Hello, I have a Bolens STG125, I bought it as a non runner, I've changed the battery, topped up the oil, drained the petrol and put in fresh got a new spark plug. When I turn over the mower it turns over at a good speed and after about 10-20 seconds of it turning over the engine starts making very loud bangs over and over again. Sometimes when turning over the mower sounds to have started but when I stop the mower turning over it just dies with no attempts to even keep running. Please let me know if you have any experience in the situation or may be able to help in any way!

Thanks
Luke

Hello, thanks for your help, there is new fuel in the mower, the mower is having no problems regarding air intake and there is a good strong spark.
Thanks
Luke

Well I guess you know why it was a non;runner. The gas and air mixture is failing to combust on one stroke of the piston so on the next stroke you have double the amount in there which then does ignite and the loud backfiring occurs.Seriously, sitting for awhile before you rehabilitated it, maybe there is some rust or sludge on the inside of the engine that is causing the valves to stick. With engine knock in a car you'd try high test fuel to eliminate it. There are also I think fuel additives you can add to the gas to clean out deposits. Problem could be you need to tinker with the screws that set the fuel amount going in, maybe regrind the valves, check the rings.. Could be a worn or deformed camshaft. Make sure the fuel line is free of any dirt or obstuctions. Even try a hotter plug.

Troubleshooting any internal cumbustion engine you have to inspect for obvious damage and then determine if you have:
1) fuel
2) air
3) spark

And then you start looking for other things.

Based on the limited amount of information, the first place I'd look is the flywheel key.

Sounds like a good candidate for a tear down inspection to me.

Check for a wet plug. See if engine sputters if carb cleaner is used in the spark plug hole. Don't use starting fluid.

Check flywheel key.

Remove carb. Clean and rebuild.

Yeah the flywheel key. I didn't think of that, and that's the most obvious thing that might be wrong. And it's easy enough to check too.