New T stat & new radiator

Your welcome.
Sounds like a stuck/bad float or needle and seat not sealing (Could be just badly out of adjustment). Now days all this stuff is manufactured in far off places by minimum wage workers. I have seen all kinds of manufacturing defects in new stuff and it is not limited to the cheep ones. I would pull the bowls off and inspect/replace the needle and seats, and while you have it apart I suggest installing a blow proof power valve. You should use one that is rated to open below your cruising vacuum level. If your running any cam you should probably stay below 10. the stock carb is probably 14 or 15.  


In the end there is no substitute for the old Q-J. The only bad thing about it is it's exceedingly arduous to tune (correctly that is). They were application specific designed. If you change anything ; cam , headers, compression ratio, ect.-- they need to be “worked on”. You can’t just turn a screw, but most guys do just that and they’re never quite right. That is where they got the bad reputation, but it's hard to beat a properly tuned Q-J (on the street).

Hope I have helped.  
.

Good Judgment comes from Experience...Experience comes from Bad Judgment

You have been a great help.  I can't thank you enough!  The Goat had some major electrical problems a couple years ago when we parked it.  We kept trying to start it with this new carb that we thought would fix the problem.  We used starting fluid and it would backfire and pop and bang etc.  Maybe we damaged the power valve or other parts while doing this??

Should I pull the bowl covers off and have a look?

My Dad is saying that we could just get a new carb and be done with it.  It would be nice to buy a square bore manifold and bolt on a nice square bore carb like a Holley Avenger or a Demon.  Since the Q jets are such a pain to tune, I'd rather not bother with that right now.  Just as thought.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

"You should use one that is rated to open below your cruising vacuum level."
Sorry ...that should be idle vacuum. you don't want it to open while you are at idle.

"We kept trying to start it with this new carb that we thought would fix the problem."  

Yes, that will damage the power valve. You can repair that carb and it should run fine. If you want the square bore you can get an adapter plate. Good luck!  
.

Good Judgment comes from Experience...Experience comes from Bad Judgment

I screwed the mixture screws all the way in with the engine idling, and the engine died, which means the power valve is functioning properly, correct?  But, I had one screw all the way in, and the other about one turn out, and the engine did not die..

Maybe the power valve is okay then.  Maybe the other items you mentioned (stuck/bad float, needle and seat not sealing) are causing the problem??  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

maybe,

but power valves are cheep and easy to change. I would change it. Holleys are easy to work on.  
.

Good Judgment comes from Experience...Experience comes from Bad Judgment

I probably will change the power valve, but I do not think that will fix my problem, but it probably help the engine's performance.  I called the Summit tech support line, and the guy was very knowledgeable and helpful.  He told me that power valves are very commonly falsely blamed for air/fuel related problems.  He has me convinced that I have an intake manifold gasket leak either on the underside of the intake or between adjacent cylinder ports.  I am going to order a pair of Mr. Gasket Ultra Seal intake gaskets.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

It's up to you but I would go with the felpro printoseal if you have a aluminum intake or the permatorque blue if you have a iron intake have not had much luck with those mr gasket gaskets.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Oh also you may want to check the mounting surfaces for straightness.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Yea that had crossed my mind.  I am thinking if I use a high quality thick gasket it should make up for any warping or roughness in the intake gasket surface.

I used Mr Gasket 502G 10 dollar gaskets last time.  When I installed the intake I snugged the intake bolts, then tightened the long water pump bolt that I neglected to tighten first, and then torqued the intake bolts.  So, I didn't actually install it correctly, and I used a really cheap gasket.  I should go back, use a high quality gasket, and carefully tighten all the bolts in the proper sequence.  II'll try the fel pro gasket you suggested.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

Some of the gaskets specify that they do not fit Ram Air applications.  I have a 400 with #13 heads.  Are those Ram Air heads?  

Also, I am using a Holley with an electric choke, so I want to seal off the exhaust cross over.  I need to make sure the new gaskets will have provisions for sealing the cross over.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

This will be what you are looking for FPP-1233 intake gaskets for pontiac I have used these gaskets and haven't had any problems with them they also come with the restrict and block off plates.
http://www.summitracing.com/parts/FPP-1233/  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Perfect.  I am currently on vacation, but I look forward to ordering a set and installing them.  It should really run like a champ with the new gaskets!  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

Did you get it fixed.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

I am still waiting for the parts I ordered to arrive.  
1969 GTO, YS code 400, 350hp/445 ft lbs of torque (stock specs), LT headers, H-pipe, 750 Holley 4160, Ignitor II ignition.  Complete Painless wiring.

Ok keep use posted on the outcome.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races