I've got a 300EX.. bought a 400EX carb with the flow mod with intentions of putting it on.. finally got around to it couple days later.. when i start it up it idled at about half throttle.. i just figured that the pilot jet was too big because it came off of a 440 or big 400 .. changed the pilot from a 50 to a 42.. and the main from a 200 or something down to a 142.. threw it back in.. still idled at about half throttle.. the float isn't stuck.. needle isn't broken or anything.. slide goes down and sits perfectly.. i tried seeing if there were leaks around the intake mani but were none.. threw the 300 carb back on.. worked like a dream.. something in the carb is messed up.. any suggestions please let me know.. thanks - adam

 

You'll probably have a hard time jetting a 38mm carb to run right on an engine that came stock with a 32mm carb. I'd guess you'll have to drop all jets quite a few sizes to get close, especially at idle. What are the smallest jets available for the 38? You might be better off rejetting the stock 32mm.

 

have you checked the idle screw under the carb?

 

it is the 35.5mm carb off of the older years.. but yeah.. the air screw is backed out far enough that it isn't even touching on the inside.. i took the black plate off of the side to make sure

 

Actually.. i just tried putting in a 38 pilot jet.. didn't work.. but i did happen to put the 50 pilot in my 32mm carb and it idles just fine.. in theory.. you would think that it would be fine.. i just got off the phone with honda.. told them what I had.. what was going on.. "uhh, it could be anything" I was like.. do you have any suggestions of what I should look for or try.. "no, like I said it could be anything".. honda is worthless anymore.. at least the one where I live.. they are always a bunch of douches when you try to ask them questions.. half the reason why i'm on atv connection