Does not accelerate.
Okay, here is your standard troubleshooting checklist for a 1901 (this also goes for 885, 2140 and 4000):
-- Is fuel sufficiently high in the bowl that gas flows through the "weep hole" at top of the pump cylinder and stands on top of the plunger?
-- Is pump cup made of leather or blue fluoroelastomer (instead of obsolete black buna rubber)?
-- Is there a 1/8" steel intake check ball in the bottom of the well secured by a metal retaining ring? If so, does it move freely?
-- Is there a discharge check needle in the proper bore? If so, is it the longer length needle as it should be? Has it been seated in its bore by lightly tapping with a hammer and small drift punch?
-- Is the bowl lid truly flat on the bottom? Or is it warped so badly at the corners that the pump charge is going everywhere (inside the bowl) except out through the nozzles?
-- Are the nozzles open?
-- Is the pump adjusted for the longest stroke (to be compatible with modern "gasoline")?
-- Is the strong spring on the pump plunger and the weaker spring on the pump arm?
On a 1901, the accel. pump nozzles are clearly visible with the air cleaner removed. With the engine off, advise the customer to work the throttle lever and see if gasoline shoots out of both nozzles the instant that the throttle lever is pushed toward "open" and keeps flowing for about 1/2 of the lever's travel toward wide open. If not, then the problem is most like one (or more) of those above