rust in tank

my tank is pretty much lined with rust on the inside. a friend told me about a product that eats away the rust and lets it burn up in the engine ... but that sounds a bit far fetched and doesnt sound good for my engine... any tips?

That don't sound right to me either. I dunno what to do, but there are smart folks here that I bet do.

Drain the tank into the lawnmower's gas can. Remove the petcock and filter. Remove the gas tank. Get a well nut, washer and bolt the right size to plug the petcock hole. Unless it is the three hole type petcock, then you will have to use duct tape. Now is a good time to replace the petcock filter. It is also a good time to repair or replace the petcock if it leaks and/or fuel lines if they are cracked or brittle. There is a method if the tank is very rusty using a known quantity of steel nuts to scour out the inside of the tank. If it is that badly rusted, or if it gets a hole, it may be better to take it to a radiator shop to be creamed. Go to the pool supply and buy 1 gallon Muratic acid. Have plenty of water handy. A garden hose is best. Pour in about a pint, seal it up and shake it all around. Try not to get any on you or the paint. If so rince with plenty of water. Pour the muratic acid out onto a grease spot on the driveway. Inspect the inside of the gas tank with a flashlight. Try not to breathe the fumes. If it got all the rust out rince it with plenty of water, If not pour in another pint of muratic acid, seal it up and shake again. Be sure to rince everything with plenty of water. Shake as much water out of the gas tank as you can. Shoot some AMSoil MP Metal Protector into the gas tank. Put the gas tank in the sun to dry with the gas cap and petcock still off. When it is dry it will be hot. It is important to get done before the dew sets. Put the petcock with new filter on and the tank back on the bike. Hook the fuel lines back up with clamps. Pour very slowly enough gasoline to go get a fill up of premium. Use all but the dregs in the lawnmower.

I found this:

http://f6.grp.yahoofs.com/v1/MHsvQ541V2SMmM77YV3P02hldg4qGV7OxXzHm3M8ba8wyi4wEh sljjJqGTrCtKU8q6aynm14H1TKsclBr-SBQFP82wjJwK0BRVs/Electrolytic%20De-rusting%20method.htm

that looks like a much better method than muratic acid.

http://autos.groups.yahoo.com/group/hondacb900cb1000/
Created with IMS Web Dwarf from Virtual Mechanics Inc.

(See Chemical Tank De-rusting for the tried-and-true brit-iron method.)

Here's a repost of a method I used quite successfully. There's no dangerous chemicals to spill; a consideration for me since I: a) live on a lake, and b) drink from a well. There's also no ball bearings or nuts to recover afterwards. You need a 10 amp battery charger, duct tape, a box of salt, water, a funnel, a piece of steel rod, electrical tape, methyl hydrate and light oil. I adapted it from a technique used by antique outboard restorers:

Prep the tank by removing the petcock and gauge sender, clean the paint work around the holes well, and cover the holes with duct tape. Stuff rags inside the hump to re-inforce the duct tape over the sender holes.

Pre-mix the saline electrolyte using most of a box of table salt to 5 gallons of water, and fill the tank. Tape up the end of a steel rod (I was told re-bar, but used all-thread ready rod which worked just fine) so it cannot short out against the tank bottom. Connect the negative lead of a 10 amp battery charger to bare metal on the tank (I used the gas cap mounting screw so as not to damage any paint). Place a wide-mouth plastic (non-conducting) funnel in the fill hole and put the rod in the tank through the funnel. Connect the positive lead to the electrode. It's best to use a charger with an ammeter. Mine just has a silly voltmeter gauge, so I connected the
positive lead through a separate ammeter. Add salt as required to bring the current up to 10 amps. (The best I got was 9.4 amps, more salt didn't help after that). Check every 2 minutes, and stop when the rust is gone. (It got so murky, I couldn't see the hump after a while unless I sloshed out some of the water and tipped the tank. I don't know how they do this with a marine tank and still check it ;-)

Total time for me on this tank was about 10 minutes.

I was amazed at the results. Where there was heavy rusting, clean shiny metal magically appeared. I used a piece of cad-plated ready-rod for the electrode, and it turned black. The saline turned a weird shade of green,
with chunks of rust floating around. I was concerned that the left half of the tank wouldn't get equal treatment, and I made a point of draining off some water and sloshing it around to evenly distribute the salt, and an examination through the fuel gauge hole confirms it worked on that side too.

None the less, I think next time I would start off with a saline solution instead of plain water, now that I know how much salt to use (just about 1 box to get 9.4 amps). I also blew a fuse in my ammeter when I inadvertently touched the rod against the hump in the middle of the tank trying to stir the salt with the electrode. I guess with a motorcycle tank, not only the bottom of the rod should be insulated with electrical tape, but part way up where it might contact the hump as well.

I flushed everything out thoroughly with water after, followed with methyl hydrate. Even so, I could see a faint patina of rust starting to form again within 15 minutes. I quickly coated the inside with oil, and that took care
of that.