Mountfield SP46 won't start: troubleshooting guide (UK)

Before you start: what you should know

The Mountfield SP46 uses a Stiga ST120 OHV 123cc engine — the same engine fitted to most Mountfield walk-behind petrol mowers. It's a simple, reliable single-cylinder design that has very few failure modes. When it refuses to start, the problem is almost always in the fuel or ignition system, both of which are accessible without specialist tools.

You'll need: a spark plug socket (16mm or 21mm depending on year), a fresh container of E5 unleaded petrol, a clean rag, and 30 minutes. If you don't have a spark plug socket, an adjustable spanner works on most years.

Important safety note: petrol vapour and electrical sparks don't mix. Do every step outdoors with the petrol cap on except when you're explicitly draining or filling the tank. Disconnect the spark plug cap before anything that involves spinning the engine by hand.

1. Stale fuel — the cause of 50% of all SP46 starting failures

Modern E5 and E10 petrol absorbs water from the air and degrades within 4–6 months. A mower left over winter with a tank of last-year's fuel will refuse to start — the petrol has lost its volatility and the carb passages have begun to varnish.

Diagnosis: open the fuel cap and sniff. Fresh petrol smells sharp and slightly fruity. Stale petrol smells of varnish, paint, or old shellac. If your mower hasn't run for 4+ months, assume the fuel is stale even if it looks normal.

Fix: drain the tank into a metal container (not plastic), wipe it clean, and refill with fresh E5. The drain plug is on the underside of the carb — turn it 90 degrees with a screwdriver. Replace the cap, prime the carb three times, and try to start. If the mower fires and runs but stumbles, run it for 5 minutes at full throttle to clear the carb passages. Most SP46s start cleanly after this with no further work.

2. Varnished carburettor — the next 25%

If fresh fuel doesn't fix it, the carb passages have already varnished. The Stiga ST120 carb is a straightforward float-bowl design with three jets that gum up in this order: idle jet first, main jet second, accelerator passage third.

Diagnosis: with fresh fuel in the tank, prime the carb 3–4 times and try to start. If it fires and runs only at full throttle but dies at idle, the idle jet is blocked. If it won't fire at all, the main jet is blocked.

DIY fix: remove the air filter cover (two screws), unbolt the air filter housing, then unbolt the carb (two 10mm bolts on the head). Drop the float bowl by undoing the brass screw at the bottom of the carb. Use carb cleaner spray and a strand of copper wire to clean each jet. Reassemble in reverse. Total time: 45 minutes. Total cost: £6 for carb cleaner. This fixes about 95% of varnished-carb cases.

Dealer alternative: £80–£100 carb service. If you're not confident with hand tools, this is the right move — Mountfield dealers do hundreds per spring.

3. Fouled spark plug — 10% of failures

If fresh fuel and a clean carb don't help, the spark plug is next. The Stiga ST120 uses an NGK BPR4ES or equivalent — replacement is £4 at any motor factor or £8 at a Mountfield dealer.

Diagnosis: pull the spark plug cap, unscrew the plug. A healthy plug is light tan in colour. A fouled plug is black and oily, or covered in white deposits, or has a wet sheen of unburnt fuel. Any of these means replace it — fouled plugs can sometimes be cleaned with a wire brush but the success rate is mixed and a new plug is £4.

Fix: gap the new plug to 0.7mm (the Stiga ST120 spec) using a feeler gauge or eyeball. Hand-tighten the plug, then 1/4 turn with a spanner. Reconnect the cap firmly. Try to start.

If a brand-new plug doesn't fire, the ignition coil is failing — that's a £45 part and a 20-minute replacement, but rare on the Stiga ST120.

4. Blocked air filter — 8% of failures

An over-rich mixture from a clogged air filter looks identical to a blocked carb. Always check the air filter before you spend an hour on the carb.

Diagnosis: pop the air filter cover. The foam element should be a uniform yellowy-orange colour. A grey or black filter is choked with dust and grass. A wet filter has been over-oiled (Mountfield dealers occasionally over-do this).

Fix: wash the foam in warm soapy water, squeeze dry, then re-oil with a few drops of motor oil and squeeze again to distribute. Reassemble. Replacement filter is £6 at any Mountfield dealer if the foam is shredded.

5. Engine flooded — 5% of failures

If you've been priming repeatedly and pulling without firing, the cylinder is full of unburnt fuel. The plug is wet, the engine won't fire even on a perfect carb because the spark is being shorted by liquid petrol.

Diagnosis: pull the plug. Wet electrode = flooded.

Fix: hold the throttle wide open and pull the rope 5–6 times with the plug out. This blows the cylinder clear. Wipe the plug with a rag, refit, and try again. The mower should fire on the first or second pull.

Prevention: stop priming after the third squeeze. The Stiga ST120 needs three primes from cold, not ten.

Diagnostic flowchart — order of operations

Work through these checks in order. Each one takes 5–10 minutes. If you complete all five and the mower still won't start, the problem is electrical (ignition coil, kill switch, or operator presence bar) and worth a dealer visit.

1. Sniff the fuel. If stale, drain and replace.

2. Pull the spark plug. If wet from flooding, dry it and pull-start with throttle wide open.

3. If plug is fouled black, replace it (£4).

4. Check the air filter. If clogged, wash or replace.

5. If 1–4 don't fix it, clean the carb yourself or take to a dealer.

If the engine still won't fire after a clean carb and new plug, the ignition coil is the next failure point — £45 part, 20-minute job. Past that, it's an internal engine fault and the mower is probably finished.

Cost summary: DIY vs dealer

DIY total for stages 1–5: under £20 (fresh fuel £6, carb cleaner £6, spark plug £4, air filter £6, plus 1.5 hours of your time). This fixes about 90% of starting failures.

Dealer service: £80–£120 for a full carb-and-tune service. Worth it if you're not confident with hand tools or the mower has multiple issues.

Replacement: a new SP46 is £279 at B&Q. A clean used one on Marketplace is £100–£140. If your SP46 is over 10 years old and needs more than a £80 fix, replacing is sometimes the right call.

Prevention for next year

Add fuel stabiliser (Briggs & Stratton, Stihl, or a generic — £6 a bottle, treats 20+ tanks) to your last fill of the season. Run the engine for 5 minutes after adding it so the stabilised fuel reaches the carb.

Or: drain the tank and run the carb dry at end of season. With no fuel in the system, varnishing can't happen. Keep the tank empty and the mower in a dry shed.

Replace the air filter every two seasons. Replace the spark plug every three seasons. Change the engine oil every 25 hours of running (most domestic users hit this once a year). Total annual maintenance cost: under £20.