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I have a 1971 2wd with a 390. truck has always running very smooth and strong but recently it has started running very rough. the strane part is it seems to run fine once I get to 40mph and beyond. Things I have done to trr and solve it are:
1 added a fuel tank additive as I though it may be bad gas / water in the tank = no change.
2 done I complete tune up, mostly because it was due = no change.
3 checked all vacuum lines to see if there was a leak or one got disconnected = all was fine, no change.
I am at the end of my rope, does anyone have any thoughts of what to try next?