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Here goes I have been working on a 1970 GTO for quite a while. Had the motor all rebuilt the guy that had it before me used two motors to make one the block and heads are out of a 1967 Grand-prix 400 HO I have fixed a lot of stuff come to find out the intake manifold is a 9799068 which is right for a 1970 GTO and has a 7040263 carburetor which is correct for the intake the one for the 1967 400 is a 9786286 the carburetor should be a 7027263 the problem is it don’t want to start very good after you run it and let it sit for 15 minutes. I have heard of heat soak where the gas boils when you shut off the car and the other problem is the gasket that goes from the carburetor to the intake manifold is not for a 9799068 and the gasket I am using has a place for the tab. I look up the gaskets and they make one for each intake. I have talked with Cliff at High Performance Quadrajets and he is sending me the right gaskets I have attached Pictures of what I am talking about. I hope this is the problem I am beat up Thanks for any input.
 

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I had a similar thread and solutions depending on your root cause.

You can definitely help reduce the chance of boiling by adding a phenolic spacer between the intake manifold and carb. You can also buy a heat shield for the quadrajet to reduce boil cause by radiant heat (GM designed this many years ago aware of the problem).

https://www.gtoforum.com/f170/another-hot-start-not-starter-solenoid-131981/
 

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Depending on the condition of your carburetor, it's also possible that the bowl is draining due to leaking jet well plugs. This was a common problem "back in the day".

Also, in case you didn't know --- no Pontiac ever left the factory with one of those "400-4" stickers on the air cleaner lid, if that matters to you.

Bear
 

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Thanks cij911 this has helped waiting for some gaskets from Cliffs High Performance should be here Monday.
 

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Thanks BearGFR The Carburetor should be good Cliffs High Performance did it for me that will be the last thing I do and I didn't know about the sticker on the Air Cleaner may have to take it off. I should be getting the new Gaskets Monday and I am going to see if that helps Plus Cliff told me to wire the choke or Butterfly open. I will give you an update. I am starting to hate cars Running into lots of problems and it is hot here!!! Thanks again
 

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Okay I change the Gasket that goes from the carburetor to the intake got rid of 1” spacer and I still have the same problem if you leave the car sit for about five hours it will start right up but if you are running it and shut it off then come back 30 minutes later it has a tough time starting. I had a friend come over and see this happen and he told me to install an Electric fuel pump because it is Vaporizing and it is taking time for the gas to get to the carburetor, I guess it is boiling the gas into vapor because it is so hot so I have air in the line instead of gas so it takes some time to get the gas back to the Carburetor .
 

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Okay I change the Gasket that goes from the carburetor to the intake got rid of 1” spacer and I still have the same problem if you leave the car sit for about five hours it will start right up but if you are running it and shut it off then come back 30 minutes later it has a tough time starting. I had a friend come over and see this happen and he told me to install an Electric fuel pump because it is Vaporizing and it is taking time for the gas to get to the carburetor, I guess it is boiling the gas into vapor because it is so hot so I have air in the line instead of gas so it takes some time to get the gas back to the Carburetor .

Think about what you just said. It is boiling the gas off and is hard to start 30 minutes later. BUT, if you let it sit for 5 hours, it will start up. If the engine/fuel pump is not pumping the gas to the carb, how does the carb fill up on its own 5 hours later so it will fire right up?

Pontiacs can become hard to start when hot. The cam selection can do this. Too much cylinder pressure being built up and when the engine is hot, the starter has to work harder to kick it over. It could also be a weak starter, low battery charge, corroded battery cables, or even some of those aftermarket store bought battery cables which are really too small in wire gauge to give the starter a good solid 12 volts/maximum battery AMPS to spin it over.

Next could be timing. Timing can play a big role in how the engine fires up when hot. Have you played around with the timing to see if it fires up better when hot at different settings? Maybe timing is too far advanced or distributor issues. Could even be a heat soaked coil or some other electrical situation that needs to be checked.

Check all rubber lines & connections for cracks or deterioration - front and back at the gas tank. Had bad starting problems with my '73 Fury. Turned out to be dried out rubber hoses that had become hard as rock and the replacement type spring clamps that hold the hoses to the fuel line. The spring clamps are fine when the rubber is new & soft, but when they get hard, the spring clamps can't clamp tight enough. The rubber lines were somewhat loose and sloppy and when I grabbed one to check, I had gas pouring into my hand. No leaking at all from the line, but when I moved it around, it leaked pretty good. So the gas line was both sucking in air at times and probably pushing some of it out under heat/vapor pressure. Once I installed new rubber lines, band clamps, and a filter, problem solved. Car fires right up every time I stop and re-start. Now I still have to pump the gas a few times to get it to start when it sits a few days or weeks, but I know the gas evaporates sitting - and Plymouth's run hot under the hood.

