here's my 2 cents.

To replace the drive belt you have to remove the motor. You will probably have to replace the exhaust gasket. If you are going to do all this, you may as well replace the bottom seal whether it needs it or not. I did this a few years ago and don't recall it being a terrible job, but it took all afternoon.

In my opinion the transmission / clutch on this model is the real problem. I have two of these mowers, both want to move forward when the drive is disengaged. I took one of the transmissions apart and throughly cleaned everything, greased and reassembled. Very little improvement. Took tranny apart again and wet sanded the clutch plates with 1000 grit paper (they did not look bad), then buffed the plates and reassembled. There was some improvement, but the mower can not be left running when you are away from it as it will still want to creep forward especially on a smooth surface like driveway The clutch does not slip as well as it should. It's better in warm weather. I can't say if replacing the clutch plates would correct this, but new clutch plates are expensive. I used Lucas red and tacky grease, perhaps I should have used a much thinner grease so the clutch would slip easier.

The other mower behaves the same way but not as badly. I was going to service the tranny in it too but am leaving well enough alone.

You can spot this type of drive when you pull the mower backwards when it is not running. You will feel resistance when pulling back.

I am always looking for Lawn-Boys to add to the herd, but this is one model I pass on.
 

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Thanks for the reply Dave.

I did the same routine as you regarding the transmission. Only difference is that I filled the entire tranny case with STENS 00 grease. Its a very light grease, almost like a gel. The transmission works great. Regarding the drive belt replacement: I completely rebuilt this machine over the past 3 months ( Carb, Muffler, tranny. blade brake clutch) I only used the mower for four years and then it sat in my garage for 22 years. After the rebuild I emptied one full can of Sea Foam to clear out all the carbon deposits. The black slurry (disolved carbon) that poured out of the muffler was unbelievable. I left the original drive belt because it looked in great shape, ( Might of been a mistake). I'm just concerned about replacing the belt when the time comes. The BBC is a real SOB to remove. Long story short, I mowed over an acre of lawn with it last week and it works great.

Thanks
 

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Almost forgot to mention, I did replace the shaft seal with an SKF 8624. DO NOT use the OMC 611396 they will pop out quickly. I did a lot of research regarding this problem. It seems that LAWNBOY did not do their homework is specing out this flimsy 611396 seal.
 

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Thanks Waynieboy.

I'll pick up some of the 00 grease and try it with the unserviced model. Appreciate the suggestion. It sure is a time consuming and messy job to pull the transmission so I only want to to do it once.
 
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