i have been fighting vapor lock issues since 1983 or so.
a few tests..
disconnect the fuel hose at the carb... blow into it. if you can blow backward thru the mechanical fuel pump.. the check valves are full of sediment. won't hold pressure. being able to blow bubbles in your gas tank is a sure give away..
symptoms.. cold starts and runs for 15 or 20 seconds and dies.. requires almost a minute of frantic cranking and pumping the gas pedal to get it to restart..
vapor lock when sitting in heavy traffic.. where the engine will start to hesitate and stall.. will be really really hard to crank..
also really hard to restart.. hot soak issues.. as old bogie described.. carb boiling over and vapors coming out.
with the cold start and stall. the issue is the fuel pump did not hold pressure and the fuel drained all the way back to the tank. this can leave carbs like the quadrajet with a needle and seat down low in the fuel bowl almost empty.
with the heavy or stopped traffic vapor lock and hot start..
the heat of the engine on the fuel pump and the line to the carb cause the fuel to boil in the lines. the fuel pushes backwards thru the pump. expanding into vapor as it does.. the fuel level in the carb drops and the engine goes lean.. this expands the pistons and rings till they grab tight in the cylinder walls. this is why the engines don't like to crank.. also lots of fuel vapor lights really fast in a hot chamber so this can also be fighting the pistons coming up..
similar issues with the heat soak.. 2 to 20 minute restart issue..
the fix.. simple.. a 10 micron fuel filter installed directly before the mechanical or electric fuel pump inlet.
i usually disable the ignition system.. disconnect the fuel line to the carb.. extend that into a clean clear soda bottle and have somebody crank the engine for 15 seconds... i usually do that 3 times.. i watch the pulses of fuel into the bottle.. each should be full and equal to the others.. not diminishing
the fast pulses of fuel will usually clean the sediment out of the check valves. 15 seconds of cranking.. three times.
magnets inside the fuel tank are always a good idea.. sediment can come from gas stations also.. i try to use neodymium magnets in the tank.. stick them away from where the end of the float arm comes down. i have stuck a float arm to the bottom.. these will collect any sediment ..
a friends van had issues with sediment in the fuel tank.. he actually kept clogging 10 micron filters but he drove at least 200 miles a day.. the van had a welded on tow hitch.. so there was no easy way to get the tank out.. he stuck a big speaker magnet to the bottom outside of the tank.. he went for almost 2 years without clogging a fuel filter.. while under the van for something. he saw his old speaker magnet and yanked it off. what was this doing there.. then he recalled.. and stuck it back.. he only made it six blocks before the filter clogged. then it did not clog again.
why watch the pulses into the bottle. if the screen in the tank is clogged.. if the fuel line is kinked or smashed someplace along.. you may have 5 or 10 seconds of good fuel flow.. but by 15 seconds the pump and lines have been emptied and the pulses reduce to a trickle.. or even drops.. i have seen that happen several times..
once on a long motor home that would overheat going up long hills.. everything was changed in the cooling system.. water pump, thermostat, radiator, fan.. turned out that a 10 second full flow test was not accurate.. the trouble was the camping world generator installer used a multi sized vacuum tee in the fuel hose coming out of the tank for the generator install.. he did not cut the smallest size off. that poor 440-3 was trying to pull fuel thru a 1/8" hole in the NOT FOR FUEL marked Tee.
another the car would not go over 25 MPH for more than a few seconds.. when i did the full flow cranking test.. the first few seconds were great.. but getting close to 15 seconds of cranking. the fuel out the hose completely stopped.. i hooked up my brake bleeder hand vacuum pump directly to the hose from the tank.. i was able to pump 22" of vacuum in that line. eventually i got some fuel but it still had 15 inches of vacuum. the line was full of sediment. a new speedometer cable core was used to snake it out then flushed with a garden sprayer .. did the same thing to the guys other car.. almost the same result.. this time the fuel line was smashed flat by improper jack stand and floor jack contact....
why put a fuel filter before an electric pump.. to stop sediment from wearing out the bushings and shafts.. from holding open the internal pump bypass piston causing the pump to not move any fuel under pressure.. just circulates it back into the pump inlet.. its a strange sound when this happens. the pump speeds up as the pressure output drops.. seconds later the engine dies.
in this odd picture.. i blew out a clogged fuel filter before throwing it away. i blew it into a white drain pan.. just for kicks i dropped a super magnet into it. notice the clean spot around it in the top of the image.. i was like.. WHAT.. i used my finger and pushed it across the bottom of the tray..
so magnets actually do work on fuel lines.. in fuel tanks... myth busted.
will you get better fuel economy.. perhaps.. if the needle and seat are not dribbling extra fuel into the float bowls raising the fuel level in the carb creating a rich condition.. i bet you will..
this big ugly filter is what i use on many applications...
why.. i can push on 5/16 or 3/8 fuel hose and clamp it on.. its also available in almost every parts and big box store nation wide.. and it has a large filter media area..
i even replace the stinky little nissan plastic fuel filters with them.
the fuel pump guts.. a check valve for the inlet side and one for the outlet..
after installing the fuel filter before the carb.. and doing the three 15 second full flow cranking into a bottle tests.. please hook the ignition and the carb back up and get the engine started.. with one hand.. use a pair of needle nose pliers to pinch the rubber hose between the fuel filter and the pump inlet closed.. with the other hand.. take the engine rpms to 1800 to 2500 RPMs.. when the engine starts to stall because of low fuel levels .. release the pliers. the needles and seats should be wide open with the low fuel levels. the fast surge of gasoline thru them should clean any sediment out of them and prevent the dribbles..
a warning. please have a wet bath towel around.. or preferably a CO2 fire extinguisher. once in a great while.. you can start a carb fire.. smoother it out with a wet towel.. zap it with a Co2 extinguisher.. dry chemical types will screw your day..
with holley carbs its harder to perform this.. as you have to open the primary slightly and then open the secondary.. so you use fuel from the secondary bowl.. this has resulted in a roaring intake manifold fire on my 460 ford with a duel plane intake and a 4 hole carb gasket. one side was on fire.. sounded like an jet engine on afterburner. the engine was still running on the other side of the carb.. i kept it revved up and snapped the throttle shut a few times and got it to go out.. but talk about scared.. my extinguisher was 50 feet away in my other van.. my towel had some gasoline spilled on it so i had tossed it out the door.
hope this helps a few rodders.. it works on daily drivers also..
and if you don't believe me.. disconnect the fuel line to the carb.. disconnect the ignition.. extend the fuel line with some clear vinyl hose up over the hood latch.. crank the motor till you get fuel in the clear section.. then try to explain to me where the fuel went when you stopped cranking.
the cure.. a 10 micron fuel filter... and magnets in the tank or on the outside bottom of the tank..