89 mazda b2200, sputtering and falling on its face while under load

Hello peoples, new to forum after a 6 year break.. had and built a lot of minis in my past, .. anyways I have a 89 b2200 5 speed I just picked up, new webber carb, timing belt and I just put a fuel pump, fuel filter on it.. along with new cap plugs wires coil... the problem I have it, the truck runs beautifull while sitting still and in neutral... but when I take it out for a drive, over 1200 rpm, it bucks and sputters and will not let me rev it up.... I can work the pedal a lil and gain a lil ground but its un driveable... if anybody can point me in right direction, it will be greatly appritiated.

Check the timing, and also make sure that the distributor is advancing. Also: a vacuum leak (not uncommon with Weber install) will play havoc with the timing.

The timing is spot on.. and think the prob im having is its not advanceing..... like I was saying, when the truck is just sitting there , it runs great, cuts out a lil around 3000 rpm, but revs threw it, but soon as ya put it in gear and try to drive, falls flat on its face.... also, when I disconnect the choke, I can get it somewhat driveable....thanks cusser for the help

First, is the vacuum line to the distributor connected to the carb and where (should be above the primary baffle plate)? If yes, continue.
Disconnect the advance hose from the carb, cap the carb vacuum port. Start the engine, watch the timing mark and suck on the vacuum hose. If the timing does not advance, the distributor advance is bad.
Next (leave vacuum line unattached, carb port capped). Look at the timing mark as you rev the engine to 2 grand. If the timing mark does not advance, the mechanical advance IN the distributor is stuck/frozen in place. Have to disassemble and clean all mating surfaces (buff with some 1000 grit wet of dry sandpaper).