OutdoorKing Repair Forum Forums Repairs and Maintenance Questions on Walk Behind Lawn Mower Frames Question On Scott Bonnar Cylinder Mowers SB45 Cutter Clutch Engaging Automatically

Hi All,

Not sure if this is the right place for this thread as it probably relates more to 'Repairs and Maintenance' for a reel mower - and I didn't notice a section for this. But I am hoping someone may be able to help me. I have a SB45 17" reel mower (that I am still intending on restoring some time soon!).

Lately (the last 4 or 5 times I've used it), I've noticed what I consider to be a fairly serious issue, that seems to be getting worse. Essentially, the cutter reel engages even when the cutter clutch handle is disengaged. It seems to be an issue with engine vibration as initially it happened at high revs (and presumably maximum machine vibration) but it now occurs even when the engine is running as slow as possible. I can actually see the reel 'twitching' when I disengage the clutch, and at full revs within about 10 seconds the cutter reel makes a 'snapping' noise and kicks back into full spin (even though the clutch lever is still in the disengaged position). I feel it's a bit of a safety issue, and now when I empty the catcher or wheel the mower over concrete areas, I have to actually kill the engine, rather than just lowering the revs and having the cutter disengaged.

I've had a cursory glance and I suspect it may have something to do with the adjustment of the cutter clutch and the engine drive shaft, but unfortunately that's about as far as my current knowledge will take me. I did notice what seems to be a noticeable amount of free movement in the cutter clutch lever from full vertical (disengaged) to the right (if standing in front of the machine looking at it) - i.e. the opposite direction the clutch lever is intended to move. I've also noticed the screw directly beneath the engine half of the cutter clutch - does this have anything to do with the issue? I also noticed what I thought may be a bit of wear on the clutch lever where this screw meets it - it appears to have ground the surface flat (unless this is originally how it was manufactured). I have attached some photos below, so hopefully they describe better what I am talking about.

Has anyone come across this issue before? Is there an easy solution? I am intending on doing a full restore to this mower, so if the issue is difficult to repair I am considering just packing the mower away until a time comes where I can complete the full restore and hopefully diagnose the problem in more detail.

Appreciate any thoughts or advice. Thanks in advance.

Kenny.

Hi Festing,

I don't feel there's that much wrong there apart from a bit of adjustment and lubrication issues.

Your black plastic thrust pad is on it's way out and should be replaced on rebuild, but most of it is in your lever adjustment.

You mention a fair amount of free play in the lever, well that's it. You have to adjust the set screw in the fork assembly so that there is about half a millimetre of play between the clutch fork and the thrust pad. This will then take out the excessive amount of slack in the lever and push the clutch cone further inwards, thus no chance of it grabbing. The flat part is designed to be there but put a dob of grease on the contact points where you can see shiny metal.

Also while we are working in the same area I would check to see if the two square headed set screws that pin the clutch body to the engines PTO shaft are nice and tight.

See how you go and get back to us.

Also a bit of an observation and that is the clutch half should be a bit closer to the engine, about two to three millimetre clearance.

BTW thanks for the photos.

Cheers,
BB.

Hi bonnar_bloke,

Thanks for your reply - that did the trick! There was a bit of mucking around as when I first tighten the set screw I must have gone too tight as I could no longer manipulate the lever - then it went back to too loose. Got it where I thought was right and started it up and it ran like a dream. I suspect that adjustment actually had impacts on the cutting reel itself when it was engaged, as it seemed to run much faster/better post adjustment. I did note that the lever still moves to the right about the same amount (i.e. past full disengaged) but the handle is certainly a lot 'tighter' to operate - is this correct? I feel the lever shouldn't really go past full vertical, the only explanation I can think of is that the flat part on the lever is overly worn and allows a bit of free play, even when the set screw is adjusted.

A quick query - which piece is the 'black plastic thrust pad'? Is that the first circular black piece with notches in it at the top of (what I assume is) the fork assembly? Should this piece not be as notched/worn as it is?

Also, can you please clarify your statement about the clutch half being closer to the engine - is that the gap between the silver clutch half and the orange side wall of the engine (shown in picture 6)? IS that as simple as undoing the two square headed set screws and sliding the clutch half closer towards the engine on the drive shaft and then re-tightening them as well as adjusting the clutch fork set screw accordingly to move the 'other' clutch half equally closer? Sorry if that is confusing, but this mechanical stuff is all fairly new to me!

Thanks again for your advice - I feel like I've had a small victory on a problem that's been nagging me for weeks!

Kenny.

Hi Festing,

See post below.

Cheers,
BB

Hi Festing,

Yes the part I am referring to is that black plastic component . The notches on the side are there to accommodate the clutch fork and thus stop the pad from rotating. It's just showing a bit of heat stress.

Yes the clutch half is a fraction to far away from the side of the engine casting. all you need to do is loosen the two square headed set screws and slide the body towards the engine so that there's about a 2 to 3 mm gap between and then adjust the clutch fork accordingly as you did previously.

Also remember that when the clutch is engaged (driving Cutter Reel) that you should have a small amount of free play between the fork and thrust pad so that there's no actual residual pressure on the 3 components involved (Clutch Cone, Thrust Pad and Clutch Fork)

I'm glad you've felt a sense of achievement as it's always good to be able to learn something new and understand its fundamentals.

Cheers,
BB

Hi bonnar_bloke (and others),

At the risk of reviving an old thread, I'm looking for more advice on the above issue. I did adjust the clutch fork and had some instant relief from the issue - unfortunately, it was only short-term and started reoccurring again. I adjusted again on the weekend just gone to the point where the cutter clutch lever was reasonably hard to manipulate - this solved the problem but only in the short-term again. The cutter blade seems to always want to engage when the engine is running, to the point where if the engine is at full speed and the cutter is engaged, when I disengage it using the lever it doesn't even slow down (keeps rotating at full speed).

I'm at a loss as to why it's suddenly doing this but can only assume it may be an issue with the worn black plastic thrust pad on the cutter clutch? Could this be correct and is this part hard to replace?

Thanks again.

Kenny.

Hi Festing

BB is unwell at the moment.

Whether the clutch engages or disengages is dependent upon
the condition of the components. The clutch lever has an eccentric
slot that determines the throw (travel) of the clutch cone.

If the cutter keeps cutting 'at full speed' when disengaged then
the issue will be:

- no or little cone travel
- worn thrust bearing
- worn thrust pad
- incorrect adjustment
- poor set

Could you please confirm that the reel is making contact with the bedknife.
They require contact to create slight drag that prevents reel free-spinning.

I feel it may be time to unbolt the motor and have a good look.
Hope this helps in BB's absence.

--------------------------
Jack

Hi Jack,

Thanks for your reply. I agree regarding unbolting the motor and having a look - I've been keen for some time to do a full rebuild and this may be the necessary catalyst. There is a fair amount of cone travel in the clutch (in my opinion), and I believe the cork cutter clutch lining was replaced when I purchased the mower (some 18 months ago). I'm going to go make a few adjustments now but I think the best course of action is a rebuild - I suspect there may be a few parts that are on the way out so I'd feel more comfortable just doing the whole lot!

Will keep you updated.

Kenny.