1.3 on Bike Carbs bubbling up bt not overheating on gauge!

Hello all,

So I've finally got a MK1, by far my favourite VW of the various ones I've had even though I've had quite a few problems so far, had to change alternator due to battery draining within an hour of switching it off, I've recently flushed the coolant, due to being brown (I only used garden hose, no flush) and changed the thermostat to try and resolve the bubbling up problem, I thought I'd solved it until today.

The flush and thermostat were changed on Friday, on Saturday I did an 80 miles round trip with no problems, but today after driving around 15-20 miles today (Halifax to Leeds, 80% motorway) when stopping it was bubbling up and spitting water out of the expansion tank overflow. The water has brown sediment in it again, it's not oily, it doesn't smell and it's not slimey, just brown. A few things I've noticed is that the heater seems to get very hot very quickly, although when slid over to cool it does also blow cool air which is a good sign, also the temperature seems to fluctuate fairly quickly, it rarely gets to half, but when parking for example it can quickly get to half way then the fan comes on and takes it back down. So the temp gague never goes over half but it's still bubbling up!

Any help would be greatly appreciated… Thanks in Advance  :?

Anybody any ideas? thought someone on here might be able to help..I suppose next stop....edition38 forums...