Dad passed away in 2020...I inherited his 1963 62 series had been sitting since 1997...I put in new gas fuel filter...had a new head installed with hard valves (to run on unleaded fuel)..had the Rochester 4GC when the outside temp gets above 85F..the car vapor-locks and no fuel seems to be getting to the carburetor...have to prime it like crazy to get it started...when its cool/cold outside I can drive it all day long with no problem....out of answers.....FUEL PUMP?????????????????

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What temp does your temp gauge show when you have the  vapor lock problem ?

Be sure that your heat riser located at the end of your driver side exhaust manifold ( a flap inside the exhaust pipe ) is opening when the car get hot after a start up - it should go to open from closed 

You can wire it open if not opening and closing as it should 

If not opening it will cause the intake manifold  and carb to get excessively hot and vaporize the fuel in the carb 

Some have covered the metal fuel lines from the fuel pump to the carb in aluminum foil to insulate the fuel  lines from the under hood heat 

Do be sure you have all the correct stock gaskets between the carbs base and the intake 

Your problem is not the fuel pump In this case 

Try running your  heater to remove some heat from the engine and under the hood to see if that helps the problem when hot out —just to determine if under hood heat is causing the issue 

Be sure —-your points and timing are correct —as retarded timing will have the engine producing excessive heat under the hood 

Be sure your tune up has new parts and the specs are correct for points and timing in particular 

Vapor lock is a difficult issue to solve —for some —but the cause should have something to do with heat build up under the hood and engine  and comes up from time to time on this site 

Run only 93 octane in our year Cadillac engines 

Some suggestions that could be the cause of the vapor lock but no 100% solution fits all with a vapor lock issue 

If your coolant is old it will not transfer  the heat from the cooling system to the radiator  as it should be doing causing heat issues 

Problem mostly caused by under hood  and engine heat build up due to outside temps as you explained 

Best advice I can offer but no one solution fits all in this case 


THAX Tony...I even put a 1/2 inch insulated carb gasket on and it still evaporated the fuel on hot days...Temp gauge is normal/cool...????  I will try that exhaust flap like you suggested

1/2 inch phenolic carb spacer rather...well you know what I mean..LOL

temp gauge seems to be about 1/3 to the cool side...not that hot ?????

My 429 runs a  just  a bit past the first mark on the temp gauge  or about 180 degrees

The center mark on the temp gauge is 200 degrees and staring to get a bit high if it always runs there  


I agree with Tony.  I'm dealing with this right now, on another classic car I own.  So far, I've:

Wrapped the fuel lines (hard and soft lines) in the engine compartment with heat-radiating insulation you can get at any auto parts store. Made sure that none are close to the exhaust manifolds. (Keep the fuel lines away from the heater and radiator hoses, too.) 

Made sure no crimped fuel lines anywhere. 

Installed a phenolic spacer between the carb and intake manifold. Wood spacers work too...but don't use aluminum. Aluminum transfers heat very well...right into your carb. 

Made sure the correct gas cap (vented or non-vented) is on the car. 

Replaced the fuel filter.

Replaced mechanical fuel pump. 

Added an electric "helper fuel pump" near the gas tank. 

Checked timing.

Checked to make sure no vacuum leaks.  (Replace the hoses if they look cracked at all.) Installed worm clamps on every vacuum fitting to assure a tight fit. 

Made sure choke is operating correctly. 

Changed spark plugs with correct heat range. 

Made sure cooling system, fan, thermostat, hoses, are tight and working properly. 

Tip to try: Get an infra-red temp gun and aim it at various places in the engine compartment when the engine's misbehaving.  That can help isolate the cause.

A tough problem to remedy without modifying the engine compartment is to keep the engine compartment from building up excess heat. In the car I'm working on, the V8 is wedged into the small engine compartment and there's just about no room for the hot air to get out.

I'm next going to install a pair of small electric exhaust fans to duct hot air out from the underhood area. I don't want to cut the hood for air vents or scoops (definitely not ever on our old Cadillacs!!) so I'm having to be inventive to get a setup that works, fits and hides. This will be the toughest thing to accomplish, and I'm only doing it IF after all the remedies above don't fix the vapor lock. 

Vapor lock is often caused by more than one thing. So, take that mind-set when you try the next possible solution.

