An engine-driven sprayer that will not start on a spray day is expensive, because the spray window rarely waits. The good news is that most no-start faults come from a short list of causes, and you can work through them in order with basic tools.
Work in this sequence. Do not skip ahead, because guessing usually means replacing parts that were never the problem.
Before you pull the cord
Three things stop more sprayers than any mechanical fault:
- Fuel switch or tap closed. Obvious, and still the most common.
- Engine switch off. Many units have a kill switch that is easy to knock.
- Choke in the wrong position. Cold engine needs full choke; a warm engine usually needs none. Repeated pulling on full choke floods a warm engine.
Step 1: Fuel
Old petrol is the single biggest cause of starting trouble on seasonal machines. Petrol left in a tank for a few months absorbs moisture and loses its lighter fractions, and it will not fire reliably.
Drain the tank, dispose of the old fuel safely and refill with fresh petrol. On two-stroke machines mix fresh oil at the ratio in your manual. Check the fuel filter and the fuel pipe at the same time — pipes go hard and crack where they bend, and a cracked pipe draws air instead of fuel.
If the machine has a primer bulb, press it until you see fuel and it feels firm. A bulb that stays soft means air is getting in somewhere upstream.
Step 2: Spark
Remove the spark plug and look at it.
- Wet with fuel — the engine is flooded. Leave the plug out, open the throttle fully, pull the cord several times to clear the cylinder, dry the plug, refit and try again with no choke.
- Black and sooty — running rich, or a choked air filter. Clean or replace the plug and clean the filter.
- Oily — on a two-stroke, too much oil in the mix. Re-mix at the correct ratio.
- White and blistered — running too lean, which will damage the engine if ignored. Stop and get the carburettor checked.
To test for spark, reconnect the plug cap, hold the plug threads against bare metal on the engine and pull the cord. You should see a clear blue spark. A weak yellow spark or none at all means a new plug first, and if that does not fix it, the ignition coil or the kill switch wiring.
Step 3: Air
A clogged air filter starves the engine. On a sprayer working in dusty fields this happens fast. Foam filters can be washed in soapy water, dried completely and lightly oiled if the manual says so. Paper filters are replaced, not cleaned.
Never run the machine with the filter removed, even briefly. Dust reaching the cylinder does permanent damage.
Step 4: Compression
Pull the starter cord slowly. You should feel firm resistance as the piston comes up. If the cord pulls through easily with almost no resistance, compression is low and the fault is internal — worn rings, a damaged cylinder or a leaking gasket. That is a workshop job, not a field fix.
When the engine runs but nothing sprays
This is a different fault, and it is in the pump, not the engine.
- Air in the suction line. Check that the suction pipe is fully submerged and that the strainer is not sitting in silt. Check every joint for a loose clip.
- Blocked strainer or nozzle. Remove and clean. Never clear a nozzle with wire.
- Worn piston seals or valves. The classic sign is an engine running normally with pressure that will not build or that fluctuates badly. These are wear items — see HTP pump spare parts.
- Pressure relief set wrong. If the bypass is open, pressure escapes back to the tank.
- Damaged pressure hose. A hose that balloons under pressure is about to fail and should be replaced immediately. Our spray hose range lists the correct pressure ratings.
How to avoid most of this next season
Before storing the machine at the end of a season, run the tank dry or drain it, flush the pump with clean water, and store it dry and covered. Remove the spark plug, add a few drops of oil into the cylinder, pull the cord twice and refit the plug. That single habit prevents most of the starting problems that appear the following year.
If you have worked through all four steps and the machine still will not start, stop there. Repeated pulling on a flooded or low-compression engine makes the repair bigger. Tell us the model and what you found at each step and we will tell you what to check next.