Bob,
Overheating at higher speeds is usually a capacity or tune issue so
adding a fan won't help. Either the radiator can no longer reject
enough heat, the water pump can no longer pump enough coolant, or the
engine has started to make more heat. I would first check the engine to
make sure it isn't putting out too much heat.
The first place I would check is the spark plugs. If it looks like the
car is running lean, fatten up the mixture a bit and see if that helps.
Evaporating the fuel takes a lot of heat so a rich engine will run
cooler.
Next I would look at the distributor. Assuming you have a vacuum
advance, make sure it is working. Make sure the mechanical advance
under the points plate works as well. After that, start advancing the
timing until you hear pinging or you get kickback against the starter.
With late ignition, the engine puts out less power. Since you are
burning the same amount or more of fuel, the extra energy ends up as
heat.
If neither of these fix the problem, pull the valve cover and check the
valve lift and timing. If the cam is worn or incorrectly timed, the
engine will make more heat. Cam lift is easy to measure with a dial
indicator and a magnetic base- both available from Harbor freight for
less then 25 bucks combined. Using these and a child's protractor, cam
timing can be checked close enough to see if you have a big problem. A
more quick and dirty check would be to try and judge when the intake and
exhaust valves are both half open on overlap- this should occur at TDC
if the cam is installed "straight up."
If you have sufficient oil pressure, try running a slightly thinner
synthetic oil. I run Mobil 1 15W50 in my MGA. This will reduce
internal friction and help the engine run cooler.
Last but not least, exhaust problems can cause overheating. An open
exhaust leans the mixture while an obstructed exhaust increases heat
transfer from the spent gasses to the head.
After all of this, then check the cooling system. Compared to the
engine, the cooling system is bone-head simple and, as such, offers
little opportunity to "tune." As often as not, cooling system repair
involves taking old parts off and bolting new parts on.
One more comment. If the engine is newly rebuilt, it may generate more
heat for a while until it breaks in.
Good luck,
Bill Eastman
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