Re: Inlet needle on OMC carbs

[frankr]

quote Adam1961:

Well I’m stumped. I had at it again tonight. Tried adjusting the drop limit, swapping floats, swapping needles, all without improvement. If I leave the bowl off and apply light pressure on the float, it cuts the fuel flow well, even against the primer bulb pressure.

Frank, where does the bowl vent to? I am not sure that I fully understand what you mean about the bowl vent. It seems to me that the fuel is getting forced into the high speed Venturi through the high speed needle seat/valve. If I seat the HS needle, then the bowl will build pressure and force the float up hard, stopping the fuel flow.

Imp have done 20+ of these carbs, and none have given me this much trouble.

Man, if I were a gambling man, I would bet that vent is plugged. Maybe a dirt dauber got in there. Look at where the low speed needle screws into the carb. See that big hole to the right and slightly below there? That is the bowl vent. The hole goes way in then down into the float chamber. If it is plugged up there is a bubble of air in the float chamber which cannot escape, which prevents fuel from flowing in and raising the float. Instead, it flows into the bottom of the bowl, through the high speed needle, and up through the main nozzle into the venturi, with nothing to even slow it down. It’s rare to see this on the Big Twin series of carburetors, but my goodness how many times have I seen it on Tillotson MD carbs where somebody has jammed a toothpick in the vent and broken it off to keep gas from squirting out. Took me all day to find it the first time.