'52 200, ultramantic, runs rough at about 50mph or more

Posted by Tim49 On 2012/9/9 21:56:14

hello all:
been some time since i have checked in. but i always read the post to see what i can learn

about 2 wks. ago i had to take my packard in for my yearly safety check recqrd. in hawaii. on the way i decided to drive to fill up the tank at a station that sells alc free gasoline. the car ran fantastic. i was very excited. (btw i ad a little diesel fuel to the tank as per this site, but did not on this day) when i left the gas station on a 55mph Hwy the car got real rough at about 45mph . I babied the car but it was not doing well at all. I got home very slowly and let the car sit and tried to think about what to do and read stuff in the shop manuel. etc. after a bit i put 1/2 cup of diesel in the tank and was on my way. still the same problem except i could get up to maybe 50 or 55mph.
Today i took the car out and got her up to 55 with no problem . ok for about 20 miles then back to the same symptoms.
I have recently done a tune up , plugs all wirre. cap rotor etc. timing
1. does the carb need adjusting? it sounds good at idle.
2. Is the next step to check the fuel pump??
i would appreciate any advice.