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1965 396/425 Hard hot starts

Anyone know how to fix a vapor lock problem? My Corvette starts fine when cold but when I try to start it when its at operating temp it cranks but won't start. I suspect vapor lock. Anyone with suggestions?

Submitted by: John Forster


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The first order of business is to determine if you really are having a vapor lock problem. To do that, begin by checking for proper spark as lack of spark will do the same thing as lack of fuel. Keep in mind that ignition components will often succumb to the extra heat the engine generates when it is shut off. Using a basic spark tester from the auto parts store follow directions and check for a minimum of 20,000 volts of clear blue-white spark at two spark plugs; one on each side of the engine. If the spark is less than 20K or yellow orange test the components in the ignition system. Pay close attention to the coils and if still equipped the condenser. Also make sure the starter isn’t drawing battery voltage too low as this will cause lowered spark and make the car harder to start. Minimum hot cranking voltage at the battery should be 9.6 volts. And close doesn’t count as even 9.5 is a fail. If the spark checks out properly remove the air cleaner and shine a light into the front throttle bore. While closely watching slowly depress the accelerator pedal and you should see two tiny streams of gas squirting into the throttle area if there is fuel in the system. If you do see fuel spray fuel is probably not the issue. If no fuel you next have to find out why. To do that you will need to know if you have a one line or two line fuel system. If the car has a one line fuel system look for places where the fuel line is close to the exhaust, especially critical if the car has tube headers. You might also consider installing insulating heat wrap or fire sleeve material on the line back to about the door. If the car has a two line system make sure it has the proper fuel pump and that the proper fuel pump is working right. If the fuel pump is okay do the checks for a one line system. In many cases it will be fuel percolation which is caused by too much heat getting into the metal of the carburetor. One of the main causes of that is an improper or seriously hardened carburetor base gasket. One thing that is often wrong is a lack of steel gasket on models originally equipped with them. So check the carburetor base gasket to make sure it is correct has not become hard and brittle and has a steel gasket if required. Posted by Administrator on 5/13/2014 1:28:53 PM

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