I'm thinking mud dobbers in the coolant passages.

Those F'n mud daubers get into EVERYTHING. I just pulled a set of 1/4 drive sockets out of my tool box. Every one of the deep sockets over 5/16" was completely plugged by those little bastids. The little worms were still alive when I poked them out of the sockets. I've had to use caps on every single air tool fitting because they plug those up. Open bolt holes in blocks and heads in the garage get plugged. It's a royal PITA. :evil:

 

I'd fill with straight water an drive it a few days then drain it. Maybe do that a couple of times.

 

A good flush is needed. I take out the thermostat and loop the heater hoses together to keep junk out of heater core. Pour in about a half cup of liquid laundry detergent. Top off with plain water and drive it about 5 -10 miles or even more if you want. Block off some of the front of the radiator with card board to get to normal temperature if needed. Remove both radiator hoses and remove block drain plugs and watch the junk come out. Allow engine to cool and flush block, radiator and heater core with garden hose to further flush junk and detergent out. Repeat if needed and then fill with 50/50 antifreeze and water.

 

Liquid laundry soap! That's a good idea! I've been trying to think of something. I've been thinking more along the lines of something that would help dissolve the bug bits.

 

I would recommend dishwasher soap (not dish soap) before laundry soap.
(Less sudsy)
However, I don't recommend any of them really.
You may need to remove the radiator to back flush it, if it's plugged.

 

I would just run the engine with the top rad hose disconnected and a hose in the rad to flush the engine, I think any sufactant may be overkill and be hard to flush out completely later.

The only additive I would use is a mild phosphoric acid like CLR which has sufactants mixed with it and add a teaspoon at a time as it ran with the top hose dumping to the ground. If you can, use hot water to keep the thermostat open or remove temporarily to allow unrestricted flow while you flush.

Once you get large bits out I would think of taking it in for a professional flush, cheap insurance if you ask me. Reverse flowing the rad and heater core is done on a flushing machine which will save you the aggravation of trying to do a good job with limited options in your driveway.

You may even decide to just let the garage do the entire flush, tell them the issue and let them handle it...they do it for a living and have equipmemt designed to do the job.

 

I've used laundry detergent many times. Everything in the cooling system ends up squeaky clean after a through flush/rinse. It doesn't get sudsy until you open the radiator cap and release all the system pressure.

 

If you want to flush the whole system you need to knock out at least 1 freeze plug on each side. You will never get the crap out of the bottom of the water jackets if you don't.

 

Thanks for all your input. Sorry I'm being so wordy.
I need to mention, this over heating started after first longer drive. Ran quite a bit before over heating started. Even sat idling while charging the vintage air, without getting hot.
Here's where I'm at. After removing radiator, water pump and t-stat flushing everything with water hose, decided that while I'm waiting on parts, I'd put everything(except t-stat) back together and run some laundry detergent/water in it. Left the rad cap off ran it mostly idling, fans kicked on at about 175, temp just kept climbing. Had a huge squirrel cage fan in front. Shut it off at about 210. Let it cool down, removed rad and back flushed, this time I got what looks like an odd cricket. So I figured I'd better remove some freeze plugs. Took out the rear one on each side. Both had a considerable amount of rust chips and crud at the bottom. Not a lot, but thought I'd mention it. Flushed out again, rust stuff cleaned up nice, didn't see any bug bits come out. New pump should be here tomorrow. I'll put it together again and pull the top hose and try the CLR idea.
My concern is that I didn't see nearly enough stuff come out to stop the overheating.
Any thoughts on anything else I should do before I put it together?

 

What kind of fans are you running? The age of the car would dictate mechanical but I had to ask to be sure. If mechanical are you sure you put the blade on the correct way? I've seen it many times on older cars where a non-thermostatically controlled mechanical blade was put on backwards and it was pushing air towards the radiator, instead of pulling air through the radiator.

 

I would not run the engine with the radiator hooked up! Whatever breaks loose in the form of bugs, or nests will plug the radiator! Put a hose directly into The radiator hose at the bottom. Since coolant flows out of the top of the engine, you'll want to wrap a rag around the hose, to keep the water flowing into the lower hose. Direct the upper hose to a point it wont be a problem. Also remove the thermostat, to allow free flow. You can do this without the engine running, but a little warmth may help break the junk loose, so you could fire it up and idle it also.
Once there's clear water flowing, and no sig of the bugs, you should be good to put it all back together again.

 

Is the radiator blocked with bug bits?

 

This would be the biggest problem I would think.
There would have to be huge amounts of stuff in the block to be a problem.
As I mentioned earlier, have you back flushed the rad.?

 

So here's an update. After flushing everything forward, backward, inside out, upside down, about 6 times, including removal of two rear freeze plugs. Installed milodon high flow pump. Changed to zerex coolant with water wetter. Still over heating. Removed the two 10 inch electric fans and the shroud that came with rad/fan kit. Installed the biggest mechanical fan I could fit. Over heating problem got better. So then I made a custom fibreglass fan shroud and now it stays around 180 when driving, still climes up over 210 at a stop. I'm hoping that after the engine gets broke in and it loosens up that the temps will continue to calm down.