My son rebuilt his 350 and now is having problems with overheating, I'll run down what I know is new and what is the same as before the rebuild when it didn't overheat and what we've tried.

Radiator, fan, clutch, shroud the same as before.

Rebuilt the 350 mild, slightly warmer cam, roller rockers, 9 or 9.5:1 CR, .030 over. Added a rebuilt Q jet that he rebuilt, cast iron GM manifold.

New water pump, new t-stat.

It was overheating with the first Q jet but it was a poor core and we couldn't get it to tune so we focused on that first. We borrowed a Qjet off a good running car to get the car running right, again would overheat but we discovered the first carb was no good. Bought a rebuilt Qjet from the parts store, runs good out of the box with minor adjustment. But still will overheat. I mention the issues we had with the carb only to rule out any one carb problem, we had three on it.
Timing is 9* initial, 18* total.

A temp guage is going to go on it so we can monitor the temp closer and see what is going on. And a new rad cap.

Can it be an air bubble in the block, how likely is that? Any other ideas as to what it could be? Can dwell affect the running temp? We didn't recheck that.

Thanks!

Timing can affect the temp but I'm not sure about dwell. Air pockets are possible, but if you've had it running chances are any air pockets would have been purged once things started moving.

How sure are you that the thermostat is working properly? They are cheap and easy to replace, I'd start there. Buy a new one and before installing it test that it opens when it should based on it's rated opening temp. Then install it and see if you still have an overheating problem. And make sure your radiator cap is the correct pressure for your application and that it is in good shape. Did you replace the water pump when you rebuilt it?

Another obviously question: if you don't have a temp gauge how do you know it is overheating? Is it barfing coolant? If so, did you leave the proper expansion room at the top of the radiator? Even an engine that is working at the right temp will barf coolant if the radiator is filled to the brim.

Also, how do you define overheating? A freshly rebuilt engine which is in the process of breaking in the rings, especially one that has been bored, will tend to run somewhat more hot than it did before the rebuild.

Keep in touch a let us know what you find.

What temps? Puking out the overflow? Most important, when does it overheat? Idling in the driveway or at a light? Cruzin down the road?

You have timing problems if you are only getting 18 degs total when the initial is 9 degs. If your dist is functioning properly but just doesn't have much mechanical advance built into it (9 degs per your post) Id say adjust the total up to 32-36 degs if it will still start as the initial will be in the 23-27 range. If it won't start then set the initial as high as you can and the car still starts...

Hi Rich
The pump was a rebuilt one installed during the rebuild. The t-stat was new name brand one, new at teh time of the rebuild.
By overheating; one time after driving then idling sitting still a few minutes we had major spewage. Checked the timing and pulled it back (it was more advanced and I thought that may have irritated the problem).
This last time (I wasn't there) I think it just burped, which could be the normal expansion but he said it "overheated".
DjD,
I'll look into the timing further. He was wanting to get rid of the points, I'm trying to encourage him in that direction now.

I do think that with out a temp guage I can't really see the whole story, or even if it's a real problem on normal break-in and expansion. Thanks for the tips. I'll keep you posted.

I would put a temp.gauge on so you could see at what temp.the the car spews.By what you have posted total timing is not near enough I would shoot for a least 32*total,then check to see what your initials running at, with stock dist.should put your initial around 12*.Make sure RPMS are low enough that they dont bring in mechanical timing when checking initial, mild engine 800 or lower.

With the engine at operating temp and the thermostat open (or removed and run cold), check to see if there is proper flow through the radiatior. If no circulation is seen, a reverse rotation water pump may have been installed.

When you pull the thermostate out again drill a 1/8 in the flat area. This will let the trapped air out of the system.

This is where we are on this; new rebuild with less than 100 miles on it.
I checked the timing, I was incorrect earlier. 9 initial, 30 total so I didn't change anything there.
16 pound cap and a Robert Shaw T-stat put in yesterday, new at the time of rebuild; rebuilt water pump.
Installed temp guage and went for a ride last night (plenty of air flow, warm night but not "hot" out). 230* running on the hiway, 240 in town. Burps when we parked it twice (w/o a catch can) temp went to 265 parked.
On a new rebuild, could this be normal break-in temps? Now that he has a temp guage in it we can see what temps are and that will really help. This is his first major project and he's a little frusterated. I'd like to keep his interest and help him work through the issues. Last night he said he wished he hadn't done the rebuild. graemlins/sad.gif I'm hoping we're just being over cautious.

I checked the pump rotation my looking in the radiator with the cap removed (it was half full from the last burp). I could see the tank to the lower hose pull low when the engine revs, that would appear to be the inlet to the pump (lower hose).
The radiator condition isn't what I would want to see, some build-up. He's not wanting to get the rad worked on unless he "knows" that's the problem. Personally, I'd do it but it's his car. So, are there any pictures that illustrate what a radiator should NOT look like inside? It'd be better coming from someone else.

I'd have the radiator completely cleaned and acid flushed. This will get rid of the build up. Next I would check the flow around the engine. When he did the rebuild, did he clean all ports and channels. Something could be stuck in there and not letting coolant flow as it should. Also, try taking the thermostat out. I know you said it's brand new, but not necessary to run it. Lastly, I'd recheck the pump. You may need to go to a higher volume pump.

I think I found a good part of the problem and wanted to pass it along to every one that helped.
The local radiator shop pointed out that many times the radiator is fine before it's taken out and not after it's put back in after and extended time. They said the build up inside is soft until it's allowed to dry. Then, it becomes hard and will restrict flow.
He recommends to have the raditor cleaned while the engine is being rebuilt.

For what it's worth, I had a '69 Nova about 15 years ago, had basically the same engine when I bought it. Always ran a little too hot for my comfort. One day, I swapped the intake for a better breathing one, and with no other changes, the engine cooled right down, ran 180 all day long. My '79 K10 runs about the same engine today, and I actually installed a thermostat to help warm it up, since it runs about 170, 180 in bumper to bumper summer traffic. Just a thought, if the radiator flush doesn't cure it.

Doug,
do you have a fan shroud? is there a 1/4" between the fan and the shroud?

Originally posted by MalibuMike70:
Doug,
do you have a fan shroud? is there a 1/4" between the fan and the shroud? plus what kind of fan do you have?

I'm no expert, but I just installed a new 355 with the same problems. I was using no shroud, a 190 degree stat, and incorrect atifreeze mixture. I changed stat to 160 degree, replaced rad cap, put shroud on, and put in the correct mixture, dropped temp from 230 to 175 to 180 average.
Now this may be way off, but I had a friend that rebuilt his marine 350 for his boat. He was overheating all the time. Within minutes of a run, temp would sky rocket. After trying everything he could think or, he took the heads off and found the head gasket were backwords and blocking some water galleys. Could this happen on a 350's? I had never heard of this. Food for thought.