I have a '68 XR-7. It’s a 302, No A/C and an Auto. It idles fine and will run gently for hours. However, When driven harder it overheats. I’ve started with the basics, New thermostat and flushed the system. But I can’t get a lot of coolant in it. I’ve read that the coolant capacity is 12 litres and I can’t get more than 8 in. I don’t seem to have any airlocks in the system
Any ideas?
Do you have a fan and shroud properly installed?
Many of us drill a couple of 1/8" holes in the thermostat to help with filling. Check coolant after running a while (wait for the system to cool down) and top up if necessary.
A bottle of Redline Water Wetter added to the coolant can help with cooling issues
You can also upgrade the radiator to the larger one used on air-conditioning-equipped cars or a big block application.
On my Cougar I run an external transmission cooler that is not plumbed into the radiator at all.
I have a stock 4 blade solid fan, fan shroud properly installed, 1/8 hole drilled in the Stat and I’m running distilled water with water wetter in already.
I’ve had the car nearly 7 years and the problem has only started in the last few weeks. 
How hot does it get?
When you say “driven harder”, are you talking about sustained, high speed operation?
I can leave it idling for hours, cruise around at 45-50mph and not use more than 1/3 throttle and it’s fine. Anymore than that and the gauge starts climbing. How hot does it get? Dunno yet, I have and infra red heat gun on the way…
Based on your excellent description, I suspect that you are not getting all the advance that you should be. I’d check that, give it a full tune up, adjust the carb, and see if the overheating condition still exists.
PS - the other 4 liters of coolant are probably in the block.
Coolant still in the block? Drained it by taking the bottom hose off. So should have drained the whole system right?
No, a good bit of coolant is still in there.
^^^ boy, you find that out when you remove the engine!
LOL Phillip, I was thinking the same.
Thanks for your advice so far.
I’ve given the car a run and got the gauge way up towards the red. Using an infra red temp gun I’m getting readings of 215 from the inlet manifold where it meets the stat housing and 206 at the top of the rad. Which from another thread on here seems to be normal.
I’m starting to wonder if the gauge is reading high…
Were these temperatures stable (as read by the gauge)? I ask because these values seem to be a bit higher than normal.
Hi Jack. Sounds like we have the same car/options. About 98k miles on mine…almost everything original (only the battery, belts and hoses look less than 50yrs old. Bought a month ago and just starting to get a feel for some of the areas that need attention. Cooling system was my first issue - overheated only after 100+ miles of 70mph highway driving (no issues idling but gets very warm). Flushed and refilled the radiator this week and took my first look at the thermostat…it’s rated at 195 degrees and works fine but thinking about changing to a 180. I could only drain and refill about the same amount of coolant you reported. Drove around this weekend…still runs quite warm but no overheating.