So I took my MC21 to the shop for an overhaul after it had been sitting for several months. It was running alright but idle wasn't great and needed some fixing in other ways. I knew a bit of rust had accumulated in the tank. He sent me these pics and said it will cost a lot to go through and fix all rust damage. My question is this: how bad can it be? I trust him well enough, but if the worst case scenario is something I can fix myself I'd rather save the cash for fun bits.

Cheers

Has he said what he is going to do and for how much? Cleaning the carbs is something pretty much anyone can do at home with some basic tools. If there is rust inside the tank, then you would want to drain the tank completely and then go through and kreem the inside of the tank (or what ever similar product you have in Japan).

Look up "Kreem tank liner" to get an idea what your looking for and how to do it.

If you have rust damage that is all the way through the tank, then it would require some extensive repairs that may be beyond the standard shade tree mechanic.

I can clean the carbs and derust the tank. But is there anything worse possible?

depends on many things.

How much water was in the tank, how long it sat, how much rust is in the tank, etc..

My guess is that since it's only been sitting for a couple months, it's not that bad. I would drain the tank, pull the petcock/filter and see what is in the tank.

Also, it looks like you have the reed valve/blocks off so you can see down into the cases and get an idea if there has been any water sitting in the bottom of the cases, see if there is any damage to the bearings.

But I really doubt its going to be that bad if it's only been water separating from the fuel (not going to have that much water).

Be really careful if you decide to line the inside if your tank. If you have ever run pre mix then you have oil in there which will make adhesion a challenge. Most of the companies that offer tank lining products will not guarantee their product if the tank used premix.

The second issue with these products is that most of them will not tolerate ethanol based fuels. Even E85 will break them down. It is common in the aircraft industry to use sloshing compound. Most of these products will stand up to Avgas, but will break down over time from ethanol. There is one company that makes a product that will not break down when exposed to pure ethanol. I haven't used this product so can't vouch for it. But when i spoke to them they claimed to have a steel paint can in their shop lined with their product and filled with ethanol for over a year with no breakdown of the material. PM me if you want the link to their website.

The two part epoxies will soften over time and will break down from ethanol.

Over 15 years ago i coated my BMW R90/S tank with sloshing compound and haven't had a problem. Two years ago my dad coated his BMW R29 tank with a 2 part epoxy and it is now starting to soften and lift. I recently had some crud in my NSR carbs and spent time researching products. In the end i put a filter on my bike as i do not know if anyone ever ran premix in it. The filter works fine and is cheap to replace.

Remember this, once you go down the path there is no turning back. If the prep work is not done right or you used the wrong product, you will ruin your tank.

Just to be clear, I would never use kreem in my tank... But it's an option that people have been using for a long time with (mixed) results (of course I think as eric mentioned, prep and application has a lot to do with results).

From the looks of the pic, I would do an inspection of the tank and see what the "damage" is first. I am guessing it's not that bad.

Not sure what the state of the fuel is in Japan, but in the US some one came up with the great idea of ethanol fuel (currently at 10%). It's absolute sh*t and has destroyed more then it's done good. I never put fuel in any of my bikes (or lawn equipment, and I have 5 acres of lawn to maintain, so I have a good sized fleet of lawn care equipment) without some form of ethanol stabilizer (Sta-bil) to help reduce the damage of crappy fuel.

I also drain my motorcycles carbs and tanks when not using them, but some people aren't willing to go through that trouble. Im just tired of rebuilding/replacing carbs so it's worth it to me.

Oh and if you have run pre-mix in your tank, rust shouldn't be that much of an issue, you should have a good coating of oil to help protect the inside of the tank.

I have a 1989 DT200R sitting under the house that I used to race back when it was new. After one particularly bad stack, the bike sat waiting for repairs for a while and the tank developed really bad rust. I ended up sticking a few handfuls of bolts and nuts in the tank, wrapped the tank in bubble wrap, wedged it in a cement mixer then ran the mixer for several hours. That got rid of the worst of the rust. I then used a Kreem tank liner kit. The first part is an etcher which dissolved all the rust and etched the metal. The second part was the liner. After pouring it in you have to continually rotate the tank around to get an even coating and drain off the excess before it gets to viscous. On tanks like the NSR that have a lip you can only really drain the excess through the hole where the fuel petcock attaches. The liner lifts paint better than any paint stripper I've ever used as well. I can't vouch for ethanol based fuels, but for fuels in Australia the liner seems to work fine. Mine is going on 24 years now and is still as good as the day I applied it.

Thanks for all the replies and valuable info. I saw the bike today and the rust was pretty bad inside the tank! Carbs were pretty creamed with rust as well, he will do the cleaning for cheap since he already has them on the bench. As for the tank, it really needs some tender loving care to get it right. He says that he will take it to an expert shop that will completely de-rust and coat it but its expensive, about $250USD. I told him to hold off on it as I am considering doing it myself.

If it were you, would you de-rust and coat it yourself or take it to the pros? Mind you I haven't done it before. Not sure on this one...

Each of us have different backgrounds so only you can decide if you can take it on. If you can fill a can with gravel and shake it up good, then you will have done the hard part.

Read the instructions on the product. Then read them again. And if you think you can shortcut the instructions, take the product back where you bought it return it.

Remember, you only get one chance with your tank. Good luck, its not very hard.

But on the other hand, you live in Japan so getting a tank isn't as hard as it would be getting one here in the states

As someone mentioned, the lining compound is a great paint remover, so be really careful not to get any on the outside of the tank or all the hard work up till then will only require additional repairs to fix the paint.

Getting a replacement tank is much cheaper than that! Well, here in Japan it is. Be careful, as polite as the Japanese generally are, most of them will rip off an unknowing 'gaijin' in a heartbeat.