Lately my 1200 Datsun pickup 1993 has been hestitating and slightly jerking when I accelerate.
It happens both when the car is cold and warm and in the beginning of the acceleration.
Even when I change gear and start to accelerate!
There is a serious lack of power and it is so annoying... sometimes the engine stalls.

I recently changed my plugs, oil, and air/fule filters, battery etc...
Imagine, joining a High-Way and want to accelerate before the approaching cars arrive, there it goes duf duf, hesitate!

I have read many articles on this problem and they say the culprits are:

1). A light stumble only when accelerating from idle could possibly be caused by the idle adjusting screw,
being turned in too far. or

2). The accelerator pump circuit. http://www.theautoist.com/weber_carb.htm or

3). Acceleration hesitation is caused by Vacuum leaks.

Point (1) cannot be my problem, because I have adjusted idling and it's good.

Point (2) might not be the problem. But it is a potential. When I opened up the carb last week-end I made sure the Accelerator Jet squirts a fine spray into the throttle.

I have cleaned my carb many times making sure there is no blockage. One time I put bigger jets (two times bigger that what are there).
The problem disappeared but fuel consumption doubled, and the exhaust pipe turned black. The link above discourages
people from putting bigger jets to cure hesitation.

Could it be Vacuum leak? I suspect this because I realised that the Primary Throttle Shaft has too much free play.
On the contrary, the Secondary Shaft is tighter. Can this be re-built?

What do you guys suggest? I am feeling so bad about this problem.
I do not want to buy another carb if this can be solved.

It is a Hitachi carburator.