I am getting vapor lock on my 72 small block (auto, no air)every time I drive it for 8-10 miles and car sits for 20 - 30 minutes. Car will start and run for 15 seconds then dies, crank the engine and their is no fuel in carb until it cools off then starts right up. Other wise Car runs perfect when driving. I have read the all the info on this and have tried the following:
rebuilt orginal Q-jet
used the orginal 9/16 thick GM gasket
installed 2 new fuel pumps
installed molded rubber hoses from fuel lines to pump
new inline tube from pump to carb
2 new fuel filters (second with valve)
blew 4 PSI air back through the fuel return line to tank (clear)
fuel flows from line to pump
tank was replaced 15 years ago (N0 rust or junk in tank)
new fan clutch
recored radiator
al new hoses and belt
new vacum hoses, controlers, rebuilt actuators (lights & wipers)
I am lost on what to do to stop this vapor lock, any help would be greatly appreciated.

 

I was having a similar issue and it turned out to be a bad fuel pump. It was a rebuilt GM unit from Ecklers. Here's how i would try and solve it. You need to find out if its a problem with the line from the tank to the pump, or from the pump to the carb.

Its quick and easy to see if its the line from the fuel pump to the carb. Run a really long temporary rubber line from your fuel pump to the carb away from all heat sources. Route it under the car, around the outside of the fender, over the top of the motor, to the carb. I used string to hang it from my hood. I installed a clear plastic fuel filter in the line to watch what was going on. Mine would run about 10 minutes then start pumping small bubbles into the fuel filter. The bubbles kept getting bigger until the fuel filter was empty. My temporary rubber line was nice and cool so I knew that wasn't it.

Next I insulated about 2 feet of the fuel line from the fuel pump back towards the tank. I unbolted the fuel lines from the frame and lifted them up enough to get insulation all around it. These lines (inlet and return line to tank) stayed nice and cool.

If your fuel filter stays full at all times, its not vapor lock from the fuel lines. It could be boiling in your carb. Not sure why that would occur, something going on with your intake. You might need a spacer/heat shield between intake and carb.

With both of these lines nice and cold I was still having my problem. With advise from this forum, replaced my new $150 fuel pump with a different supplier, problem solved. Hope you can get your problem solved.

Roger

 

How are you determining there is no fuel in the carburetor? Are you looking down the primary side of carburetor moving throttle shaft and see no gas squirting from nozzles? Did you rebuild the carburetor?

 

I knew my carb was out of gas by looking in the primaries and opening the throttle. No gas was coming out. But like yours, when it would cool off, spin it over, fuel filter would fill up right away and car would crank and run fine.

I had 2 extra carbs in my shop tried them both, same problem so I knew it wasn't a carb issue.

If you install a clear plastic fuel filter just before the carb then you can tell if the fuel supply system is the problem or not. If its not the fuel supply system (fuel pump and fuel lines) then it is either the fuel filter in the at the entrance to the carb, or the carb. I am assuming you are not seeing any fuel squirting the carb when the car dies.


Roger

 

Yes, I rebuilt carb. After 20 to 30 minutes shut down the rea=start will run for 15 seconds then die. Remove air cleaner and open throtle plates and no fuel squirts in. Pour fuel in carb and will run for a few seconds and die. Wait an hour or two and it runs like nothing is wrong.

 

Vapor lock occurs at the fuel pump, not the carb so stop change those.

Do you have the original three port style pump including a return/bypass line to the tank. Is the bypass line free and clear?

Just for trouble shooting, next time you have the problem, throw ice on the pump and give it a minute or two. If it fires right up, that's your problem.

 

Sounds to me like carb is not the issue, unless its getting too hot and fuel is boiling in it, don't really know if that can happen. I suspect its the fuel supply (bad fuel pump or dirty fuel filter at carb inlet or vapor lock or messed up fuel supply/return system).

What carb are your running?

