Scotts/Murray 6.5hp self-propelled barely propels

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I have Scotts front wheel drive self-propelled which barely moves. It is NOT the belt. It moves the drive pulley fine. I put the paint mark on it to verify that it is rotating fine. The wheels will spin in the air on the ground, it will only move on its own if going downhill. I have adjusted the drive cable as much as I can but it does not seems to be making any difference. I do not understand what exactly the cable does under that pulley. Pulley itself is stationary the cable connects to double spring contraption.

I have looked at youtube but did not see this particular mower front wheel drive mechanism. The book talks about belt adjustment and the cable adjustment which I have tried with no use.

How many friction points are there in this type of system. I do want to get out the blackbox (transmission) which drives the wheel and has the pulley on top. I do not hear the telltale sign of stripped gear but given the engine noise, I am not sure I can hear the stripped gear noise. One wheel is wobbly but other feels good. Could one slipping wheel make it not go? If it has real differential, then that is possible but do they put differential in a lousy walk behind mower?

228670x8A is the mower

Found video; looks like wheels are the most common cause
 
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Take the belt off and turn the input pulley by hand with the engine not running. With the drive lever engaged, the output shaft of the transmission should turn. This shaft goes across to both wheels under the mower. There is no differential gearing, it is solid all the way across so both sides will always turn. Make sure you're looking at the shaft itself and not a plastic sleeve that may be over it.

If that is OK the problem would be inside the wheels. Wheels are easily removed by removing the plastic hubcap (if any) then the retaining clip or pin on the axle, then slide the wheel off. The drive shaft turns a spur gear which engages plastic teeth inside the wheel. As noted, the plastic teeth commonly strip. Also there is a one-way clutch inside the each spur gear to let the outside wheel overrun when you make turns with the wheels on the ground. If that breaks, there will be no drive on that side.

There is no friction clutching involved, everything is solid drive with metal or plastic.
 

Vikas

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Both wheels have worn teeth. I think I need to start there. About $40 for two wheels but before I plunk down, is there a specific test? Does the spur gear move only in one direction when I take the wheel off?

There must be some friction mechanism or is it on/off from the pulley to the transmission? The pulley has constant belt tension in this model and is always spinning but wheels start spinning only after engage the drive cable and only when lifted up in the air. That has been the most confusing part so far.
 
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The lever on top of the transmission engages the gear inside. The input shaft always turns but the output shaft only turns when the lever is in the on position.

Do make sure that the cable pulls the transmission lever all the way when the control on the handlebar is engaged. It should then move a spring to take up the extra travel. Stretched or binding cable is a common problem.

With the front wheels assembled but lifted off the ground, they should turn freely in the forward direction but engage the one-way clutch and turn the transmission when you try to turn them in the backward direction.
 

Vikas

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So it is essentially on/off lever inside the gearbox instead of friction based power transfer? I just want to make sure that the transmission itself can not slip internally.

Off the ground, the wheels are jamming up in both directions and randomly free wheel. Looks like I need to first figure that out before ordering the new wheels. The one way pinion gear and the key way slot could be jammed.
 

Vikas

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Can't believe it! The pinion gear is attached via drive pin to the axle! The back of the pinion gear has a slot for the pin. There is nothing for one way motion in this mechanism. Oh well, drive axle rotates fine when I rotate the pinion gear. Time to order the new wheels!!

Here is the pinion gear detail https://shop.briggsandstratton.com/products/briggs-and-stratton-71792ma-pinion-gear

r65lpmx7ulfwo2kyvma9.jpg
 

Vikas

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Success!! After putting the wheels and re-adjusting the cable tension at the "undocumented" wrong end, now it self-propels fine.

Still a very bad design because the wheel do not free wheel. Any turn or with the engine off and drive not engaged, means the front wheels have to be off the ground. Otherwise, it is chewing up those plastic wheels when it tries to move the transmission internals. Those wheels are heavy but I all see is plastic including the bushing. It is a race to wear out the gears or bushing.

But at least I feel like I did complete one project successfully.
 
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