Another tip: I helped my pervious '64 Sedan deVille to run a lot better in the hot Texas summers, when I lived there, by experimenting with ducting the inlet of the air cleaner to the grille. I used simple 4" diameter collapsible hose, like what a clothes dryer uses. This was just an experiment, not for permanent use. It was tough to find a path to snake the ducting to the grille, but I finally was able to do it.

I was surprised at how much better the engine ran with injesting air that (even at 100-degrees ambient outside) was a lot cooler than the 200+ degrees under the hood. It made the engine respond more crisply and picked up some power too. I really liked the improvement, and had to convince myself to remove it (the right thing to do, of course, if you want a stock look underhood) because it DID work. 

So, yes to everything Tony wrote...and some/all of the ones I just listed, and you'll probably get 'er solved.

BTW: I always carry a can of starting fluid. Where the engine won't start when hot, or begins running crappy as the temps climb, I can quickly tell (by squirting the fluid into the carb) if it's fuel starvation/percolation or something not fuel-related at all.

Good luck, and be prepared to try a lot of these suggestions before you have it fixed for good. It can be a lot of little things that add up to a big problem. 

Let us know what you find out. 

PS: Run good-quality straight gasoline, not anything with ethanol.  

A good tip to cool the engine compartment is to remove the splash material that covers the front suspension arms from under the hood on the wheel wells —-if it is still there 

This is an old hot rod trick —and our Cadillacs will not get any debris  in the engine compartment by doing that just  some more  cool air flow 

A —lean fuel mixture  —will creat excessive heat in the cylinders and under the hood 

Check the color of your spark plugs 

They will be burning a whitish color if you have  a lean fuel mixture 

Plug  color SB  a brown paper bag when  it is correct 

Using todays ethanol 93 octane ethanol fuel will make the stock fuel mixture leaner than when new with higher octane fuels available back then  —- 103- 110 octane then 

My 64 - 429 does run perfect on 93  octane from the pump but I did have to rejet my 1406 Edelbrock carb two settings richer —than it came— on the primary jets and metering rods with the help of the Edelbrock techs 

Rejetting  the stock Carter or Rochester carbs is doable as well 

The fuel mixture needs to be richer than when our  stock Cadillacs were new due to the fuels available  we are burning from the pump today 

A number of things combined that will raise the heat level of the engine bay 

From my  Super Stock Drag racing days I can tell you that any engine will perform stronger when keeping the under hood temps down 

My Drag race car ran at 160 degree with every hot rod trick available  in the book 

Super  Stock Class track record holder —1/8 mile drag racing 


All rubber fuel lines need to be changed up  to hoses for ethanol fuel to run the ethanol fuels in our year Cadillacs from the pump today 

These Cadillacs can run perfect on 93 from the pump with ethanol —but you  do need a carb jetting and fuel hose upgrade to do that correctly 

You can trust me on this as I am doing just that 

That could be good new for some who might be wondering 

My 64-429 simply could not run any better than it does today— but then it is mine -a  passionate gearhead for life 


THANX all for your input...NICK

You can use  a 160 degree thermostat in the cooling system ( located in the coolant cross over pipe)  to lower the under hood temp 

That will lower the engines internal temp from the stock 180 to  approx 160 degrees 

Get the kind  that when it fails —it fails in the open position - -  Slant makes the best I know of 

The 160 degree will work fine 

Do use— two of the gaskets they sell for the thermostat housing  today as the  stock original is twice as thick as the one they sell for it  today 

I like the performance of the engine a tad more with the stock  180 or 195 degree thermostat but very little difference  in performance ———if heat reduction under the hood  is a must have 

This could be a  very helpful solution  for a vapor lock condition but it is always a problem trouble shooting a vapor lock condition because of the many possible variables involved 


I am sure there are two different fans in our year Cadillacs 

I think a 5 and 7 blade —one for AC and one for not 

You can reduce your under hood  with the 7 blade if you have the 5 

Could help with the vapor lock issue 


Here"s something simple to try, put  5 or 6 wooden spring loaded clothes pins on the fuel line between the fuel pump and the carburetor.The wooden pins will absorb the heat from the steel  fuel line preventing the fuel in the line from vaporizing Sounds crazy I know, but i had a 63 Pontiac Star Chief with AC when i was 18 and it would vapor lock, our hired man on the farm told me to do that, I thought he was CRAZY, that Pontiac  never vapor locked again.



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