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I really hope someone can help me out here, I'm having a real struggle to get this engine running.

The symptoms are:
Very hard to start - it "catches" readily but won't run;
- it splutters, misfires very badly and spits & pops back through the air cleaner;
- with much perseverance I can get it to the point where it'll run at idle but as soon as you open the throttle a little it starts spluttering & missing again (this happens with or without air cleaner fitted).
The spark plug is very sooty, indicating very rich running.

What I've tried so far:
- fresh fuel, new spark plug, checked fuel flow OK, checked spark OK, choke flap operating normally;
- twice I've stripped the (Huayi) carb and cleaned it with carb cleaner, blown it out with an airline, and it's had two sessions in an ultrasonic tank.
- I've also checked the engine oil for the presence of fuel and it's OK.

The engine is a Mountfield 7-500 and it's a 432cc OHV single (they have them made in China I think) and it's pretty much a clone of a Briggs & Stratton OHV single.
It's fitted to a 2012 Mountfield 1538H-SD lawn tractor.

Thanks for reading and I hope one or two of you super mechanics can give me some pointers.

Two things come to mind. Check the flywheel key to make sure it isn't sheared, and also check the valve clearances.

In addition to the above recommendations, I would also check the carburetor float to see if it has a hole and is "sunk" and verify that the needle & seat are completely sealing off the fuel supply. I say this because you mentioned that the spark is very sooty. This symptom begs the question how is extra fuel getting into the combustion chamber? In picture three the choke shows to be wide open. Is is possible the choke is closing off while the engine is running? Looks like the linkage is connected properly.

How many hours on this engine?

I go with both replies. Timing or fuel. Also check the needle / seat. The slightest mar can allow fuel to flow even though the float is good. Regarding the needle, is the tip metal or the hard rubber - like material? If it's metal and the tip is black, the black can be from ethanol (if that's what you use) when the mower sits for a period of time. I'd consider replacing the needle if the float is good and installing an inline shut-off valve. If you store the mower for a period of time and don't run the engine until it starves out from lack of fuel, use the shut off valve to starve it out so that fuel doesn't remain in the bowl. Don't forget to turn off the key when it dies. If you find and correct the timing / fuel problem, I'd proceed with a new set of plugs and be done with it.

Thanks fellas, for your insights and ideas. You inspired me to go back to the beginning and be methodical.

I'd already checked for spark and it looked OK. But this time I used my B&S spark tester inline so I could watch the spark with the engine running whilst trying to increase the revs.

To my surprise the spark was faltering in unison with the misfiring!!

The coil was only delivering a good spark at idle (low demand), but as I tried to increase the revs there was too much internal resistance for it to supply the required spark. Thus leading to incomplete combustion and a sooty plug - does that sound logical to you engine gurus?

I tested the coil (better late than never, eh?) and it was reading 11.29 K Ohms resistance whereas I understand it should be less than 5 K Ohms.

I did a continuity test on the kill wire, including various connectors and the ignition switch with the engine off and with it running and it all checked out OK.

But, here's the weird thing, with the kill wire detached from the coil the engine runs perfectly throughout the rev range, but if I touch it to the kill-tab on the coil the engine starts to misfire again.

What's happening here, has the kill tab on the coil gone closed-circuit or something?

There is a diode in the wire that bridges the two coils.
That diode is bad so one side earths out the other.
Not particularly common but does happen.
You replace the complete earth wire as a sub loom down to the plug.

bertsmobile1 said:

There is a diode in the wire that bridges the two coils.
That diode is bad so one side earths out the other.
Not particularly common but does happen.
You replace the complete earth wire as a sub loom down to the plug.

Click to expand...

It's a single cylinder so only one coil. Plus, it is not a Briggs & Stratton engine. I have had to replace the diode in a B&S twin in the past but do the single cylinder engines even need a diode?

Sorry, brain fart time.
In that case you have a problem with safety system .
Either one of the switches is going short or one of the connecting wires is earthing out.
This is PIA job, easy to do but very time consuming .
I had one where the PTO spring was slack which allowed the PTO lever to bounce around and the switch flicked on & off with engine vibes.
It will be best if you get a wiring diagram, work your way through it, jumping individual switches one at a time till the problem goes away.
So then you are looking at that switch or the wires leading from that switch.

Check the flywheel key to make sure it isn't sheared,
check the valve clearances
check that the carb float does not have a hole in it.

Micah Haarhoff said:

Check the flywheel key to make sure it isn't sheared,
check the valve clearances
check that the carb float does not have a hole in it.

Click to expand...

Doing it again, are ya? Tsk, tsk. LOL

I've narrowed it down to the coil, or a kill-wire problem. New coil going on today - fingers crossed . . .

In summary: carb, flywheel key, and valves are all good.

New coil made no difference so it's something affecting the kill wire that's causing the poor running.

I'll try bertsmobile1 suggestion of bypassing each safety switch in turn to see if the problem goes away.

It's weird though, that the spark is only weakened by the problem and not cut off completely :confused2:

That is the sort of thing a bad switch does.
It is not hard, but you will be lost without a circuit diagram as nearly all the wires are each wires, most will be black and some work by themselves while others work in groups.