Hey guys, I waited until the very end before I posted this topic. I own a small engine shop and am not new to carb work, but man do I have a head scratcher here. Maybe yall can help. I have an hsr42 that will no quit flooding. As of right now, the carb only floods at idle, at which time, fuel starts to come out of the overflow. If i idle the engine up a bit, flooding quits. Also, if I squeeze the primer bulb and put pressure on the fuel inlet into the bowl, the carb will flood out the overflow until I removed the hose from the fuel tank to shut off flow. Here is what I have done, new float needle, then new float seat and o-ring, ran the carb and all parts through an ultrasonic cleaner, washed, dried and reassembled as I do all of my carb jobs at the shop. Adjusted the float adjustment tank as per mikuni specs, readjusted float needle tank three more times aftterwards (gradually closing the float sooner and sooner). I inspected the float and put in a can of fuel. It floated just fine and is a solid float which makes me believe that even if there were a pinhole, the solid float would not sink. I inspected the float hinge and hinge pin for uneven wear, held the carb upside down and blew air in it to see if it would leak passed the float needle or float needle seat o-ring. Old needle seat o-ring - leaked air, new needle o-ring - no leaks. I am not using ethanol blended fuel anymore since I swapped the needle and seat in the carb. Wtf? Any suggestions? Only thing I can think now is to check the brass overflow tube in the bowl for holes or damage. But if that was leaking, I feel like it would be a constant leak and not just at idle or under primer bulb pressure.

Which float needle and seat are you using? The 4.2 size that come standard with the carb (on the right in the picture below) are switched out for this application. I believe the correct one is the 2.3 size for pressure feed (Mikuni part # 786-27001-2.3):



Also just for reference, properly adjusted float looks roughly like this:

just squeezing the bulb can put upwards of 10lbs of pressure on the needle which it isn't designed to handle. if your engine has a bigger cam than stock it can aggravate the problem also. when you try to idle one down too slow it will actually put pressure back into the bowl thru a condition known as intake reversion. when you idle it up the condition is less or goes away to an extent. try and lower the float level as much as you can without going below the pilot jet suction. you may want to put a pressure gauge on the fuel line and see where its at. if it's more than 2 or 3 pounds max its too much. also if you just have a leaky needle when you speed the engine up it will use more fuel than it leaks hence no dribble out of the carb. but of course some mikunis are stubborn and just like to leak

Longer duration cam could cause some reversion. That sounds feasible. I will check fuel pressure and set float again. Hell of an ordeal lately. 4 wheeler had new float needle and started leaking as well. It was wear in the float hinge and pin that allowed the pin to slide back and float to hang uneven. Have a friend with a new boss motor with mikuni upgrade who was complaining of carb overflow as well. Adjusting float did not cure his issue either. Will need to look a bit further into that when he brings it into the shop. Old float needle seat was smaller inner diameter than the replacement I installed. But he old one leaked around the o-ring and the new one doesn't seem to do so. Maybe I need to installed smaller I.d. float seat with new o-ring and see what gives. Thanks guys!!!!!

Is there pressure in your tank from thermal expansion causing pressure. Just a thought..