G'day Tim,
First thing I’d do, if you haven’t already, is take the fuel hose off the fuel tank. There’s usually a fuel filter strainer in the outlet of the tank. Sometimes you take the hose off and the filter is still in the end of the fuel hose. Then clean the filter and the tank out.
The carby has a fuel tap on it, sometimes I’ve had to push a wire into the fuel pipe inlet to dig the corrosion out, and then pull the fuel tap apart to clean the gunk out of it. Undo two screws and the fuel tap comes apart.
With fuel in the tank and the fuel bowl off and the float and needle removed you can check to make sure the fuel flow is correct.
With this, 5 mins of mowing it starts to run rough and surge before stopping altogether.
It's usually fuel or spark problem. Also you do the easiest fixes first like replacing the spark plug with a good known plug.A bad plug can fail when hot.
When a small petrol mower like a Sanli runs fine cold, then after ~5 minutes starts surging, running rough, and finally dies, that usually points to a heat- or fuel-delivery–related problem, not basic tuning.
Here are the most common causes, roughly in order of likelihood:
1. Fuel tank not venting (very common)
Symptom match:
Runs fine for a few minutes → fuel flow slowly stops → surging → stalls.
What happens:
If the fuel cap vent is blocked, a vacuum forms in the tank as fuel is used. After a few minutes, fuel can’t flow.
Quick test:
Next time it starts to surge, loosen the fuel cap slightly or remove cap.
If the engine smooths out or restarts immediately → blocked fuel cap vent
Fix:
Clean or replace fuel cap
Make sure vent hole isn’t clogged with dirt or grass
2. Partially blocked carburettor (main jet / emulsion tube)
Symptom match:
Runs OK at first
As demand increases and fuel level changes → leans out
Causes surging, hunting, then stalling
Why tuning “helped” temporarily:
Adjusting mixture can mask a blockage, but once warm and under load, the restriction shows again.
Fix:
Remove carburettor
Clean main jet, emulsion tube, and all passages with carb cleaner
Blow through with compressed air (don’t use wire)
3. Debris in fuel line or tank
Symptom match:
Debris floats around
After a few minutes it gets sucked into the fuel outlet
Engine starves, then stops
Debris floats away again → restarts later
Check:
Drain tank into a clear container
Look for dirt, rust flakes, plastic shavings
Check inline fuel filter (if fitted)
4. Ignition coil breaking down when hot
Symptom match:
Runs perfectly cold
Once hot → misfire, surging, stall
Often won’t restart until cooled
Test:
When it dies, immediately remove spark plug
Check for spark
No spark hot, spark returns when cool → coil failing
Fix:
Replace ignition coil
5. Blocked fuel cap pickup screen or carb bowl vent
Less common, but can cause:
Gradual leaning out
Surging under load
Stalling after several minutes
6. Overheating (less likely but possible)
If:
Cooling fins packed with grass
Flywheel shroud clogged
Check:
Remove top cover
Clean cylinder fins and flywheel area
Cheers
Max.
