I believe that ethanol in fuel is pure evil.  It causes various plastics to swell, various rubber to distort, and attaches to water molecules which then restrict fuel flow through tiny holes such as idle jets. 

 

Fortunately there are some fuel stations that sell ethanol free here in the states.  That is the only fuel I use in all my non-highway, gas (petrol) powered equipment now. 

 

I have found that if you drain the fuel and let the tank air out (cap removed), they sometimes will shrink back to normal size.  Problem is, this can take a couple weeks.  You could try draining the tank after each ride and leave the cap off so it can air out, then refilling fuel just before you are going to ride next.

Gasgas TXT Racing 2017

My fuel tank has stretch and lifted off the frame from the middle upwards and is a pain to refit, as a lot of these models with the dip in the tank seem to do, and wondered if the earlier models with the same frame and more rounded/convex tank would fit in place. Not sure if the earlier ones stretch as much or have the same problem?

Edited December 28, 20232 yr by Tootsie

1 hour ago, Tootsie said:

Gasgas TXT Racing 2017

My fuel tank has stretch and lifted off the frame from the middle upwards and is a pain to refit, as a lot of these models with the dip in the tank seem to do, and wondered if the earlier models with the same frame and more rounded/convex tank would fit in place. Not sure if the earlier ones stretch as much or have the same problem?

@Tootsie  If you are planning on keeping your bike for a long time you could have a fabricator make you an aluminium tank using an old second hand one from an early pro as the base then cut the top off your ruined plastic tank  to use as a cover over the aluminium tank for aesthetic purpose. 

Edited December 28, 20232 yr by Tr1AL

Mine started to increase in length and then I realised, after some research, that the fuel I was using had ethanol in it, although it wasn't advertised. I started to used ethanol free fuel and surprisingly it went back to its original dimensions.

Ethanol is the Devil's drool.

It has swollen the tanks on 2 of my bikes, rotted the fuel hoses and caused ignition problems on my classic car and means that chainsaws, mowers etc need to be drained if they are going to be left unused for any length of time.

I use Super (5% ethanol or less) whenever possible, but it still needs thinking about to minimise problems.

I run either race gas or non-ethonal in all my bikes.  No swelling issues, they run better and it is more tolerant of sitting which happens sometimes during ski season.... If you are going to run ethenol then I guess Beta's alumni frame tank would be a real advantage.