Car won't start hot - Vapor lock?

Took Challenger out and made a couple of local stops.( 71 340 / stock 4bbl carb set up) .

After first stop ( after driving car for 20 minutes ) , just for a few minutes , car started back up, no issues.

After second stop driving another 5 minutes, but leaving it for about 15 minutes, car would crank over but not start.
As it was warmed up, I initially gave it no gas upon start.
But then tried a couple of pumps and still no start.
I had this happen when car was stopped when it was inspected last year, and we ended up spraying starter fluid in carb and it started.
Remembering this, and thinking Vapor Lock at first, I checked to see if gas was coming out of primaries when moving the throttle manually, and there was.
So I tried again, and it would not start.
I then sprayed starting fluid in the carb, and it started up.

I don't think with gas coming out of primaries that it is vapor lock, is it?
I also took gas cap off , and I did not hear any pressure noise upon opening.

Any feedback appreciated .

I've had that happen before and it was heat soak. It boils the gas in the carbs causing it to flood the engine. If it happens hold pedal all the way to floor and crank it. If you let go of the starter to rest don't let up on gas pedal so you don't add more fuel to the problem. It takes some cranking but should start without starting fluid.

Makes sense.
I did try to put gas pedal all the way to floor and crank but it wouldn't start, probably didn't crank it long enough though.

So how do you prevent heat soak?
Spacer between carb and manifold? If so how thick?

Heat riser off. non ethanol gas. wrap headers. As said likely fuel related but you might have a helper crank it & see if the coil wire makes a good spark to elim that area.

I was thinking something heat riser related, but not sure how to check or eliminate it.

Agreed on the coil check , but didn't have a helper at the time.
As the car started with fluid , figured coil was ok.

Car has stock exhaust manifolds.

The bimetallic spring on my heat riser broke the ear off one day. The flapper wouldn;t open so all teh exhaust crossed over the intake to go out the working side.

That was an eyeball check there, might need a mirror to see though.

They also tend to carbon up and no longer rotate either, that's a hand check. They made a special cleaner to address this.

Here's a video showing the setup on a slant six, same deal for you but easier to see.

Rust Penetrant #04318039AC

Thanks for the video Sniper.
Will check that as well

Make sure your tank vent system isn't clogged somewhere. Fuel in the tank gets hot, vaporizes and builds pressure. With a clogged vent system, the path of least resistance to relieve the pressure is through the carb. Took me forever to sort that out. Acts just like a heat soak/fuel boil-in-the-bowls scenario. Exacerbated by ethanol blended fuel.

So are you running a Carter TQ carb?

If so, make sure you are venting it to the charcoal carnister and make sure that at idle throttle setting the vent valve is actually open. There is a specific pre-set you need to adjust for. Any carb rebuild instructions have the dimensions.

Heck, if you can find the carb # I will look this up for you, I have quite a few different docs around here.

Thanks Chris.
Wouldn't I have had a pressure release when I removed the gas cap in this scenario?
I didn't seem to get any.

Thanks Diplomat, I'll check and confirm the carb # and type.
Not sure what you mean by the vent valve.
Let me take a look .

I had very similar hot start problems with my 360/Eddy Performer Intake/Eddy 600 carb.

Ended up putting a carb insulating gasket on and it made a HUGE difference. There are several different patterns depending on your carb/intake combo.

https://www.edelbrock.com/catalogsearch/result/index/?p=1&q=carb+insulator+base+gasket

You would indeed. I'd jump out and pop the cap right after a hot shut down, just to be see what happens and confirm or deny that clogged evap could be a problem.

What clued me in (I have a 73, different system with a charcoal canister and a vent line from the tank that I don't *believe* your car would have) is that the cap started venting. Troubleshooting led me to the liquid/vapor separator shuttle valve being gummed up. An overnight dip in a one gallon carb cleaner can took care of it.

Another trick I have used for vapor lock successfully on cars without the charcoal canister setup and no 1/4" tank return line is to plumb in one of those three legged fuel filters (goodmark sells a factory repro that can be had on rock auto for a little less impact on the wallet and has a tab that can go under a fuel pump or other convenient accessory bolt) and run a 1/4" rubber fuel line back to the tank. You can put a hole close to the fuel outlet on the sender, install a fitting and connect the new return line there. Make sure the fitting goes far enough inside the tank, preferably with a 90* bend) so that any liquid fuel in the return doesn't just dribble down the inside of the sender plate/tank. Or, put in a sending unit with a return but getting a properly functioning fuel gauge with aftermarket senders is hit or miss in my experience.

This won't help fuel boiling in the bowls though. A 1/2" wooden or phenolic spacer (open or 4 hole depending on your combo) will help with that. I've been tempted to run a carb heat shield as well, but squaring away the evap system and my coated quick fuel carb cured my problem with the same symptoms. With hotter weather coming on, I'll try an insulating gasket or spacer if I have a problem.

Knowing what type of evap system your car is equipped with, which carb you are running, how close to stock your engine is and if you have a return line of any sort will help the brain trust focus its advice.

Thanks for the link vdriver.

I'll double check the intake , but it is stock 71 340 manifold .
Went back and found model number on back DS tab, looks like model # is 4973S

Chris, I popped the cap right after it wouldn't start hot, for any pressure, and there didn't seem to be any.
adding vent lines seems like a lot of work than I want to tackle right now on the car.
I am going to start with a phenolic spacer once I figure out which carb/ intake set up I have at what spacer I need.

This should work, no?

THERMOQUAD SPACER

Not sure how that is better than a FEL-PRO 60273, which is 1/6th the price. However, either one will affect your stock choke linkage setup since they will space the carb up more.

My Fifth Avenue was doing that this spring and got worse with every day that it got warmer which ended up being the mechanical fuel pump

Agreed Sniper, seemed quite expensive.
The FEL PRO you listed seems to be only the base gasket though, not a spacer.

No kidding, so how did you end up figuring out it was the fuel pump?

In 71 the TQ base gasket was thin. In 78 it was about 1/4" thick same as that spacer, that's the Felpro 60273

Ok gotcha.

Forgot to reply about the choke linkage comment.
So will the spacer not work with it then?

My problem seemed like vapor lock but the pump just wasn't getting fuel through, wasn't leaking so it must have been a one way valve or whatever they use

It'll work, but you may need to lengthen the rod coming up from the choke well. Just needs looked at and addressed if there is an issue is all.

Ok , understood.

I hear that, no reason to fix a problem you easily ruled out. I'm with you on the spacer at this point, as a solution to fuel bowl boil (as was pointed out early on).

Yeah will keep you posted, thanks for the feedback.