First of all, fresh fuel is your very best friend 2nd to confirming the oil injection is working top notch and using the correct oil. Many people have reported great results even with the Wall Mart brand TC-W3, with hundreds to thousands of hours under their belt. Make sure it's the right type, I tend to use the OEM oils myself b/c then nobody can say the oil was the wrong type or poor quality. Honestly, I really don't think there are many bad oils out there anymore, just using the wrong type is more what I see is the case.
As far as the fuel tank goes, 1st rule is you don't want water and trash in your fresh fuel. A quick way to find out if there's a problem is by emptying the contents of your spin-on fuel filter into a clear jar. 1st, shake the contents of the filter up good first to loosen any dirt, and pour the contents of the spin-on filter into a clear glass jar and allow it to settle for a couple hours. There's you proof of how much trash that filter kept from going into your injection system, hopefully it's perfectly clean!!! I know in my boat with steel tanks I've been getting plenty of trash from one side, that's why I installed a spin-on filter, my small little in-line paper fuel filters kept plugging up sometimes after about 30 minutes. Hail to the mercury style spin-on fuel/water separator canister!
(The larger the boat, the bigger the canister)
If the results look clear with no dirt, water, or foul looking gobs of goo then you can be pretty confident your fuel tank doesn't have much trash in it if at all. The spin on filter really should be changed annually though, they're a very effective design and the best thing mercury did since installing electric starters and fuel injection. Fuel Injection is the reason though, it's necessary to keep your fresh fuel from being contaminated by junk that shouldn't be in there. The reason is, a clogged fuel injector will not fuel a cylinder properly and that cylinder can be damaged as a result. This is a characteristic of 2-stroke engines, it's very important to not run them if one cylinder isn't making power like it should, take care of the issue before damage occurs. If you feel the power dropped off stop and investigate the reason, keep a spare known-good set of spark plugs on board so you can change them immediately at the first sign of trouble. If that doesn't resolve the problem then know that if that engine is run further it's at risk of being damaged.
So what to do if you do discover trash in the fuel? Well if you can remove the fuel tank then you can swab out the trash by using clean rags on the end of a stick to mop up the dirt and debris, sometimes it's possible to do this while the tank is mounted in place by accessing through the inspection covers.
Occasionally I'll run into a boat, usually one of those huge cabin cruiser type boats, you know, the type that's 25~30 feet long with dual fuel tanks made of aluminum (subject to internal corrosion) and no cap on the fuel fill so rain water is going right in there. Sheesh, those tanks are deep in the bilge you know, there's just no getting them out. Fortunately, most have inspection access covers where I can remove and reach in with my bag-o-rags on a stick and mop up the water and brown scum.
Then, the fuel system can be partially filled and the fuel cycled through a big spin on filter using an electric fuel pump, circulating the fuel from the tank through the spin on filter and back into the fuel tank, this process is called polishing the fuel and while the boat is sitting in the water the fuel tends to slosh around so the trash doesn't just lay stagnant in one corner of the fuel tank.
So I bet you get the idea, make sure those kids aren't pouring buckets of sand down your fuel fill while you're busy cooking hot dogs and burgers on the beach, I caught them doing that a couple years back, about to pour a soda into my fuel tank, LOl....
For more great info, have a GOOG look for articles on polishing your fuel, there are a number of free articles out there all about the importance of keeping your tanks clean and how to accomplish that.
