Mountfield ride on, engine will not stop

[marksman702]

Hi All and thank you for accepting this newbie to the forum.
I have a mountfield ride on mower that starts and runs perfectly fine and turns off ok when cold, however once it get hot it will not turn off and also none of the safety cut out features work either.

Does anyone have any idea about why this is happening please?

Thanks in advance

Mark

 

[transam]

Pre ignition ? ???

Fuel ?
Timing ?
Plugs ?
May be ???

 

[Harry Bloomfield]

Pre ignition ? ???

Yep!

Fuel ?
Timing ?
Plugs ?
May be ???

Or carbon build-up in the combustion chamber.

First, before investigating further - does the note of the engine change when it is switched off, but continues to run?

 

[marksman702]

No change when I turn the ignition off, what I used to do to get it to cut out was engage the blades, this would cause it to stall, however when I tried this again it didn't work, it tried to stall but carried on and the only way to stop it was to turn the fuel off, I also had it serviced a few months ago and highlighted the issue to them but they said they couldn't find anything wrong, probably because they only tried it when cold.

 

[marksman702]

I am just in the middle of getting it out of the garage and get access to the motor etc

 

[marksman702]

What I don't get is the fact that it functions great from the start and all the safety features work like getting off the seat stops it, but once I have been using it and its hot it stops working, lets say that it is an over heating problem for example, how would that stop the electrical side of it, must say its puzzling me.

 

[Harry Bloomfield]

No change when I turn the ignition off, what I used to do to get it to cut out was engage the blades, this would cause it to stall, however when I tried this again it didn't work, it tried to stall but carried on and the only way to stop it was to turn the fuel off, I also had it serviced a few months ago and highlighted the issue to them but they said they couldn't find anything wrong, probably because they only tried it when cold.

No change in engine note, sort of discounts the pre-ignition/running on theory.

Rather than points and coil, they usually use magneto ignition, so the switch doesn't switch off - it shorts the ignition/magneto to ground, to stop the engine. My guess would be, that the shorting wire looses contact, when things heat up. As a test, you could add a bit of wire from the magneto, and take it to somewhere convenient, where you could short it to a known ground.

There is maybe a break in the wire somewhere, it will likely go around several places on the machine, like the ignition switch, seat, brake, forward and reverse lever.

Do you have some sort of multi-meter and could check continuity of the wire, around the machine. Type the make and model, plus circuit diagram into a search engine - you might be lucky, and find a diagram, or one near enough to work with.

 

[carmanmemoranda]

Or even something is getting too hot and expanding, failing to make, or break a contact

 

[lostinthelight]

Theres often an electronic fuel shutoff solenoid in the bottom of the float chamber thats supposed to combat pre ignition..check if you have one.

 

[Harry Bloomfield]

Theres often an electronic fuel shutoff solenoid in the bottom of the float chamber thats supposed to combat pre ignition..check if you have one.

It also helps prevent evaporation from the carburettor, mine has one.

 

[Burnerman]

If the safety interlocks don’t work either I’d consider the circuit board beneath the dashboard......unfortunately they aren’t cheap to take a gamble with!
Check also the push on connector on the back of the ignition switch, and check the switch securing nut isn’t too tight!
John (mountfield 1436, 2002 model).