When i start bike from cold, after about a minute or so the downpipe/header gets very hot then the fan kicks in. i have only just got the bike. i am not sure if this right or not.

Low coolant, if there is an air pocket in the engine it will make it to the heater core. Once there , the air coming out is cold/cool. Once the coolant fills the heater core, it gets hot. If the coolant is fine hot or cold, then have the HVAC system scanned.

There are other things to test before we have the engine spread all over the yard. Cooling fan operation. Yes the thermostat hot air in heater just means hot water. start with a cold engine then start and run at a high idle if heats quickly upper radiator hose hot lower cold good but reverse no flow. remove and test thermostat (not replace) put on stove boil water and watch it open once center piece is moving check with thermometer it should agree with stamp number on thermostat. if good check for slipping fan belt. check for air restrictions on radiator and water flow.

The fan is only supposed to come on when there is excess heat .. in normal driving conditions it doesn't start. When you start your car in the morning the fan won't come on because the engine is cold.

With a hot engine - and it depends how hot the engine is - the fan will only kick in to prevent overheating. In heavy slow moving traffic or a traffic jam, the fan will kick in if needed.

The air flow through the radiator is usually sufficient -in normal driving conditions - to cool the engine.

When you park up and the car has a hot engine, the fan will kick in. This is perfectly normal. It is caused by a surge of hot coolant triggering the fan sensor. Perfectly normal.

The fan then should cut out after a while .. maybe 2-3 minutes at the most. If the fan runs continuously that suggests a fault in the temperature sensor switch.

Next time - wait by the car and see how long it takes for the fan to quit after you have parked up. It should stop of its own accord after a couple of minutes.

If the fan cuts off after a short while, and your battery is becoming drained, that's probably a battery or alternator fault.

Remember: when you start the car the fan won't kick in at idle speed because the engine is COLD, and there is no reason for the fan to start up ..
The fan WILL kick in for a couple of minutes after you have parked the vehicle and the engine is hot

Run the engine and look at the header tank , is there lots of air bubbles coming back out of the water if is then the head gasket may have split and that is down to either a can of Radweld or a strip down repair

If the radiator is not uniformly hot them I would go for that as your problem

Normal operation is that the engine starts the thermostat is closed , engine block heats up and opens water starts to circulate around the block

So to check that you need to fill the radiator with cold water run it for a few minutes check the bottom hose it should be cold

The stat opens and the bottom hose should get hot as the water from the block is pumped into the radiator for cooling.

The radiator should get hot all over and the cooling fan should kick in when the sensor hits the over limit stop.

If that does not happen then get someone to remove the stat altogether (It is only there to get the engine block warm quickly in cold regions )

In which case the bottom hose should go from cold to warm to hot

Dont twist the throttle at all when starting the warm/hot engine. The carb has a diaphragm fuel pump that gives a shot of gas each time you crack the throttle open. It is very easy to flood the bike.

The heater will not work.Do you pull the choke cable?Is your carb assemblies cleaned and adjusted properly?I assume that your fuel pump works fine.

Check the coolant circuit that does not have air at all kept inside.
With the bike cold and the radiator cap open on the bike's kick stand start pressing again and again the coolant pipes and fill in coolant if you have to.Then start the bike without closing the radiator cap and keep pressing-releasing (like a sponge) the pipes until the radiator's lower pipe start getting hot.Then close the cap and wait with the engine running until you hear the radiator fan turn on.If you reach 227 F (108 C)and it hasn't start turn it off and check the temp switch on the radiator or the fan.
If the radiator's lower pipe never get's hot but the temp gauge shows you the temp reaching above 194 F (90 C) then you have a thermostat problem and you should check your cylinder head gasket too for compression failure.

So what you need to address is the overheating issue.

First check radiator fluid while engine is up to operating temperature, add if neccessary.

Then check to make sure your fan is kicking on when your bike starts to get hot. If your fan doesn't kick on that is your problem. If your fan does kick on and your fluid is full move on to next step.

Remove thermostat from bike and test to see if its working properly. You can put the thermostat in a pot of water on the stove. Use a candy/meat thermometer to see at what temp the thermostat opens up. It should open at around 180 degrees.
If the thermostat doesn't open up before the water starts boiling that is your problem. Or you can just change it because your bike has sat for 1.5 years and now the thermostat probably is sticking causing no radiator fluid to enter your engine to cool it down.

Hope this helps,
AadoubleA

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1. The carb has 2 choke knobs,red and black one,black for cold start and red for hot start,I found on my 426 that by pulling the black one every time I try to start the bike,the bike starts hot or cold.When it started press the choke knob back.
2. A dirty air filter will also make the bike difficult to start.
3. Do not open the throttle even a bit whilst kicking it,the carb tends to flood very easily and then you are going to kick the bike for ever and ever.
4. The exhaust turning red hot I'd suggest you get the carburator's jetting checked out,it can be because your feul/air mixture is not right and this could also be a factor in your starting problem,but another reason can also be when the bikes stands still whilst idling or even being revd whilst not in motion the engine tends to get very hot very quickly,the bike does not have a electric fan blowing threw the radiator,and if you did not know your exaust system removes between 60-70% of your engines heat,so it will turn red hot if your are not riding so the cold wind can blow over the exhaust and threw the radiator.
5. You say you get good spark,where did you test it?Did you check the spark plug itself,or only the sparkplug lead,I find the 426 to be very hard on spark plugs,when I find my 426 difficult to start the first thing I change is the spark plug and it solved my problem many times.
6. You also mentioned you have feul "to" carb but do you have feul - carb to engine.
7. How does the compresion feel when kicking the kickstarter,any lack in compresion could indicate a sticky valve in the open position,the unexperianced rider will not kickstart this bike due to the amense compression these bikes have (Ratio 12.1).

Just a few suggestions I hope one works for you.