Self propulsion (drive) on Mountfield Lawnmower not working

[Muddy42]

I have a second hand Mountfield SP535 HW with a Honda engine. I got it cheap because of various minor issues.  I've fixed everything apart from the drive. Is meant to drive both rear wheels when you engage the lower handle, the upper handle being the 'dead man's handle.

 

At the moment only one wheel turns quite weakly.  I've had the blade and lower cover off, cleared out all the grass and oiled everything.  The belt is intact and working.  The gearbox seems ok and is turning the axle.  The axle is connected to a small cog which turns a large cog inside the wheel. ive had the wheels off but I can't find anything wrong.  The cogs do slip a bit, but surely something has to slip so you can push the mower without drive or even to reverse or turn the thing?

 

Any advice gratefully received.

 

 

 

 

[pleasant]

On the end of the gearbox axle shaft inside each wheel is a drive pinion. These have a tendency to seize on mountifields. Its a ratchet system to allow for pulling the machine backwards so the wheels arent permanently locked into drive. They will be held in place with a circlip. Remove circlip and any washers and gently slide off the pinion.....i maybe seized on the drive shaft so some release agent and a pry bar will help. Depending on the set up the ratchet system will either be built into the pinion or there will be a keyway in the driveshaft with a spring loaded woodruff style key. If theres enough crud and rust the woodruff keys sticks in the keyway so doesnt lock with the pinion so it will freewheel. Needs to be removed, cleaned and lubricated.....use a light oil, not grease as that attracts the crud again. Then reassemble.

 

It could also be wear inside the gearbox as the gearboxes are pretty lightweight for a heavy machine. Trace the end of the drive cable to where it attaches to the drive activation arm on top of the gearbox and then with a clamp, clamp the lower drive lever on the handle in the engaged position. Then get your hand on the lever on the top of the gearbox and fully push that lever right up to its stop. Then with your free hand move both rear wheels....if theres still little or no drive to both wheels then the gearbox is shot. Do this as a last resort as cleaning out the pinions may well sort it. 

[Muddy42]

thanks for the comprehensive explanation. I’ll try both approaches on Friday.

[Muddy42]

@pleasant

 

So a wee update.  I got the drive pinions off and both of them were moving freely.  The internal parts for the wheel that is working are a bit worn, so they may need replaced in time.  Behind the wheel that wasn't working, there were two problems. This plastic spacer is cracked, but I'm not sure that is causing any issues yet, its more to do with raising and lowering the height of the machine.  Maybe I can glue it.  Secondly the retaining wheel nut was stripped (not shown in these photos).  I suspect this was preventing the large cog in the wheel from aligning with the pinion gear. I repaired the thread, but still no drive on that wheel!  The investigation continues, maybe something wrong with the gearbox.

 

 

Edited Friday at 13:09 by Muddy42

[pleasant]

That split spacer is nothing to worry about regarding the loss or poor drive. That roll pin in the axle needs to slide easily from side to side. In the pic of the drive gear where you have circled in red on the inside of it, those three 'teeth' need to be nice and clean. Debris can build up in the corners and then the pin in the axle cannot get a good grip in the corners and can slip. Yours looks pretty clean though. It's a simple system as that's all there is to it. Assuming the corresponding larger gear inside the wheel hasn't got teeth missing then I cannot see an issue with the drive mechanism you have shown. If you are sensible, then remove both rear wheels plus the drive mechanism on the end of each axle rod...so all you have are the two rods protruding. Then lift the rear end of the mower (don't ground the deck) start the engine and engage the drive lever and then watch both axles...they should rotate. if not, then it will be the gearbox (or the cable has come unattached from the top of the gearbox- unlikely though) If there is some rotation...which there may be with zero resistance from the mower not being on the ground....then stop the mower. Re-fit the roll pins and both gears, lift the weight off the back end of the machine, start the engine, engage the drive again but get someone with a decent glove on to put gentle pressure on the drive pinions as they rotate....if you can stall the movement with your gloved hand then its the gearbox.

 

As an aside.....you stated the belt is in place. However there should be a degree of tension on the belt otherwise it will simply slip on the pulley on the crankshaft. To check this- remove spark plug. Tip it on its side with the exhaust side facing the ground. Then rotate the blade by hand......there should be a positive grip by the belt in the crank pulley. If you can move the blade and the belt has little or no movement then it won't be transmitting the drive to the gearbox pulley. There is a tensioning mechanism on all this type of mower for the belt....mostly a tension spring on the gearbox to pull the gearbox on it's pivoting axles backwards to tension. Some have a metal bracket, which when fitted also lifts the front of the gearbox up and back...again to add tension. The springs have a habit of coming off in use, thus losing tension and then drive.

