Overheating problem again

[emagt980two]

Hi!
I'm having an overheating problem on my bike.

I bought a 454 ltd that had broken piston rings.
Once I changed piston rings and gaskets and all the stuff. I noticed that my bike had not thermostat on it. So I put an aftermarket t-stat that opens at 81 C degrees. The temp gauge takes around 20 minutes to reach 3/4 of the level (riding on the avenues and streets). Once it reach that temp the fan comes on and the temp comes down just a little bit. If I continue ridding , the fan started kicking on on and on for shorts periods of time and the temp does not comes down. Yesterday the coolant started to boils and I inmediately shut the bike off. The temp gauge was before the last white line. Today I put a new radiador cap on it and I flush the cooling system with a special fluid that I bought on AutoZone. The system and the rad seems to be really dirty.
However after flush the system twice the bike do the same thing.
Regards from Mexico dudes!

[ArturHawk]

It sounds like the coolant system is working...but not well.

Did it overheat before you put the thermostat in?

If not, I would start by checking the radiator for obstructions.

[bikeman]

hi.  bit late here not been on the forum for a while. you could have one of three issues. the rad is blocked (coolant flow) try back flushing it take it off turn upside down force water through it  see what comes out and if it free flows. 
look through the rad core (outside) to see if it's clear (air flow) blow it out with the same hose. 
lastly you had the head off so did you grind both the head and barrels flat (lapping) while they were off procedure here www.ex-500.com/threads/lapping-the-head-to-repair-a-head-gasket.67003/ it is important on these engines. and should be done automatically if the heads are removed. you could have a warped head and failed gasket. this pressurises the cooling system blowing the coolant out of the overflow chamber. causing the overheating. do the simple things first.