How to inspect a used Mountfield SP46 — UK guide
The SP46 is the best-selling self-propelled petrol mower in the UK. It's also the most-listed used mower on Marketplace — meaning condition varies wildly. Here's the 10-minute inspection that catches every common fault.
Before you go
Bring: spark-plug socket (16mm), a torch, and a screwdriver. Optional: a feeler gauge.
Insist on a cold start. A warm engine hides every starting problem. Confirm with the seller before you travel that the mower has been cold for at least 30 minutes.
1. Cold-start the Stiga ST120 engine
Three primer squeezes, full throttle, pull. A healthy SP46 starts on the first or second pull cold.
Won't fire after four pulls = stale fuel or varnished carb. £35 dealer fix or £6 DIY with carb cleaner — knock £40 off the asking price.
Knocking or rattling at idle = walk away. The engine is finished and a replacement Stiga is £150+.
2. Test the self-drive on an incline
Engage the bail-arm on a kerb-step gradient. The mower should pull without slipping or hesitating.
Slipping under load = stretched drive cable, the most common SP46 fault after 5 years. £18 part, 30-minute DIY. Knock £30 off.
3. Inspect the steel deck
Lift the front of the mower and shine a torch underneath. Surface rust is normal and harmless.
Rust through the deck (you can see daylight, not just brown surface) = walk away. SP46 decks are non-replaceable economically.
Welded patches = the mower has been through a serious impact. Walk away unless the price reflects it (60% off fair value).
4. Check the blade-hub and bolts
With the spark plug disconnected, tilt the mower and grip the blade. Vertical play means the spindle bearings are gone.
Rounded blade-bolt head suggests previous DIY damage. £8 fix but flag it.
Unbalanced blade (visible vibration when running) = £18 generic blade. Easy fix, but should knock £15 off.
5. What it's worth in 2026
2014–2018: £100–£140 in tidy condition with working drive.
2019–2022: £150–£200. Newer engines, less rust risk.
2023–2025: £210–£260. Near-new, sometimes with warranty remaining.
Knock £30 for: stiff drive cable, cracked primer bulb, missing service stamps. Knock £80 for any rust through the deck.
FAQs
Is the SP46 worth more than the SP46H?
The SP46H has the Honda GCV engine instead of the Stiga ST120 — it's worth about £100 more used. If the listing just says 'SP46' assume it's the Stiga; the H suffix is what to look for if you want Honda.
When is the best time to buy used?
October-November. End-of-season sellers want the mower out of the shed before winter and prices drop 15-20%. See our seasonality guide for the year-round pattern.
What does a full Mountfield service cost?
£80-£120 at a dealer including oil, plug, air filter, blade sharpen and carb clean. Worth it once at purchase, then DIY annually for under £20.
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