Classic/Antique Car Repair: Overheating chevy 216, engine flush

Question
My dad and I have a 1946 Chevy 1/2 ton pick up truck. It is all original except that it has a Carter YF carb on the original 216 engine.

A friend and old chevy mechanic overhauled the engine in-the-truck about 10 years ago.  The head was redone at NAPA. They checked it for cracks. The rest of the overhaul consisted of rings, cylinder honing, and bearing service - the journals all looked good.  The engine starts fine (except when hot) and runs fine and seems to have plenty of power.

The problem is that over the years it seems to run hotter and hotter.  We boiled out the radiator, checked the water pump, checked thermostat, belt tension, and hoses. I did a block check kit that sniffs for exhaust gas in the cooling system. Test was negative.  

Last summer it started to overheat on hot days after idling more than about 5 minutes - pushing coolant out of the overflow tube.

I installed the optional larger fan that was used on 1-1/2 ton trucks and this seems to have solved the overheating.  I suspect ths is a bandaid fix to a deeper problem which I belive is sediment and crud filling up the water jackets - since the probelm has evolved slowly.  any thoughts?  If you agree is there a way to clean the water passages effectively without dismantling the engine?

I think the hard hot starting is a seperate or lightly related issue to the hot running.  When hot, it cranks fine but won't fire.  Sometimes it'll start and run terrible for a minute and then be fine until the next hot start attempt. I haven't attempted any diagnosis for this issue.  Any thoughts?

Thanks, Jim

Answer
Hello Jim,

I agree with your diagnosis based upon what you describe. I would go with a good back flush product like Zerex, temporarily removing the thermostat for best flow. Upon reassembling, I'd
install a new thermostat as your problem could be caused by a failing thermostat, partially blocking flow. Do not run the truck without a thermostat as it will introduce a new set of problems.

If this does not work and you still suspect crud, consider the following practical fix I found on the F150 website. I've NEVER tried it but it sounds intriguing. Please let me know your results if you decide to try it:

"For what it's worth.. A few years back when I was a parts guy at a Freightliner dealership, the tech's there used Cascade automatic dishwasher soap. Those Detroits, Cats & Cummins held a lot more fluid that our trucks do but the techs would drain the coolant, refill with one whole container of Cascade and the rest was water. Run the engine until hot then drain and flush/rinse with plain water. I could not believe how clean Cascade got things. I used it on my F150 once since then, inside of the radiator came as clean as it was when new."

As far as your hot start, excessive heat can soak into fuel system components and cause all kinds of problems. From what you describe, it sounds like vapor lock either on the incoming line or in the carb. float bowl. As a test, I'd wrap some aluminum foil around/underneath both these components and see if it helps. If it does, you will need to fix the problem as above or band- aid with insulation, fan shroud or auxiliary cooling fan for the radiator. These steps just take further the known effective step of the larger fan. I'm not saying to not fix the problem, just that there are more band-aids (bandages?) out there if you don't care about the originality of the truck.

Good luck, I think you're on the right track.

Bob