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Very telling pictures !  Tank-rust - Bummer !    Obvious: So fine it can pass through a typical filter.  Enough of this will eventually compact in places that will eventually become blocked.  This will have to be addressed as it will not cure itself. Two of the three sets I rebuilt last winter had this red-oxide on the bottom (filtered?-air) side of the diaphragms.  I can only conclude the fasteners inside the air-box are badly rusted and the air-flow is distributing it.  A relatively small amount and only the finest of particles. The large vent between the carb throat intakes is large so this area is subject to suspended particles settling on and under the diaphragms.  I have also removed fine granular sand-like debris from this cavity under diaphragms.  Indicates a poorly sealed air box/filter when you have "dirt" collecting here !  This can damage/scratch the slides ! 
 

After getting the carbs back on the bike at 7:30am this morning and going for a ride to the local bike shop (about 60 mile round trip) I can report that the bike is much.......................worse. At this point it not really worth trying to figure out what I've done wrong whilst the reddish/orange stuff is floating around the fuel system. It took me a little while to figure out where this has come from but I can tell you it hasn't come from the tank or air box. Sorry, it has come from the tank but indirectly. Let me explain.

The fuel tank, before installation, had slight surface rust but this was quickly dissolved by filling the tank with Citric acid and left to soak for a few days before repeated amounts of flushing and then treated with petrol and 2 stroke to stop the tank from rusting until I filled the tank with fresh petrol. The fasteners inside the air-box are spotless with no signs of rust. I have already drained the tank of fuel and filtered it into a petrol can. The filter has turned orange. I'll flush the tank completely later on this week. I've totally stripped the carbs again and although not as bad as in the pictures there is signs of reddish/orange residue inside the carbs. These will be cleaned again and reassembled later on this week.

There is no possibility of this rust coming from anywhere else. So, where has it come from? The original tank was the peanut shaped tank which I've changed for the more conventional tank. The original tank was solid with petrol varnish and rust. I've fitted a new petrol tap but used the original fuel sender. This is were I've made a serious error. I cleaned the sender externally and wrapped it in a plastic bag ready to fit to the new petrol tank at a later date. The sender unit is basically a small box and must have been full of fine rust and petrol varnish. Fresh petrol has gradually broken down the varnish inside the fuel sender and released it into the fuel system.

I must admit that carburettors are not my strong point when it comes to motorcycle maintenance and I've had to carry out a lot of research lately (from this forum and other resources) to try to understand the science of how these things work.

More updates on progress to follow later on this the week.

Regards

Martin.

Firstly - You used a Paint-Filter ! Kudos ! Your additional info was enlightening ! Two things you now mention make sense. Rusting fuel sender and your "treatment" of the tank. Cleaning the tank is only part of the remedy. Once the original "finish" of the tank interior is compromised it will continue to rust if left unsealed and the condition will only get worse and if left, will eventually rust-through ! The tank MUST be "sealed" with POR or similar. Relying on an oily 2-stroke mix is useless as you've discovered and regardless of oil-content, it will simply wash off with the first "non oiled" fill. Until the tank and sender are treated you're situation will continue, I'm sorry to say.

Dear all,

Latest update regarding the carbs. Carbs were fully stripped earlier on in the week and I've just spent about four hours cleaning them using Scotch's cleaning tool, Lacquer thinner and compressed air and then repeating the process. reassembly starts tomorrow.

Dear all,

Pleased to say the carbs have been reassembled and put back on the bike. Used a temporary fuel tank to get the bike started and set up the carbs, set the fast idle, tick over and then synchronised the carbs. The fast idle adjustment was set on the bench as well as the initial syncing of the carbs.

This is the second time I've synced the carbs on the bench and cant believe how accurate it is. (Thanks Scotch) I used a small strip of paper about 5mm wide and inserted it under the butterfly. I then adjusted the synchronisation screw until it only took light pressure to withdraw the paper strip. if you take your time and obtain the same pressure on all three carbs its amazing how close you can get them synchronised. When I used my Carbtune to balance the carbs I was only three Hg out on the centre carb.

The next job was to see what was floating around the petrol tank. The tank only had very light rust (in isolated areas) and was treated with citric acid to remove the rust. the internal areas of the tank was inspected with a boroscope camera to ensure it was clean. The tank was then treated with a petrol and 2 stroke mix to stop it rusting any further until I filled it with petrol.

Now, I'm going to respectively ignore Scotch's advice here to treat the tank with a sealer as I believe the problem is not the tank. I will, of course, kiss his butt and beg for forgiveness if the problem returns.

Further tests will commence tomorrow so I'll update then.

Regards

Martin

Scotch is waiting