It's likely that there is dirt in the carb. I've run HP470s for the last 5 years in my little gardening business.
Take the air cleaner off by withdrawing the long screw from the top. Remove the 3/8" bolt from the front of the petrol tank. Remove the 1/2" bolt from the side of the carb inlet and pull the tank towards you, gently. Don't lose the rubber gasket(s). When the tank assembly starts to move, disconnect the throttle butterfly rod carefully and you will be left holding the tank/carb assembly.
Pour the petrol in the tank back into your can. Undo the 5 Phillips screws holding the carb to the tank. Split the carb from the tank carefully or you'll damage the gasket. Don't lose the spring from the underside of the carb. Pull off the gauze jet filter and blow through it, then replace. You can blow the jet through but I've never had a blocked jet yet; the gauze is too fine. Look at the filter on the end of the fuel pickup. Brush it with a toothbrush and rinse in petrol if it's dirty.
Look inside the fuel tank. If you can see dirt or water, rinse it out with meths and/or petrol. Use kitchen towels to make sure the tank is bone dry and gleaming inside.
Now look at the bit of the tank that you pulled the carb off. There's a hole where the fuel pickup goes and a bowl next to it with a cross moulded in the base. This is where the dirt is supposed to collect. It's probably pretty horrible in there. Clean it thoroughly with petrol, meths and a paintbrush until it's spotless. Put everything back together after you've oiled the sponge on the throttle butterfly.
Now you need to stop it happening again. Don't put fuel in the tank straight from the can. Use a funnel with coffee filter or kitchen towel in to keep the fuel clean.
Happy mowing!
Hawkeye (the gardener), August 2008