Edited Friday at 19:27 by pleasant

[Muddy42]

On 27/06/2025 at 20:20, pleasant said:

That split spacer is nothing to worry about regarding the loss or poor drive. That roll pin in the axle needs to slide easily from side to side. In the pic of the drive gear where you have circled in red on the inside of it, those three 'teeth' need to be nice and clean. Debris can build up in the corners and then the pin in the axle cannot get a good grip in the corners and can slip. Yours looks pretty clean though. It's a simple system as that's all there is to it. Assuming the corresponding larger gear inside the wheel hasn't got teeth missing then I cannot see an issue with the drive mechanism you have shown. If you are sensible, then remove both rear wheels plus the drive mechanism on the end of each axle rod...so all you have are the two rods protruding. Then lift the rear end of the mower (don't ground the deck) start the engine and engage the drive lever and then watch both axles...they should rotate. if not, then it will be the gearbox (or the cable has come unattached from the top of the gearbox- unlikely though) If there is some rotation...which there may be with zero resistance from the mower not being on the ground....then stop the mower. Re-fit the roll pins and both gears, lift the weight off the back end of the machine, start the engine, engage the drive again but get someone with a decent glove on to put gentle pressure on the drive pinions as they rotate....if you can stall the movement with your gloved hand then its the gearbox.

 

As an aside.....you stated the belt is in place. However there should be a degree of tension on the belt otherwise it will simply slip on the pulley on the crankshaft. To check this- remove spark plug. Tip it on its side with the exhaust side facing the ground. Then rotate the blade by hand......there should be a positive grip by the belt in the crank pulley. If you can move the blade and the belt has little or no movement then it won't be transmitting the drive to the gearbox pulley. There is a tensioning mechanism on all this type of mower for the belt....mostly a tension spring on the gearbox to pull the gearbox on it's pivoting axles backwards to tension. Some have a metal bracket, which when fitted also lifts the front of the gearbox up and back...again to add tension. The springs have a habit of coming off in use, thus losing tension and then drive.

 

Thanks. I was thinking of doing more something along that lines - more comprehensive diagnosis and getting closer to the drive.  For safety, I was thinking of taking the blade off and raising the mower up on a fish crate or something similar. I could even turn the belt by hand with the engine off.

 

[pleasant]

iIiiiiiiiiiiiiiiiYou could but you wont be the best results turning it by hand. If you remove the blade it will most likely not start so dont try. The blade acts as a flywheel, and trying to start a mower like yours with no blade fitted when you pull the rope you with get severe kickback and the rope handle will be snatched out your hand.....very painful.

 

If you have checked the cable is correctly attached to the operating lever on the gearbox, and you have cleaned out the pinions and lubed it all up, just reassemble it all, put the wheels back on, start it with the blade on and engage the drive if you can easily stall the drive to the rear wheels by holding it back from going forward, then stop the drive, adjust the cable up via the inline adjuster and keep doing it until the wheels continue to scrabble for traction whilst holding the mower stationary. If you can hold it back and the wheels stop rotating under load even though the cable is adjusted as far as it will go, its the gearbox........as i mentioned earlier, this is providing the belt is correctly tensioned, but if it isnt you wouldnt get any drive at all as the belt would simply slip on the engine pulley when started. 

[Muddy42]

45 minutes ago, pleasant said:

iIiiiiiiiiiiiiiiiYou could but you wont be the best results turning it by hand. If you remove the blade it will most likely not start so dont try. The blade acts as a flywheel, and trying to start a mower like yours with no blade fitted when you pull the rope you with get severe kickback and the rope handle will be snatched out your hand.....very painful.

 

If you have checked the cable is correctly attached to the operating lever on the gearbox, and you have cleaned out the pinions and lubed it all up, just reassemble it all, put the wheels back on, start it with the blade on and engage the drive if you can easily stall the drive to the rear wheels by holding it back from going forward, then stop the drive, adjust the cable up via the inline adjuster and keep doing it until the wheels continue to scrabble for traction whilst holding the mower stationary. If you can hold it back and the wheels stop rotating under load even though the cable is adjusted as far as it will go, its the gearbox........as i mentioned earlier, this is providing the belt is correctly tensioned, but if it isnt you wouldnt get any drive at all as the belt would simply slip on the engine pulley when started. 

 

Thanks for the warning!  I'll find a way to do this safely with the blade on.

 

Yes I think that's all ok  Pinions clean and lubed up.  The cable is free and engaging the drive.

What's odd is that one wheel drives and one doesn't.  When I say one wheel doesn't work, this is even if you lift the mower off the ground.  The one that works appears to have most wear to the pinions.

 

Does each side of the gearbox work independently or is it a solid axle all the way across?