You pin the throttle and it stumbles, dies, stalls. This article deals with the carbs on the 150 and 250s. The factory installed carbs are a constant velocity style or CV for short. This is a very popular carb nowadays and even comes shipped on several new Harley Models. This CV carb is touted as being the next best thing to fuel injection. Trick --this carb needs a restricted air box. Most mechanics and high-performance motor heads all were taught that more is better. Us motorheads want to rip off the air box and install a free flowing KN style air filter. NOT SO for this CV carb. Don't do it. This carb has a small venturi, a butterfly like a chainsaw carb, and a slide that raises up by vaccuum. That slide must go up to also raise the main jet needle and that lets the gas flow to rev the engine. Screw up that velocity or vacuum and it stumbles and dies.Trick ---OIL that air filter foam. For one that helps filter the dirt from going into your engine and it also creates restriction in the air box. We can't tell you how many problem carb calls we have fixed by just oiling the foam air filter in the air box. Do not remove any plugs or restrictors from the air box. We thought we would make a brand new 250 breather easier by removing one little rubber plug from the air box. We then spent several hours pulling the carb off and on and off and on before we were smart enough to put the rubber plug back in the air box. Check the seal where the stock air box is screwed together. We found several cracks even on the carb side of the air filter which means direct path for dust and dirt into engine. Silicone those joints---seal that box up. Trick #3--when starting a cold engine you can squeeze the rubber hose between carb and air filter box to richen the whole system. Some of these CV Carbs are cold natured and if you are impatient just reach down and squeeze the hose with left hand and thumb throttle it with right hand. (250 models). There will be several more articles about this carb on this site.
