Small Engines (Lawn Mowers, etc.): Lawn Mower, model numbers, varnish

Question
HI. I have a Briggs ans Stratton HP470 lawn mower and I'm having a few problems. It starts with no problem but sounds like its missing and I'm not getting any power but it continues to run. I've checked the spark plug and cleaned it up its full of oil and fuel. However the fuel is from last summer I'm heard this could make a differeance is this true. Thanks

Answer
Yes, I have seen gas from the year before so worthless, that it would
barely burn if poured on the burnpile.  What is even worse is that
it breaks down into a varnish that clogs up the jets in the carb.
Sometimes dumping out the old fuel and installing fresh will get it going and after running a bit, the varnish will dissolve and burn
on through.
    If not, you will have to clean the jets in the carb, and
for me to help much with that, I will need to know the engine's model numbers,

  Thanks,

  Fish