Jayman
09-02-2005, 10:31 PM
With all the mods OMI is doing to O.S. carbs I decided to try boring out my YS91 carb. For my mod I bored the carb barrel, body and plastic bottom plate to 11mm. Just bolt it together minus the needle assemblies hold the barrel fully open and ream it out as an assembly. I used a 11mm reamer in a hand drill with some oil on it for lubrication. After boring you need to remove the barrel and break the edges around the hole in the barrel and on the carb body with some 600 grit sand paper. This will allow smooth rotation of the barrel in the carb body. 11mm is about the absolute max you can bore it out as you will not be able to close the carb enough to get the engine to idle. With mine bored at 11mm it will idle well but I have to pinch the fuel line to kill the engine. The nice thing about boring the YS is you do not need to remove any jets and anyone who can buy a reamer and has a hand drill can do it. The reamer will cost you between $15-$40 depending on the quality you feel you need. Set the needles a couple of clicks richer to start and tune from there. Mine runs stronger than before the mod and is still very smooth and consistant. :D
Jayman
Jayman