I have a Rover Ranger 547 with 67 hours on the clock from new. The engine starts and runs fine, but after about 10 minutes as the engine warms up I get a miss fire and the engine is surging. I have fitted a new fuel filter, replace the fuel in the tank, checked the fuel cap breather. Removed and checked and cleaned the carburetor, fitted a new fuel soleniod. Fitted a new spark plug and still I have this surging problem after about 10 minutes of running. I do not know where to look next. Where I live there is no Rover service agent and the spare parts I need have to come from 6000Kms away.
With the symptoms you describe, the next step is to determine whether the problem is fuel-related or an ignition (spark) problem.
When the engine starts surging after about 10 minutes, try pulling the choke on slightly. If the engine immediately runs smoother or the surging improves, that points to a lean fuel mixture problem. It could be caused by a fuel delivery restriction, an air leak, or a carburetor issue that only shows up once the engine is warm.
Another good test is to remove the air filter and, while the engine is surging, spray a small amount of Start Ya Bastard (or a similar starting fluid) into the carburetor throat. If the engine smooths out or picks up, that strongly suggests a fuel mixture problem rather than an ignition fault.
If neither of those tests improves the way it runs, I would start looking at the ignition system. A common fault is an ignition coil that breaks down as it heats up. The engine will run perfectly when cold but begin misfiring and surging once the coil reaches operating temperature.
A cheap adjustable spark tester can be very useful here. Check the spark when the engine is cold and running well, then compare it to the spark when the surging starts. If the spark becomes weak or intermittent when hot, the coil is likely failing.
The advantage of an adjustable-gap spark tester is that it allows you to test the coil's voltage output against a specific resistance, providing a much more reliable diagnosis than simply looking for a spark in open air. These testers work on everything from modern electronic ignition systems to small engines, making them a handy tool to have in the toolbox.
The results of those tests should quickly tell you whether you need to keep chasing a fuel issue or focus on the ignition system.
When the mower starts surging after 10 minutes, hook the tester up and see if it will consistently jump a 6–8 mm gap. If it jumps strongly when cold but becomes weak or intermittent when hot, I'd be very suspicious of the ignition coil breaking down with heat. If the spark remains strong while the engine is surging, I'd go back to chasing a fuel-mixture problem.
You can get a spark tester under $20. Adjustable Ignition Spark Tester T&E Tools 3404
