TOPIC:

Hi,
Has anyone got any ideas why since a top end rebuild the engine over heats?
I've still got carb issues but if anything it's running a little on the rich side now.
The engine has only run for a few hours so things are going to be tight (new pistons)which worries me a bit when it over heats.
It doesn't help being summer as the temp is around 30 +/- here. Ive checked the obvious, coolant level and it's got hot enough to exchange with the expansion bottle. Also can see with the rad cap off movement of coolant.
A few times now in traffic I've had to pull over for 5 mins to let the engine cool down.
Am I missing something?? Should I fit an oil cooler, to me that would be masking the problem.
Cheers Pete.

is your stat working properly ? correct stat ?
just a thought, if its faulty it wont close of the bypass pipe which forces coolant through the rad.

You don't mention the rad fan in your post so I'll assume that it's not coming on? I would test this first.

Second- The thermostat is a double valve thermostat. Very common in a lot of the European cars. If one or the other valve plates is not sealing properly the coolant tends to circulate in both the engine coolant loop and the recirculation loop. It's really important that the coolant in the engine gets up to temp and the thermostat opens and the coolant in the engine exchanges with the coolant in the rad and hoses. After the thermostat closes, the coolant pump then has to circulate the coolant through the engine. If the thermostat is not fully closed, the coolant won't circulate properly through the engine.
One of the most common failures is using the wrong thermostat gasket (too thick) and it holds the thermostat valve plate off of the seat in the bypass port.

Last but not least, in these 35 year old bikes it's very possible that some of the coolant tubes in the rad are blocked.If the rad can't dispense with the excess heat then you'll get about 20 minutes riding before you're parked on the side of the road.

Hope this helps.

Thanks guys for the replies,
Forgot to mention the fan but it's working fine and cutting in when it should, but becomes a point when it can't cope anymore. The stat is only a few months old, it was replaced when I replaced the water pump ealier this year before I decided to do the engine rebuild.
I guess there is no guarantee it's working ok but should be. Is there a chance it's seating at an angle??
I'm convinced there is enough flow in the rad, it was ok pre-rebuild and flushed loads of water both ways with the hose for ages.
Cheers Pete

Make absolutely sure the cams are correctly timed, if the exhaust cam is laging you will get a big build up of heat but it wont hurt anything in the short term like bending valves but of course the long term damage can be bad news. incorrect Ignition timing can cause overheating, if it is wrong usually the engine runs like a dog and is hard to start.
Also when it overheats does it cool quickly when you are going down hill or does it take ages. Does it overheat when going along the flat at an easy throttle at a normal road speed, say 100kph (60mph).

Hi Rick,
I checked the valve timing a few times but what might be out slightly is the ignition timing, reason being when I put the cylinder and Head back on and slowly turned the crank over to make sure everything was ok I noticed the pickup rotor wasn't turning. What had happened was the small chain had managed to jump off the lower sprocket,with out me being able to see the chain must have been half on and half off. Due to the location of both top and bottom sprockets the only way I could see to get the chain back on was to drop the sump and with a bit of fiddling around got the chain back on. Now being a couple of teeth out to the top sprocket I had to extend the slots in the timing plate to compensate. Going by pics that I had taken pre-rebuild visually matched the pickups to the rotor. This however seemed to be a reason to the flat spot I had at 1.500 to 2000rpm. It's better now but not prefect, my friend has leant me a strope, not sure if one can be used here?
Anyway so there could be a reason still for overheating there but the flat spot is almost unnoticeable now and it ticks over ok.
When it does overheat it takes a long-ish time to cool down on a down hill. It's not too bad at night on a steady throttle and during the day with the summer heat it's definitely higher than pre-rebuild temp.
Cheers Pete