Datsun 2000 carb linkage issue

[two_68_510s]

My son's friend Dan has a Datsun 2000 that is giving him this issue:

Dan: "Any idea why SU carbs would have resistance right as it opens? It's fine once you're driving but starting from a stop there's pressure on the pedal and throttle valve to a point where it is very difficult to accelerate smoothly from a stop. It requires more pressure on the throttle than it should to get the engine to open up, but only from idle. And it only happens when the car is running. There isn't any throttle resistance when the car is off. Its not the pedal itself sticking or the throttle cable."


Joel : "Do you mean a flat spot in acceleration or a mechanical obstruction in the pedal?

Dan : "It feels like a mechanical obstruction, but it isnt the pedal or cable. The motor runs great it's not an issue with acceleration, just pedal feel. But Its only when the car is running. When I go to push the pedal from idle, it feels stuck and then suddenly lets go, making the car jump to around 2.5k rpm. But when the car is off, the pedal moves much more smoothly like it should."


Any ideas fellows?

[okayfine]

Always been an issue? Nothing off the top of my head would point to an issue with the carb bodies (insofar as I don't see how the carb bodies could create that symptom), but his symptom of linkage binding doesn't seem to be the case since he reports it doesn't feel stuck with the engine off. Interesting. Any info on when this started/what may have changed when this started?

The only thing I can think of is perhaps engine vacuum is affecting the throttle plates or some part of the smog equipment? Happens when the engine is one, but disappears when there's no vac.

[two_68_510s]

He reports no play in shaft

[bertvorgon]

Dry spot on the cable? I have see a throttle cable have one wire fray from the cable, and as it gets dragged into the housing, it binds for a MM or 2.

seen that a lot on bike cables, either right where they go through a ferrule, or the derailuer barrel adjuster.