1.6 td pulsating under load

[vixentd]

Car is a 1990 1.6td, idles perfectly and when you rev it up at idle in neutral, it revs perfectly.  However when you drive it as it climbs past 3500 starts to pulsate while pulling strong.  

Haven\t changed the fuel filter yet, but wonder if that could be the problem?  Anything else to check?

my other ride has a sink
www.vixenrv.org

[smutts]

Guessing mind, air being drawn through the front seal, or from somewhere else. This might make the tranfer pressure in the pump pulsate as the bubble goes through the regulating valve. Transfer pressure changes, so will the pump advance.
Or the filter is buggered.
I await my Unfeasable Theory Award.