Running rough when it hits just over 40 mph.....

[wiesket]

I have a 94 with the 3.0 and manual trans, was an automatic but I swapped it out. My problem is when I hit that magical 40+ mph it starts to buck a bit, like the darn engine doesn't fire on a cylinder. Am working on doing a tune up but would like some input as to what else I might need to do. Below 40 it runs good.

 

[bucko]

Could be fuel leaning out....if it were spark related, I'd think it would be poor at any speed/RPM range.

How old is the fuel filter? I'm not one to suggest tossing parts at the engine to locate the fix, but if you don't know how long it's been, then it's most likely overdue, and it's a cheap try, and should be replaced as a standard maintenace anyhow.