In my experience the 700 fuel gauge was always quick to respond- not at all like the one in our 240's.
I think it's more likely you lost the float ball- perhaps the arm was bent removing or installing? Also the actual variable resistor could have been damaged and finally failed- it's kind of a fine wire wrapped on a ceramic circuit board and it can break.

But certainly check all the connections first, as they can also be the source of your problems.
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Rob Bareiss, New London CT ::: 87 244DL- 249K, 88 245DL- 181K, 84 242DL, 89 244DL parts, SOLD: 86 244, 88 244GL, 87 244, 91 244, 82 245T, 88 744GLE, 86 244