Mark, Take the air cleaner cover off and have a look to see if the accelerator jet (its on the primary - left - side looking from the drivers side) is squirting a nice clean "healthy" jet of fuel down the barrel.
Do this with the engine hot - so you don't have to wedge the choke flap open.
Just open the throttle with your right hand and have a good look - you should see the jet as described above.
Flat spots as you describe - where the engine eventually pulls through - are usually caused by this "injector" either being fully or partially blocked or the "o" ring that seals it to the carb body allowing fuel to leak out - not giving the engine the full measure it requires to richen up when the throttle is snapped open.
Sometimes the pump diaphragm perishes but you have changed it i assume when you rebuilt the carb with a kit.
If the jet is blocked, poke it out with a wire bristle from a wire brush - so long as its fine enough.
Probably have to remove the top of the carb to do so and carefully "pluck" the jet out with a pair of pliers
If the above checks out - hope it does not! - make sure the float level is correct (Haynes manual covers this)
Timing belt being a notch out usually results in a lack of power, not a flat spot - the same with the jets being fitted the wrong way round.
As a rule of thumb - the smaller main jet (119 factory) F30 tube and 240 air corrector goes on the primary, furthest away from you looking from the drivers/belt side.
The larger main jet (150) F30 tube and 240 air corrector goes on the secondary, closest to you.
The tick over jets follow the same rule - smallest, "50" furthest , Larger "80" closest.
Get the tick over jet wrong and it wont idle properly, get the mains wrong and again you have a lack of power as mentioned above.
Remember that if the jets are a different size from what i mention, chances are they have been replaced with larger ones by somebody previously - it does happen!
Check the belt if it makes you feel better and check the distributor timing with a timing light - 10 degrees before TDC with the vacuum pipe removed and blocked (If early Bosch that is) idling around 900 rpm.
Personally i would put my money on the accelerator jet.
Hope you find the problem.
