I can't say alot on the wt-990 carbs but I currently own a wt-813 and have no issues with it , hold full throttle, press bulb twice, then blip the throttle a couple of times (doing all this draws the fuel in as it has a pump), then engine starts on 2nd/3rd pull. Thats on a OBR 27.2 full mod which uses a Zenoah 4 bolt head.