Here we go again. I have an 8896 1050 that had some issues like yours. I also bought a used 1250 to convert the same as you need to do, most likely not a pump issue but an excessively rich mid. I have made mods to my 1050 as well. Read the following and you will understand this is a common Dominator problem.
http://speedtalk.com/forum/viewtopic.php?t=9934
http://www.motorsportsvillage.com/fo...pic.php?t=3283
http://www.motorsportsvillage.com/fo...pic.php?t=3742
http://www.motorsportsvillage.com/fo...pic.php?t=3811