Is the fuel return line from pump back to tank hooked up?

Roger

 

when i was a kid my dads 53 merc used to get vapor lock and he would keep a gallon of water in the car to pour onto the fuel pump to cool it down. Would start right back up. But the vapor lock used to occur when driving not after sitting.

 

I am running the 7042202rochester Q-Jet. the fuel pump is the the orginal 2 port (fuel line & fuel return line). Just put air through the retuen line and can hear fuel going back to tank.

 

Sounds like your return lines are free and clear. They shouldn't be any problem unlsess they are getting heated up.

When my car is running at normal operating temperature the metal fuel lines (supply and return to the fuel pump) and the fuel pump are cool to the touch.

The metal line going from the fuel pump to the carb runs very close to the lower radiator hose and also very close to the heater hose that runs from the water pump to the heater core. Make sure your metal fuel line is not touching these hoses.

Run a short rubber hose between the carb and end of the metal fuel line with a clear plastic filter. You will need a hose barb to connect to the carb. You will be able to see if the filter starts running empty.

Roger

 

I have put a piece of rubber hose between the heater and the lower radiator hose plus sliped a piece of rubber hose over the fuel line from pump to carb with no help.

 

it sounds to me like it may be related to the accelerator pump not working when hot. At the bottom of the casting where the pump fits is a screw, underneath the screw is a steel ball. When you rebuilt the carb did you remove the screw and ball and made sure there was no dirt in the passage?

 

Is the rubber hose/shield touching the radiator hose?

Is the rubber hose/sheild touching the heater hose?

You don't want the rubber hose/shield touching the hot water hoses, they will conduct heat to the fuel line.

I believe the fuel pump acts as a one way valve. It should keep the fuel line to the carb full and not let it flow back towards the tank. Sounds like this could be an issue since your car will run for about 15 secs then die after sitting. Sounds like the carb has gas in it but then runs out and the fuel pump hasn't gotten fuel pumping yet.

 

very interesting.....if it runs for 15 secs it means its using the gas in the float.....and it stalls when that runs out.......so why is that not getting replenshied when its hot?..... cars need surprisingly little gas to keep running.....i would be looking at fuel supply issues so lets get right to the meat of it.....next time when its hot.....pull the fuel line off going into the carb and start the car and measure the quantity of gas being pumped out over 15 seconds......if it measures up to the fuel pump specs then the problem is not with the fuel supply it is carb related....and conversly if you are not getting any gas then it is the fuel supply

 

While im not sure what symptoms would occur if your heat riser was stuck closed or partially closed, if it was it would be directing more heat than required into the intake manifiold after the car has warmed up. Could it be causing the fuel to boil out? Its a long shot but it may be worth checking out. Its easy to check and youve tried most everything else.

 

Thanks Bob you have confirmed my thoughts, I will pull the gas line at the carb tomorow when I create the vapor lock problem (its easy). The heat riser is free and moves up and down freely. By the way I added a second gasket under the carb to intake (now about 3/4 thick), moved the timing up from factory setting of 8 to 12 degrees and that did not make any difference either. Do you think the fuel could be the problem? I am running shell regular (8.5 compression). Would premium and turbo blue 110 leaded added help any?

 

no the fuel grade is not the issue. do the test and report back...

 

Strange. I am having the same issue. I let it run and warm up and after about 10 min running it starts to spudder out. I have spark, thought it was coil saturation but tried another coil when it started happening and still the same. I just tried it and I am getting fuel squirting out of the boosters. So fuel is not the issue. could it be valves? Mine get to spuddering and no matter how much u pump it, it still dies.

Holley 780 cfm, pertronix ignition, summit fuel pump. It is acting up on me right now in the garage.

 

valves....like intake and exhaust valves....no they are more go/no go and do not deterioate when they get hot to the point of stopping an engine

 

Hum... Sorry for the hijack but figure maybe 2 with this issue, could help find the cause.