Check the float level on the old carb. With the float bowl off tip the carb upside down and look at the float. It should be level from the pin side accross. If it is high at the side opposite of the pin, you have found your problem. The float is closing before you get enough fuel in the float bowl, caused by the viton seat swelling. Need to replace the float needle and seat after a good cleaning. I have even seen this in new carbs.