The ethanol gas is indeed a big problem and it could be heat soak vaporizing the fuel, but it may not be. The electric fuel pump can work as my brother added one to his Mopar which would pitch a starting fit when he shut it down hot and then tried to fire it back up. BUT, it also took a little bit to get it fired up even when it cooled down - just more cranking of the engine than should be. Now he says it starts right up and he does not have the hard starting issues when it is hot and he shuts it down and then hops back in it 5 minutes later.

He also has the same issue with his '57 Caddy and it will not re-start on really hot days unless he lets it sit and cool down - which is a real pain. He has even gone so far as pouring cold water on the fuel lines that run along the engine to get it to fire up. Those older cars don't have big fuel pressures coming off the fuel pump to begin with, so he is adding an electric fuel pump to that car, as well as plugging off the exhaust heat crossover under the carb and installing an insulated carb gasket.

So, could be the gas or you may have other issues compounding the problem. :thumbsup:
 

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I still stand with my recommendation (different thread). Hold the throttle down ~1/3 the way while cranking and it will likely fire up quickly. My guess is the car is super rich (flooded from the gas bubbling into the cylinder) and needs the throttle blades open more....After it sits for five hours, it no longer is flooded and thus no longer an issue (my guess).

That said, everything Jim said is true. If your battery or starter are weak, that could be the problem when hot. If your car is cranking fine though, I doubt it is a starter or battery issue. Please try my recommendation and report back.

Thanks
 

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I still stand with my recommendation (different thread). Hold the throttle down ~1/3 the way while cranking and it will likely fire up quickly. My guess is the car is super rich (flooded from the gas bubbling into the cylinder) and needs the throttle blades open more....After it sits for five hours, it no longer is flooded and thus no longer an issue (my guess).

That said, everything Jim said is true. If your battery or starter are weak, that could be the problem when hot. If your car is cranking fine though, I doubt it is a starter or battery issue. Please try my recommendation and report back.

Thanks
X2, with last 2 posts. Just a little bump of fuel from the carb squirter and mine usually fires in 3 rotations when hot, also check your choke is fully open when its hot, if its settling back down could be flooding it.
 

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WOW Thanks for all the help I have a lot of things to try this weekend and if the gas was boiling out then why would it start so good 5 hours later makes no sense I am going to try holding the gas pedal 1/3 down when i go to start it when it's hot and check my fuel lines probably replace everything then I know that it has been done and play with the timing and wire the choke open. The Battery is new and the starter is also new no issues that I can see it cranks over good and strong. One last thing the temperature when it is hot is about 210-220 thought about putting a different thermostat in it to bring the temp down or getting a new radiator and replacing the clutch fan. I was thinking the thermostat might be a good idea currently running a 195 Degree. Oh yeah I live in Ogden, Utah and the temperature here is between 90 and 100 Again thank you for all the help.
 

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With regards to your cooling - If the system is working well and as designed, the thermostat should be selected based upon the desired operating temperature (or close). I run a 180* and appear to be running between 180 - 195 depending on ambient temperature, how hard I am running the motor, how little airflow, etc. Many run a 160* thermostat, but I think that is just hiding a different problem, but if it works, then it is likely a cheap fix.

I have a new aluminum radiator (Cold Case), clutched fan, and fan shroud - this helps significantly over the factory setup.
 

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With regards to your cooling - If the system is working well and as designed, the thermostat should be selected based upon the desired operating temperature (or close). I run a 180* and appear to be running between 180 - 195 depending on ambient temperature, how hard I am running the motor, how little airflow, etc. Many run a 160* thermostat, but I think that is just hiding a different problem, but if it works, then it is likely a cheap fix.

I have a new aluminum radiator (Cold Case), clutched fan, and fan shroud - this helps significantly over the factory setup.
I prefer to run the 160 T-stat. Not to hide a problem, but the cooler you can keep engine temps, the less prone it is to detonation. Heat contributes to engine detonation. You might also be able to advance your timing a little with the cooler engine and that means a little more power out of it.
 

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I run a 180 thermostat . I think anything less is not doing anything for you. I don't think many run below 180 when running and they run best at 180 to 210 . That's my opinion only. As far as not starting when hot I can remember my 68 in 1974 being a bear to start . I think the main problem back then were the starter getting hot and just not having the torque to turn it . . I never have that problem now but running 9.4 cr vs 10.75 might have a lot to do with it . Doug
 
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