Overheating

Hell's teeth! Ever since having the Athena std-bore barrel fitted, my bike has started overheating.

1. Fan works -replaced with a new one and checked function in diagnostics.
2. Seems inconsistent - overheats at slow speed or idle, but does it inconsistently. Does not overheat at speed.
3. Nothing, other than the new cylinder and a CR8EIX plug, is different from before. And before, the bike had never overheated, not in the worst I could throw at it. Except when it dumped the coolant due to a radiator hose attacking the scenery.
4. Is it really overheating anyway? No coolant smell, no bubbling out of the catch tank.
5. #8 plug shouldn't affect anything. Lots of slow running. And anyway, it doesn't show the overheat light once I get up some road speed.
6. Radiator is not blocked - fins are blown clear of dust and seeds often.
7. Coolant is the same as always - Yamalube 50/50 premix.

Ideas, anyone? Temp sender? Faulty thermostat? Thermostat is relatively new, as the original started drooling coolant, but it was removed during engine disassembly and sat around for a couple of weeks while the head was cleaned up. Same with the sender.

Have you put it on a dyno and/or verified the EFI fuel map for that setup? From what you wrote it appears that it might be running lean in low to mid RPM.

Thanks for your reply wwguy. But there is no difference in the setup. Standard size cylinder, standard exhaust, standard inlet apart from flapper valve mod, standard everything.
The thing I have really noticed since the repair work is that the engine no longer detonates when rolling on in top gear from, say, 60 km/r GPS. 13/48 gearing. It always used to, but NEVER overheated regardless.

Only thing I can think of is maybe the wrong head gasket (or installed upside down) blocking a coolant port?  Or possibly the thermostat is sticking closed on occasion....

I'd start by going back to the stock CR9EK plug.  The CR9EK is a significantly different design than the CR8EIX and is also a heat range cooler. (With NGK plugs the higher the heat range number, the colder the plug.)

NGK's website says:

"In identical spark plug types, the difference from one full heat range to the next is the ability to remove 70°C to 100°C from the combustion chamber."

"When making spark plug heat range changes, it is better to err on the side of too cold a plug. Running too cold a plug can only cause it to foul out, whereas running too hot a plug can cause severe engine damage."

"Caution should be made in selecting a "high performance" plug.  If your car came OE with a multi-ground plug (like the CR9EK) your engine will likely wear through single electrode plugs, especially fine wire plugs (like the CR8EIX) at a rapid rate."

Thank you for your replies. Much appreciated.

Thermostat is a very likely problem, as is the head gasket thing.

Re plugs - I used a CR9EIX for >20,000 km with no ill effect on anything that I could tell. My OEM plug was wasted after about 8,000 km. I went to the #8 because of all the slow running I do, and I'm careful not to flog the engine in a sustained way. I'm not sure that it would make much difference to go back up in the heat range - the extra heat dissipated by the #9 has to go somewhere ie straight into the coolant. But I take your point. There is much I don't know about plug thermodynamics, and you may well be right. My problem is that #9 stock plugs tend to suffer from carbon buildup because of the way our bikes are used a lot of the time. It's a cheap and easy experiment to reinstall a 9, with nothing to lose really, and everything to gain.