Mountfield Lawnmower problems

[jazzcricket]

Hi, i have a Mountfield SP 470 mower.
It will start up, run for a minute, then die.
I've cleaned the air filter, changed the oil and fuel,and put a new spark plug on, but the problems still the same. The engine sounds rough,not running smoothly.
Any suggestions would be appreciated,in case i've missed something obvious.

cheers

 

[Jackrae]

Sounds like you're not getting sufficient fuel into the carburettor bowl. The bowl slowly fills, you run it but burn fuel faster than it fills, runs out of fuel and then dies. After a few minutes the bowl has refilled and you start the cycle again.

 

[Burnerman]

I think on this machine, the tank is below the carburettor?
If so, then replace the gasket that sits between the carb and the tank - it serves as a pump diaphragm and they are very prone to failure.
At the same time, with the carb off you'll see a dip pipe that descends into the fuel. Just see that the filter on the end is clear.
John

 

[jazzcricket]

Thanks chaps.
I've ordered a new gasket, only cost a fiver, will fit it and post if it solved the problem.

Thanks

 

[jazzcricket]

Quick update, fitted the part and it hasn't made any difference.
In fact, looks like i may have made things worse.
The primer bulb now gets sucked in after the engine starts, and then the engine dies as before.
I'm thinking either i've fitted the gasket and diaghram the wrong way round,or there's some dirt in the carb. Gonna take it apart, give the carb a clean and see what happens.

cheers

 

[Burnerman]

Yep - gasket issues there!
Always compare the gaskets like for like on these - sometimes the differences are very subtle!
John

 

[jazzcricket]

yeah thanks mate
when i took the carb apart there semmed to be only 1 thin rubber gasket.
The new part came with 2, diagram and gasket. i'm wondering whether i needed to fit both?

 

[Burnerman]

I could really do with a pic of it, but if the thinner one has some 'flaps' on it, this is the pump diaphragm and the gasket proper sits above.
I'm still not too sure which carb you have though!
John

 

[jazzcricket]

Quick final update, thankfully!!
I removed the carb again, gave it a good clean with compressed air, looked like there was a tiny blockage on a filter on one of the tubes.
Anyway, refitted, and yes burnerman, the thin rubber diaphragm had flaps, fitted that first, then the gasket.
Voila!! Runs fine now..
I've attached a useful video in case someone else has the same problem.
Nice cheap repair.

http://www.youtube.com/watch?v=C5nzigcNTwo

 

[Burnerman]

Nice one mate and well done......those flaps are in fact valves that open or close passages courtesy of pulses from the engine.
These things are failing more frequently than ever due to the ethanol content of petrol today, so I'm told.
John