I have used this product as an additive to "renew" old gas in a car, and was impressed. I have read on other forums that it does wonders if you add it straight into the fuel system via the brake booster vacuum line, let it sit for a while, then restart. Supposedly it really cleans thing up. I have heard nothing but postive things about doing this.
http://www.seafoamsales.com/motorTuneUpTechGas.htm
http://www.seafoamsales.com/motorTuneUpTechGas.htm