Congratulations on your "new" seadoo!
Firstly, you need to change the lines. The lines may or may not be your problem, but if you don't change them, you'll likely have neverending problems and then need to change the engine. This is because the old grey lines degrade and release purple gunk into the fuel, which then clogs the carbureator filters, which then prevents enough gas from getting to the engine. Without enough gas, the engine will overheat (gas actually cools the cylinders), and destroy itself.
You should start by changing the lines and the fuel selector valve. I know you said you don't have the knowledge, but you're going to need it! So start watching some youtube videos and you'll see it's actually pretty easy. Go to your local auto shop (napa, preferably) and ask for 1/4" carbureated fuel line (as it's WAY cheaper than fuel injection line) and get about 25 feet of it. That'll be more than you need, but that way if you make a mistake you don't have to go back. Then get yourself a nice large pack (around 50) of stainless steel worm hose clamps (you can pick these up on Amazon) and go to town. Make sure they're stainless steel or you'll have a rusty mess in a year.
You'll need the following tools:
Small flatbladed screwdriver
Dykes or snips to cut the fuel line
For each old fuel line, try gently pulling it off where it attaches to a fitting. While pulling you can twist a bit, but always pull straight--never pull sideways, or you may break the fitting the line goes in. If it won't come off, undo the existing clamp with a screwdriver and remove it on both ends. Using the existing old hose as a guide, cut an equally long section of the new 1/4" fuel line and with a worm clamp on each end, put it back in as a replacement for the grey line you took out. Tighten both worm clamps until they're snug on the fuel line and you can't twist the worm clamp by hand. You should see it just start to dig into the rubber on the hose in between the tracks. Don't overtighten or you could snap the fittings it goes in, especially the plastic joints.
Only do *ONE* fuel line at a time so you don't get confused and hook them up wrong. There are a couple short segments that vent out the hull--you can leave these as the old grey lines, but you have to change *all* the rest, including the ones between the carbs.
While you're doing this, you should change the fuel selector valve with a brand new one. Also, gently clean the filter in the water separator, and replace the o-ring in the water separator. To identify these parts, just Google and you should get plenty of examples. They're both hooked up to the fuel lines you'll be replacing.
When you buy parts, buy only from reputable dealers (seadoowarehouse.com, osdparts.com, westsidepowersports.com, or a local sea doo dealer). Whenever possible, get OEM parts, which means that they were made by the manufacturer. They typically cost more, but there's a reason (quality). The aftermarket parts will be MUCH lower and price and tempting, but they often fail. Note: Don't buy parts from amazon, as people often mislabel and scam what's being sold. I've seen obviously non-OEM parts listed as OEM on there. Just don't do it.
Finally, once you have the fuel lines, selector valve, and water separator done, you will have to get back into the carbs. You've run the carbs with the old fuel lines and they may be gummed up with junk. Talk to your dad and see if he used genuine mikuni kits when he rebuilt the carbs. If he didn't, you're really going to need to buy those kits (osdparts.com) and rebuild the carbs again. If they were genuine kits, then check the small filters in the carbs--they're likely full of junk from the old lines. This was an excellent video seeing how to rebuild the carbs on these:
There's a lot there, and it may be daunting, but if you do this right, you'll have a fun toy that can last you a long time. Not only that, compared to modern cars and boats, the parts are cheap and the engines are simple. If you do this, you're going to give yourself mechanical skills that may be useful later in life.
If you need any help along the way, or if you get it done and you're still having issues, post here and we'll try to help you out.
Good luck!
