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My 6 cyl '66 Mustang will start fine in the morning with no problems. If I drive for 10 miles, temp gauge is just below half at this point, and park it for 10 minutes then return it is difficult to start. The car will always start if I let it crank for about 8 seconds. It will then start and idle ok but as soon as I put it in gear it will either stall or stutter and continue to idle. It will repeat this until I rev it and get the temp back down to below half. Outside temp does not seem to have an effect as it does this if it is 70 or 90. It will also stall if temp gets above half while idling in hot conditions. Car never stalls while driving down the road. Car will do this with either 89 or 93 octane gas, any brand, with lead substitute or not. Car will start as normal if left over night or for 3 or more hours. Things I have checked:
1. Choke is off when this problem occurs.
2. There is fuel in the float bowl when this occurs.
3. Carb rebuilt and float height was set.
4. New steel fuel line tank to pump, pump to carb. bent to dublicate original.
5. New mechanical fuel pump.
6. Original alum. carb spacer with heater hose tube is in place
7. All other tune up items are new.
This seems like a vapor lock heat soak problem. If it is I am going to have to install a electric fuel pump near the tank and possibly a return line, but I do not want to have to resort to this.

Best Regards,
Aaron
 

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I6's are known to have a vapor lock problem. Try wrapping the fuel line with insulation. Can you add another carb spacer to get it further away from the engine?
 

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Try disconnecting the heater hoses under the carb. Do not go thru the spacer plate. Also change to a rubber fuel line from the pump to carb or wrap the steel line and see what happens. If that doesn't help then I would say it is the carb.
 
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