Did both filters get changed? The original configuration has a filter in the tank at the fuel pickup above the large plug on the bottom of the tank with a square drive socket in it. Check the hoses coming fron the manifold up to the idle adjust valve. One of mine hade a crack out of sight on the lower side, although the symptons were rough idle and ran okay above idle. I think if somehow someone got the idle fuel setting at the regulator adjusted to idle smooth with a crack in the hoses it would mess up things above idle.

However, since you said fuel filter, not filters the symptoms sound like my car with a plugged prefilter. I'm running without the in tank filter; instead I installed an inline filter between the tank and the pump. Baldwin BF7691 fits in the line. I don't know if a 74 has enough length in the line from the tank to the pump to cut the hose and put in the filter. I have a '75 K-jet system in my '71 142e (which originally had the D-jet) and the older 140's had the fuel outlet on the right hand side of the tank. The external filter is easier to change without draining the tank.

Another possibility is that as I understand it some earlier K-jet cars got retrofitted with an in-tank pre-pump which has a sock type pickup filter. In the 240 list you will read tales of woe where someone has spent bucks on the main pump and other goodies, only to find out that the real problem is the in-tank pump. You could check there if you happen to have the in-tank pump setup for troubleshooting.

Mel